Modern Java Types
Optional
Avoiding Null
Your findUser(id) method might not find a user. Returning null leads to
NullPointerException when callers forget to check. Optional
Create Optional values
Wrap values in Optional.
CreatingOptional.java
Replay: real traced execution (multi-file project)
// Creating and checking Optional values
// Concept: Optional - container for potentially absent value
import java.util.Optional;
public class CreatingOptional {
public static void main(String[] args) {
// Create Optional with value
Optional<String> present = Optional.of("Hello");
System.out.println("Present: " + present);
// Create Optional from nullable value
String nullable = "World";
Optional<String> maybe1 = Optional.ofNullable(nullable);
System.out.println("Maybe 1: " + maybe1);
nullable = null;
Optional<String> maybe2 = Optional.ofNullable(nullable);
System.out.println("Maybe 2: " + maybe2);
// Create empty Optional
Optional<String> empty = Optional.empty();
System.out.println("Empty: " + empty);
// Check if value is present
System.out.println("\nChecking presence:");
System.out.println("present.isPresent(): " + present.isPresent());
System.out.println("empty.isPresent(): " + empty.isPresent());
// isEmpty (Java 11+)
System.out.println("\npresent.isEmpty(): " + present.isEmpty());
System.out.println("empty.isEmpty(): " + empty.isEmpty());
// Get value if present
if (present.isPresent()) {
String value = present.get();
System.out.println("\nValue: " + value);
}
if (empty.isPresent()) {
System.out.println("This won't print");
} else {
System.out.println("Empty has no value");
}
// Example with numbers
Optional<Integer> num1 = Optional.of(42);
Optional<Integer> num2 = Optional.ofNullable(null);
System.out.println("\nNumbers:");
System.out.println("num1: " + num1);
System.out.println("num2: " + num2);
System.out.println("num1 present: " + num1.isPresent());
System.out.println("num2 present: " + num2.isPresent());
}
}
// Creating and checking Optional values
// Concept: Optional - container for potentially absent value
import java.util.Optional;
public class CreatingOptional {
public static void main(String[] args) {
// Create Optional with value
Optional<String> present = Optional.of("Hello");
System.out.println("Present: " + present);
// Create Optional from nullable value
String nullable = "Java";
Optional<String> maybe1 = Optional.ofNullable(nullable);
System.out.println("Maybe 1: " + maybe1);
nullable = null;
Optional<String> maybe2 = Optional.ofNullable(nullable);
System.out.println("Maybe 2: " + maybe2);
// Create empty Optional
Optional<String> empty = Optional.empty();
System.out.println("Empty: " + empty);
// Check if value is present
System.out.println("\nChecking presence:");
System.out.println("present.isPresent(): " + present.isPresent());
System.out.println("empty.isPresent(): " + empty.isPresent());
// isEmpty (Java 11+)
System.out.println("\npresent.isEmpty(): " + present.isEmpty());
System.out.println("empty.isEmpty(): " + empty.isEmpty());
// Get value if present
if (present.isPresent()) {
String value = present.get();
System.out.println("\nValue: " + value);
}
if (empty.isPresent()) {
System.out.println("This won't print");
} else {
System.out.println("Empty has no value");
}
// Example with numbers
Optional<Integer> num1 = Optional.of(42);
Optional<Integer> num2 = Optional.ofNullable(null);
System.out.println("\nNumbers:");
System.out.println("num1: " + num1);
System.out.println("num2: " + num2);
System.out.println("num1 present: " + num1.isPresent());
System.out.println("num2 present: " + num2.isPresent());
}
}
// Creating and checking Optional values
// Concept: Optional - container for potentially absent value
import java.util.Optional;
public class CreatingOptional {
public static void main(String[] args) {
// Create Optional with value
Optional<String> present = Optional.of("Hello");
System.out.println("Present: " + present);
// Create Optional from nullable value
String nullable = null;
Optional<String> maybe1 = Optional.ofNullable(nullable);
System.out.println("Maybe 1: " + maybe1);
nullable = null;
Optional<String> maybe2 = Optional.ofNullable(nullable);
System.out.println("Maybe 2: " + maybe2);
// Create empty Optional
Optional<String> empty = Optional.empty();
System.out.println("Empty: " + empty);
// Check if value is present
System.out.println("\nChecking presence:");
System.out.println("present.isPresent(): " + present.isPresent());
System.out.println("empty.isPresent(): " + empty.isPresent());
// isEmpty (Java 11+)
System.out.println("\npresent.isEmpty(): " + present.isEmpty());
System.out.println("empty.isEmpty(): " + empty.isEmpty());
// Get value if present
if (present.isPresent()) {
String value = present.get();
System.out.println("\nValue: " + value);
}
if (empty.isPresent()) {
System.out.println("This won't print");
} else {
System.out.println("Empty has no value");
}
// Example with numbers
Optional<Integer> num1 = Optional.of(42);
Optional<Integer> num2 = Optional.ofNullable(null);
System.out.println("\nNumbers:");
System.out.println("num1: " + num1);
System.out.println("num2: " + num2);
System.out.println("num1 present: " + num1.isPresent());
System.out.println("num2 present: " + num2.isPresent());
}
}
present ← Optional[Hello], nullable ← World, maybe1 ← Optional[World]
6public class CreatingOptional {7 public static void main(String[] args) {8 // Create Optional with value9 Optional<String> present→ Optional[Hello] = Optional.of("Hello");10 System.out.println("Present: " + presentOptional[Hello]);11 12 // Create Optional from nullable value13 String nullable→ World = "World";14 //@nullable="World", "Java", null15 Optional<String> maybe1→ Optional[World] = Optional.ofNullable(nullableWorld);16 System.out.println("Maybe 1: " + maybe1Optional[World]);17 18 nullable→ null = null;19 Optional<String> maybe2→ Optional.empty = Optional.ofNullable(nullablenull);20 System.out.println("Maybe 2: " + maybe2Optional.empty);21 22 //@help h123 // Optional.of(value) - throws if value is null24 // Optional.ofNullable(value) - creates empty if null25 // Optional.empty() - explicitly empty Optional26 //@end27 28 // Create empty Optional29 Optional<String> empty→ Optional.empty = Optional.empty();30 System.out.println("Empty: " + emptyOptional.empty);31 32 // Check if value is present33 System.out.println("\nChecking presence:");34 System.out.println("present.isPresent(): " + present.isPresent());35 System.out.println("empty.isPresent(): " + empty.isPresent());36 37 // isEmpty (Java 11+)38 System.out.println("\npresent.isEmpty(): " + present.isEmpty());39 System.out.println("empty.isEmpty(): " + empty.isEmpty());outputPresent: Optional[Hello] Maybe 1: Optional[World] Maybe 2: Optional.empty Empty: Optional.empty Checking presence: present.isPresent(): true empty.isPresent(): false present.isEmpty(): false empty.isEmpty(): truevalue ← Hello
41// Get value if present42if (present.isPresent()) {43 String value→ Hello = present.get();44 System.out.println("\nValue: " + valueHello);45}output Value: Helloelse
48 System.out.println("This won't print");49} else {50 System.out.println("Empty has no value");51}outputEmpty has no valuenum1 ← Optional[42], num2 ← Optional.empty
53// Example with numbers54Optional<Integer> num1→ Optional[42] = Optional.of(42);55Optional<Integer> num2→ Optional.empty = Optional.ofNullable(null);5657System.out.println("\nNumbers:");58System.out.println("num1: " + num1Optional[42]);59System.out.println("num2: " + num2Optional.empty);60System.out.println("num1 present: " + num1.isPresent());61System.out.println("num2 present: " + num2.isPresent());output Numbers: num1: Optional[42] num2: Optional.empty num1 present: true num2 present: false
present ← Optional[Hello], nullable ← Java, maybe1 ← Optional[Java]
6public class CreatingOptional {7 public static void main(String[] args) {8 // Create Optional with value9 Optional<String> present→ Optional[Hello] = Optional.of("Hello");10 System.out.println("Present: " + presentOptional[Hello]);11 12 // Create Optional from nullable value13 String nullable→ Java = "Java";14 Optional<String> maybe1→ Optional[Java] = Optional.ofNullable(nullableJava);15 System.out.println("Maybe 1: " + maybe1Optional[Java]);16 17 nullable→ null = null;18 Optional<String> maybe2→ Optional.empty = Optional.ofNullable(nullablenull);19 System.out.println("Maybe 2: " + maybe2Optional.empty);20 21 22 // Create empty Optional23 Optional<String> empty→ Optional.empty = Optional.empty();24 System.out.println("Empty: " + emptyOptional.empty);25 26 // Check if value is present27 System.out.println("\nChecking presence:");28 System.out.println("present.isPresent(): " + present.isPresent());29 System.out.println("empty.isPresent(): " + empty.isPresent());30 31 // isEmpty (Java 11+)32 System.out.println("\npresent.isEmpty(): " + present.isEmpty());33 System.out.println("empty.isEmpty(): " + empty.isEmpty());outputPresent: Optional[Hello] Maybe 1: Optional[Java] Maybe 2: Optional.empty Empty: Optional.empty Checking presence: present.isPresent(): true empty.isPresent(): false present.isEmpty(): false empty.isEmpty(): truevalue ← Hello
35// Get value if present36if (present.isPresent()) {37 String value→ Hello = present.get();38 System.out.println("\nValue: " + valueHello);39}output Value: Helloelse
42 System.out.println("This won't print");43} else {44 System.out.println("Empty has no value");45}outputEmpty has no valuenum1 ← Optional[42], num2 ← Optional.empty
47// Example with numbers48Optional<Integer> num1→ Optional[42] = Optional.of(42);49Optional<Integer> num2→ Optional.empty = Optional.ofNullable(null);5051System.out.println("\nNumbers:");52System.out.println("num1: " + num1Optional[42]);53System.out.println("num2: " + num2Optional.empty);54System.out.println("num1 present: " + num1.isPresent());55System.out.println("num2 present: " + num2.isPresent());output Numbers: num1: Optional[42] num2: Optional.empty num1 present: true num2 present: false
present ← Optional[Hello], nullable ← null, maybe1 ← Optional.empty
6public class CreatingOptional {7 public static void main(String[] args) {8 // Create Optional with value9 Optional<String> present→ Optional[Hello] = Optional.of("Hello");10 System.out.println("Present: " + presentOptional[Hello]);11 12 // Create Optional from nullable value13 String nullable→ null = null;14 Optional<String> maybe1→ Optional.empty = Optional.ofNullable(nullablenull);15 System.out.println("Maybe 1: " + maybe1Optional.empty);16 17 nullable→ null = null;18 Optional<String> maybe2→ Optional.empty = Optional.ofNullable(nullablenull);19 System.out.println("Maybe 2: " + maybe2Optional.empty);20 21 22 // Create empty Optional23 Optional<String> empty→ Optional.empty = Optional.empty();24 System.out.println("Empty: " + emptyOptional.empty);25 26 // Check if value is present27 System.out.println("\nChecking presence:");28 System.out.println("present.isPresent(): " + present.isPresent());29 System.out.println("empty.isPresent(): " + empty.isPresent());30 31 // isEmpty (Java 11+)32 System.out.println("\npresent.isEmpty(): " + present.isEmpty());33 System.out.println("empty.isEmpty(): " + empty.isEmpty());outputPresent: Optional[Hello] Maybe 1: Optional.empty Maybe 2: Optional.empty Empty: Optional.empty Checking presence: present.isPresent(): true empty.isPresent(): false present.isEmpty(): false empty.isEmpty(): truevalue ← Hello
35// Get value if present36if (present.isPresent()) {37 String value→ Hello = present.get();38 System.out.println("\nValue: " + valueHello);39}output Value: Helloelse
42 System.out.println("This won't print");43} else {44 System.out.println("Empty has no value");45}outputEmpty has no valuenum1 ← Optional[42], num2 ← Optional.empty
47// Example with numbers48Optional<Integer> num1→ Optional[42] = Optional.of(42);49Optional<Integer> num2→ Optional.empty = Optional.ofNullable(null);5051System.out.println("\nNumbers:");52System.out.println("num1: " + num1Optional[42]);53System.out.println("num2: " + num2Optional.empty);54System.out.println("num1 present: " + num1.isPresent());55System.out.println("num2 present: " + num2.isPresent());output Numbers: num1: Optional[42] num2: Optional.empty num1 present: true num2 present: false
Optional.of() for non-null, Optional.ofNullable() for maybe-null, Optional.empty() for absent.
Optional
Container that may or may not contain a value. Makes null explicit.
Check and use values
Handle present and absent cases.
IfPresent.java
Replay: real traced execution (multi-file project)
// ifPresent and ifPresentOrElse
// Concept: ifPresent - execute action if value exists
import java.util.Optional;
public class IfPresent {
public static void main(String[] args) {
// ifPresent - do something if value exists
Optional<String> name = Optional.of("Alice");
System.out.println("Using ifPresent:");
name.ifPresent(value -> System.out.println(" Hello, " + value));
Optional<String> empty = Optional.empty();
empty.ifPresent(value -> System.out.println(" This won't print"));
// ifPresentOrElse (Java 9+)
System.out.println("\nUsing ifPresentOrElse:");
name.ifPresentOrElse(
value -> System.out.println(" Found: " + value),
() -> System.out.println(" Not found")
);
empty.ifPresentOrElse(
value -> System.out.println(" Found: " + value),
() -> System.out.println(" Not found")
);
// Process multiple optionals
Optional<Integer>[] scores = new Optional[] {
Optional.of(85),
Optional.empty(),
Optional.of(92),
Optional.of(78)
};
System.out.println("\nProcessing scores:");
for (int i = 0; i < scores.length; i++) {
int index = i;
scores[i].ifPresent(score ->
System.out.println(" Score " + index + ": " + score)
);
}
// Accumulate values
System.out.println("\nAccumulating:");
int[] sum = {0};
int[] count = {0};
for (Optional<Integer> score : scores) {
score.ifPresent(value -> {
sum[0] += value;
count[0]++;
});
}
if (count[0] > 0) {
double average = (double) sum[0] / count[0];
System.out.printf(" Average: %.1f%n", average);
}
// Log if missing
System.out.println("\nLogging:");
Optional<String> config = Optional.empty();
config.ifPresentOrElse(
value -> System.out.println(" Config: " + value),
() -> System.out.println(" WARNING: Config not found")
);
}
}
// ifPresent and ifPresentOrElse
// Concept: ifPresent - execute action if value exists
import java.util.Optional;
public class IfPresent {
public static void main(String[] args) {
// ifPresent - do something if value exists
Optional<String> name = Optional.of("Bob");
System.out.println("Using ifPresent:");
name.ifPresent(value -> System.out.println(" Hello, " + value));
Optional<String> empty = Optional.empty();
empty.ifPresent(value -> System.out.println(" This won't print"));
// ifPresentOrElse (Java 9+)
System.out.println("\nUsing ifPresentOrElse:");
name.ifPresentOrElse(
value -> System.out.println(" Found: " + value),
() -> System.out.println(" Not found")
);
empty.ifPresentOrElse(
value -> System.out.println(" Found: " + value),
() -> System.out.println(" Not found")
);
// Process multiple optionals
Optional<Integer>[] scores = new Optional[] {
Optional.of(85),
Optional.empty(),
Optional.of(92),
Optional.of(78)
};
System.out.println("\nProcessing scores:");
for (int i = 0; i < scores.length; i++) {
int index = i;
scores[i].ifPresent(score ->
System.out.println(" Score " + index + ": " + score)
);
}
// Accumulate values
System.out.println("\nAccumulating:");
int[] sum = {0};
int[] count = {0};
for (Optional<Integer> score : scores) {
score.ifPresent(value -> {
sum[0] += value;
count[0]++;
});
}
if (count[0] > 0) {
double average = (double) sum[0] / count[0];
System.out.printf(" Average: %.1f%n", average);
}
// Log if missing
System.out.println("\nLogging:");
Optional<String> config = Optional.empty();
config.ifPresentOrElse(
value -> System.out.println(" Config: " + value),
() -> System.out.println(" WARNING: Config not found")
);
}
}
// ifPresent and ifPresentOrElse
// Concept: ifPresent - execute action if value exists
import java.util.Optional;
public class IfPresent {
public static void main(String[] args) {
// ifPresent - do something if value exists
Optional<String> name = Optional.empty();
System.out.println("Using ifPresent:");
name.ifPresent(value -> System.out.println(" Hello, " + value));
Optional<String> empty = Optional.empty();
empty.ifPresent(value -> System.out.println(" This won't print"));
// ifPresentOrElse (Java 9+)
System.out.println("\nUsing ifPresentOrElse:");
name.ifPresentOrElse(
value -> System.out.println(" Found: " + value),
() -> System.out.println(" Not found")
);
empty.ifPresentOrElse(
value -> System.out.println(" Found: " + value),
() -> System.out.println(" Not found")
);
// Process multiple optionals
Optional<Integer>[] scores = new Optional[] {
Optional.of(85),
Optional.empty(),
Optional.of(92),
Optional.of(78)
};
System.out.println("\nProcessing scores:");
for (int i = 0; i < scores.length; i++) {
int index = i;
scores[i].ifPresent(score ->
System.out.println(" Score " + index + ": " + score)
);
}
// Accumulate values
System.out.println("\nAccumulating:");
int[] sum = {0};
int[] count = {0};
for (Optional<Integer> score : scores) {
score.ifPresent(value -> {
sum[0] += value;
count[0]++;
});
}
if (count[0] > 0) {
double average = (double) sum[0] / count[0];
System.out.printf(" Average: %.1f%n", average);
}
// Log if missing
System.out.println("\nLogging:");
Optional<String> config = Optional.empty();
config.ifPresentOrElse(
value -> System.out.println(" Config: " + value),
() -> System.out.println(" WARNING: Config not found")
);
}
}
name ← Optional[Alice], empty ← Optional.empty
6public class IfPresent {7 public static void main(String[] args) {8 // ifPresent - do something if value exists9 Optional<String> name→ Optional[Alice] = Optional.of("Alice");10 //@name=Optional.of("Alice"), Optional.of("Bob"), Optional.empty()11 12 System.out.println("Using ifPresent:");13 name.ifPresent(value -> System.out.println(" Hello, " + value));14 15 Optional<String> empty→ Optional.empty = Optional.empty();16 empty.ifPresent(value -> System.out.println(" This won't print"));17 18 //@help h119 // ifPresent(Consumer) executes lambda only if value present20 // Safer than isPresent() + get() pattern21 // No risk of forgetting to check22 //@end23 24 // ifPresentOrElse (Java 9+)25 System.out.println("\nUsing ifPresentOrElse:");26 27 name.ifPresentOrElse(28 value -> System.out.println(" Found: " + value),29 () -> System.out.println(" Not found")30 );31 32 empty.ifPresentOrElse(33 value -> System.out.println(" Found: " + value),34 () -> System.out.println(" Not found")35 );36 37 // Process multiple optionals38 Optional<Integer>[] scores = new Optional[] {39 Optional.of(85),40 Optional.empty(),41 Optional.of(92),42 Optional.of(78)43 };44 45 System.out.println("\nProcessing scores:");46 for (int i = 0; i < scores.length; i++) {outputUsing ifPresent: Using ifPresentOrElse: Processing scores:index ← 0
pass 1 of 445System.out.println("\nProcessing scores:");46for (int i0 = 0; i < scores.length4; i++) {47 int index→ 0 = i;48 scores[i]Optional[85].ifPresent(score -> 49 System.out.println(" Score " + index + ": " + score)50 );51}All 4 passes — pass 1 is the card above pass iscores[i]index1 0 Optional[85] 0 2 1 Optional.empty 1 3 2 Optional[92] 2 4 3 Optional[78] 3 int[] sum = {0};
53// Accumulate values54System.out.println("\nAccumulating:");5556int[] sum = {0};57int[] count = {0};output Accumulating:for (Optional<Integer> score : scores)
pass 1 of 459for (Optional<Integer> scoreOptional[85] : scores) {60 score.ifPresent(value -> {61 sum[0] += value;62 count[0]++;63 });64}All 4 passes — pass 1 is the card above pass scorecount[0]sum[0]average1 Optional[85] — — — 2 Optional.empty — — — 3 Optional[92] — — — 4 Optional[78] 3 255 85.0 value ->
pass 1 of 359for (Optional<Integer> score : scores) {60 score.ifPresent(value -> {61 sum[0] += value;62 count[0]++;63 });64}All 3 passes — pass 1 is the card above pass count[0]sum[0]average1 — — — 2 — — — 3 3 255 85.0 average ← 85.0
66if (count[0]3 > 0) {67 double average→ 85.0 = (double) sum[0]255 / count[0]3;68 System.out.printf(" Average: %.1f%n", average85.0);69}config ← Optional.empty
71// Log if missing72System.out.println("\nLogging:");7374Optional<String> config→ Optional.empty = Optional.empty();7576config.ifPresentOrElse(77 value -> System.out.println(" Config: " + value),78 () -> System.out.println(" WARNING: Config not found")79);output Logging:
name ← Optional[Bob], empty ← Optional.empty
6public class IfPresent {7 public static void main(String[] args) {8 // ifPresent - do something if value exists9 Optional<String> name→ Optional[Bob] = Optional.of("Bob");10 11 System.out.println("Using ifPresent:");12 name.ifPresent(value -> System.out.println(" Hello, " + value));13 14 Optional<String> empty→ Optional.empty = Optional.empty();15 empty.ifPresent(value -> System.out.println(" This won't print"));16 17 18 // ifPresentOrElse (Java 9+)19 System.out.println("\nUsing ifPresentOrElse:");20 21 name.ifPresentOrElse(22 value -> System.out.println(" Found: " + value),23 () -> System.out.println(" Not found")24 );25 26 empty.ifPresentOrElse(27 value -> System.out.println(" Found: " + value),28 () -> System.out.println(" Not found")29 );30 31 // Process multiple optionals32 Optional<Integer>[] scores = new Optional[] {33 Optional.of(85),34 Optional.empty(),35 Optional.of(92),36 Optional.of(78)37 };38 39 System.out.println("\nProcessing scores:");40 for (int i = 0; i < scores.length; i++) {outputUsing ifPresent: Using ifPresentOrElse: Processing scores:index ← 0
pass 1 of 439System.out.println("\nProcessing scores:");40for (int i0 = 0; i < scores.length4; i++) {41 int index→ 0 = i;42 scores[i]Optional[85].ifPresent(score -> 43 System.out.println(" Score " + index + ": " + score)44 );45}All 4 passes — pass 1 is the card above pass iscores[i]index1 0 Optional[85] 0 2 1 Optional.empty 1 3 2 Optional[92] 2 4 3 Optional[78] 3 int[] sum = {0};
47// Accumulate values48System.out.println("\nAccumulating:");4950int[] sum = {0};51int[] count = {0};output Accumulating:for (Optional<Integer> score : scores)
pass 1 of 453for (Optional<Integer> scoreOptional[85] : scores) {54 score.ifPresent(value -> {55 sum[0] += value;56 count[0]++;57 });58}All 4 passes — pass 1 is the card above pass scorecount[0]sum[0]average1 Optional[85] — — — 2 Optional.empty — — — 3 Optional[92] — — — 4 Optional[78] 3 255 85.0 value ->
pass 1 of 353for (Optional<Integer> score : scores) {54 score.ifPresent(value -> {55 sum[0] += value;56 count[0]++;57 });58}All 3 passes — pass 1 is the card above pass count[0]sum[0]average1 — — — 2 — — — 3 3 255 85.0 average ← 85.0
60if (count[0]3 > 0) {61 double average→ 85.0 = (double) sum[0]255 / count[0]3;62 System.out.printf(" Average: %.1f%n", average85.0);63}config ← Optional.empty
65// Log if missing66System.out.println("\nLogging:");6768Optional<String> config→ Optional.empty = Optional.empty();6970config.ifPresentOrElse(71 value -> System.out.println(" Config: " + value),72 () -> System.out.println(" WARNING: Config not found")73);output Logging:
name ← Optional.empty, empty ← Optional.empty
6public class IfPresent {7 public static void main(String[] args) {8 // ifPresent - do something if value exists9 Optional<String> name→ Optional.empty = Optional.empty();10 11 System.out.println("Using ifPresent:");12 name.ifPresent(value -> System.out.println(" Hello, " + value));13 14 Optional<String> empty→ Optional.empty = Optional.empty();15 empty.ifPresent(value -> System.out.println(" This won't print"));16 17 18 // ifPresentOrElse (Java 9+)19 System.out.println("\nUsing ifPresentOrElse:");20 21 name.ifPresentOrElse(22 value -> System.out.println(" Found: " + value),23 () -> System.out.println(" Not found")24 );25 26 empty.ifPresentOrElse(27 value -> System.out.println(" Found: " + value),28 () -> System.out.println(" Not found")29 );30 31 // Process multiple optionals32 Optional<Integer>[] scores = new Optional[] {33 Optional.of(85),34 Optional.empty(),35 Optional.of(92),36 Optional.of(78)37 };38 39 System.out.println("\nProcessing scores:");40 for (int i = 0; i < scores.length; i++) {outputUsing ifPresent: Using ifPresentOrElse: Processing scores:index ← 0
pass 1 of 439System.out.println("\nProcessing scores:");40for (int i0 = 0; i < scores.length4; i++) {41 int index→ 0 = i;42 scores[i]Optional[85].ifPresent(score -> 43 System.out.println(" Score " + index + ": " + score)44 );45}All 4 passes — pass 1 is the card above pass iscores[i]index1 0 Optional[85] 0 2 1 Optional.empty 1 3 2 Optional[92] 2 4 3 Optional[78] 3 int[] sum = {0};
47// Accumulate values48System.out.println("\nAccumulating:");4950int[] sum = {0};51int[] count = {0};output Accumulating:for (Optional<Integer> score : scores)
pass 1 of 453for (Optional<Integer> scoreOptional[85] : scores) {54 score.ifPresent(value -> {55 sum[0] += value;56 count[0]++;57 });58}All 4 passes — pass 1 is the card above pass scorecount[0]sum[0]average1 Optional[85] — — — 2 Optional.empty — — — 3 Optional[92] — — — 4 Optional[78] 3 255 85.0 value ->
pass 1 of 353for (Optional<Integer> score : scores) {54 score.ifPresent(value -> {55 sum[0] += value;56 count[0]++;57 });58}All 3 passes — pass 1 is the card above pass count[0]sum[0]average1 — — — 2 — — — 3 3 255 85.0 average ← 85.0
60if (count[0]3 > 0) {61 double average→ 85.0 = (double) sum[0]255 / count[0]3;62 System.out.printf(" Average: %.1f%n", average85.0);63}config ← Optional.empty
65// Log if missing66System.out.println("\nLogging:");6768Optional<String> config→ Optional.empty = Optional.empty();6970config.ifPresentOrElse(71 value -> System.out.println(" Config: " + value),72 () -> System.out.println(" WARNING: Config not found")73);output Logging:
ifPresent() runs code only when value exists. Avoids null checks.
Provide defaults
Get value or use fallback.
Defaults.java
Replay: real traced execution (multi-file project)
// orElse, orElseGet, orElseThrow
// Concept: default values - providing fallbacks
import java.util.Optional;
public class Defaults {
public static void main(String[] args) {
// orElse - provide default value
Optional<String> name = Optional.of("Alice");
Optional<String> empty = Optional.empty();
String result1 = name.orElse("Unknown");
String result2 = empty.orElse("Unknown");
System.out.println("orElse:");
System.out.println(" name: " + result1);
System.out.println(" empty: " + result2);
// orElseGet - compute default lazily
System.out.println("\norElseGet:");
String r1 = name.orElseGet(() -> {
System.out.println(" Computing default...");
return "Generated";
});
System.out.println(" Result: " + r1);
String r2 = empty.orElseGet(() -> {
System.out.println(" Computing default...");
return "Generated";
});
System.out.println(" Result: " + r2);
// orElse vs orElseGet difference
System.out.println("\nDifference:");
System.out.println("With orElse:");
String x1 = name.orElse(expensiveDefault());
System.out.println("\nWith orElseGet:");
String x2 = name.orElseGet(() -> expensiveDefault());
// orElseThrow - require value
System.out.println("\norElseThrow:");
try {
String value = name.orElseThrow();
System.out.println(" Got value: " + value);
} catch (Exception e) {
System.out.println(" Error: " + e);
}
try {
String value = empty.orElseThrow();
System.out.println(" Got value: " + value);
} catch (Exception e) {
System.out.println(" Error: " + e.getClass().getSimpleName());
}
// orElseThrow with custom exception
try {
String value = empty.orElseThrow(() ->
new IllegalArgumentException("Value required")
);
} catch (IllegalArgumentException e) {
System.out.println(" Custom error: " + e.getMessage());
}
// Practical examples
System.out.println("\nPractical:");
Optional<Integer> port = Optional.empty();
int serverPort = port.orElse(8080);
System.out.println(" Server port: " + serverPort);
Optional<String> env = Optional.ofNullable(null);
String environment = env.orElseGet(() -> {
System.out.println(" Reading from config...");
return "development";
});
System.out.println(" Environment: " + environment);
Optional<String> apiKey = Optional.empty();
try {
String key = apiKey.orElseThrow(() ->
new RuntimeException("API key not configured")
);
} catch (RuntimeException e) {
System.out.println(" " + e.getMessage());
}
}
static String expensiveDefault() {
System.out.println(" Expensive computation!");
return "Default";
}
}
// orElse, orElseGet, orElseThrow
// Concept: default values - providing fallbacks
import java.util.Optional;
public class Defaults {
public static void main(String[] args) {
// orElse - provide default value
Optional<String> name = Optional.empty();
Optional<String> empty = Optional.empty();
String result1 = name.orElse("Unknown");
String result2 = empty.orElse("Unknown");
System.out.println("orElse:");
System.out.println(" name: " + result1);
System.out.println(" empty: " + result2);
// orElseGet - compute default lazily
System.out.println("\norElseGet:");
String r1 = name.orElseGet(() -> {
System.out.println(" Computing default...");
return "Generated";
});
System.out.println(" Result: " + r1);
String r2 = empty.orElseGet(() -> {
System.out.println(" Computing default...");
return "Generated";
});
System.out.println(" Result: " + r2);
// orElse vs orElseGet difference
System.out.println("\nDifference:");
System.out.println("With orElse:");
String x1 = name.orElse(expensiveDefault());
System.out.println("\nWith orElseGet:");
String x2 = name.orElseGet(() -> expensiveDefault());
// orElseThrow - require value
System.out.println("\norElseThrow:");
try {
String value = name.orElseThrow();
System.out.println(" Got value: " + value);
} catch (Exception e) {
System.out.println(" Error: " + e);
}
try {
String value = empty.orElseThrow();
System.out.println(" Got value: " + value);
} catch (Exception e) {
System.out.println(" Error: " + e.getClass().getSimpleName());
}
// orElseThrow with custom exception
try {
String value = empty.orElseThrow(() ->
new IllegalArgumentException("Value required")
);
} catch (IllegalArgumentException e) {
System.out.println(" Custom error: " + e.getMessage());
}
// Practical examples
System.out.println("\nPractical:");
Optional<Integer> port = Optional.empty();
int serverPort = port.orElse(8080);
System.out.println(" Server port: " + serverPort);
Optional<String> env = Optional.ofNullable(null);
String environment = env.orElseGet(() -> {
System.out.println(" Reading from config...");
return "development";
});
System.out.println(" Environment: " + environment);
Optional<String> apiKey = Optional.empty();
try {
String key = apiKey.orElseThrow(() ->
new RuntimeException("API key not configured")
);
} catch (RuntimeException e) {
System.out.println(" " + e.getMessage());
}
}
static String expensiveDefault() {
System.out.println(" Expensive computation!");
return "Default";
}
}
name ← Optional[Alice], empty ← Optional.empty, result1 ← Alice
6public class Defaults {7 public static void main(String[] args) {8 // orElse - provide default value9 Optional<String> name→ Optional[Alice] = Optional.of("Alice");10 //@name=Optional.of("Alice"), Optional.empty()11 Optional<String> empty→ Optional.empty = Optional.empty();12 13 String result1→ Alice = name.orElse("Unknown");14 String result2→ Unknown = empty.orElse("Unknown");15 16 System.out.println("orElse:");17 System.out.println(" name: " + result1Alice);18 System.out.println(" empty: " + result2Unknown);19 20 //@help h121 // orElse(default) - return value or default22 // orElseGet(Supplier) - compute default only if needed23 // orElseThrow() - throw exception if empty24 //@end25 26 // orElseGet - compute default lazily27 System.out.println("\norElseGet:");28 29 String r1→ Alice = name.orElseGet(() -> {30 System.out.println(" Computing default...");31 return "Generated";32 });33 System.out.println(" Result: " + r1Alice);34 35 String r2 = empty.orElseGet(() -> {36 System.out.println(" Computing default...");37 return "Generated";38 });39 System.out.println(" Result: " + r2);outputorElse: name: Alice empty: Unknown orElseGet: Result: Alice() ->
35String r2 = empty.orElseGet(() -> {36 System.out.println(" Computing default...");37 return "Generated";38});output Computing default...r2 ← Generated
35String r2→ Generated = empty.orElseGet(() -> {36 System.out.println(" Computing default...");37 return "Generated";38});39System.out.println(" Result: " + r2Generated);4041// orElse vs orElseGet difference42System.out.println("\nDifference:");4344System.out.println("With orElse:");45String x1 = name.orElse(expensiveDefault());output Result: Generated Difference: With orElse:static String expensiveDefault()
100static String expensiveDefault() {101 System.out.println(" Expensive computation!");102 return "Default";103}output Expensive computation!x1 ← Alice, x2 ← Alice
44System.out.println("With orElse:");45String x1→ Alice = name.orElse(expensiveDefault());4647System.out.println("\nWith orElseGet:");48String x2→ Alice = name.orElseGet(() -> expensiveDefault());4950// orElseThrow - require value51System.out.println("\norElseThrow:");output With orElseGet: orElseThrow:value ← Alice
53try {54 String value→ Alice = name.orElseThrow();55 System.out.println(" Got value: " + valueAlice);56} catch (Exception e) {output Got value: Alicecatch (Exception e)
62 System.out.println(" Got value: " + value);63} catch (Exception ejava.util.NoSuchElementException: No value present) {64 System.out.println(" Error: " + e.getClass().getSimpleName());65}output Error: NoSuchElementExceptioncatch (IllegalArgumentException e)
71 );72} catch (IllegalArgumentException ejava.lang.IllegalArgumentException: Value required) {73 System.out.println(" Custom error: " + e.getMessage());74}output Custom error: Value requiredport ← Optional.empty, serverPort ← 8080, env ← Optional.empty
76// Practical examples77System.out.println("\nPractical:");7879Optional<Integer> port→ Optional.empty = Optional.empty();80int serverPort→ 8080 = port.orElse(8080);81System.out.println(" Server port: " + serverPort8080);8283Optional<String> env→ Optional.empty = Optional.ofNullable(null);84String environment = env.orElseGet(() -> {85 System.out.println(" Reading from config...");86 return "development";87});88System.out.println(" Environment: " + environment);output Practical: Server port: 8080() ->
83Optional<String> env = Optional.ofNullable(null);84String environment = env.orElseGet(() -> {85 System.out.println(" Reading from config...");86 return "development";87});output Reading from config...environment ← development, apiKey ← Optional.empty
83Optional<String> env = Optional.ofNullable(null);84String environment→ development = env.orElseGet(() -> {85 System.out.println(" Reading from config...");86 return "development";87});88System.out.println(" Environment: " + environmentdevelopment);8990Optional<String> apiKey→ Optional.empty = Optional.empty();91try {output Environment: developmentcatch (RuntimeException e)
94 );95} catch (RuntimeException ejava.lang.RuntimeException: API key not configured) {96 System.out.println(" " + e.getMessage());97}output API key not configured
name ← Optional.empty, empty ← Optional.empty, result1 ← Unknown
6public class Defaults {7 public static void main(String[] args) {8 // orElse - provide default value9 Optional<String> name→ Optional.empty = Optional.empty();10 Optional<String> empty→ Optional.empty = Optional.empty();11 12 String result1→ Unknown = name.orElse("Unknown");13 String result2→ Unknown = empty.orElse("Unknown");14 15 System.out.println("orElse:");16 System.out.println(" name: " + result1Unknown);17 System.out.println(" empty: " + result2Unknown);18 19 20 // orElseGet - compute default lazily21 System.out.println("\norElseGet:");22 23 String r1 = name.orElseGet(() -> {24 System.out.println(" Computing default...");25 return "Generated";26 });27 System.out.println(" Result: " + r1);outputorElse: name: Unknown empty: Unknown orElseGet:() ->
23String r1 = name.orElseGet(() -> {24 System.out.println(" Computing default...");25 return "Generated";26});output Computing default...r1 ← Generated
23String r1→ Generated = name.orElseGet(() -> {24 System.out.println(" Computing default...");25 return "Generated";26});27System.out.println(" Result: " + r1Generated);2829String r2 = empty.orElseGet(() -> {30 System.out.println(" Computing default...");31 return "Generated";32});33System.out.println(" Result: " + r2);output Result: Generated() ->
29String r2 = empty.orElseGet(() -> {30 System.out.println(" Computing default...");31 return "Generated";32});output Computing default...r2 ← Generated
29String r2→ Generated = empty.orElseGet(() -> {30 System.out.println(" Computing default...");31 return "Generated";32});33System.out.println(" Result: " + r2Generated);3435// orElse vs orElseGet difference36System.out.println("\nDifference:");3738System.out.println("With orElse:");39String x1 = name.orElse(expensiveDefault());output Result: Generated Difference: With orElse:static String expensiveDefault()
pass 1 of 294static String expensiveDefault() {95 System.out.println(" Expensive computation!");96 return "Default";97}output Expensive computation!x1 ← Default
38System.out.println("With orElse:");39String x1→ Default = name.orElse(expensiveDefault());4041System.out.println("\nWith orElseGet:");42String x2 = name.orElseGet(() -> expensiveDefault());output With orElseGet:static String expensiveDefault()
pass 2 of 294static String expensiveDefault() {95 System.out.println(" Expensive computation!");96 return "Default";97}output Expensive computation!x2 ← Default
41System.out.println("\nWith orElseGet:");42String x2→ Default = name.orElseGet(() -> expensiveDefault());4344// orElseThrow - require value45System.out.println("\norElseThrow:");output orElseThrow:catch (Exception e)
49 System.out.println(" Got value: " + value);50} catch (Exception ejava.util.NoSuchElementException: No value present) {51 System.out.println(" Error: " + ejava.util.NoSuchElementException: No value present);52}output Error: java.util.NoSuchElementException: No value presentcatch (Exception e)
56 System.out.println(" Got value: " + value);57} catch (Exception ejava.util.NoSuchElementException: No value present) {58 System.out.println(" Error: " + e.getClass().getSimpleName());59}output Error: NoSuchElementExceptioncatch (IllegalArgumentException e)
65 );66} catch (IllegalArgumentException ejava.lang.IllegalArgumentException: Value required) {67 System.out.println(" Custom error: " + e.getMessage());68}output Custom error: Value requiredport ← Optional.empty, serverPort ← 8080, env ← Optional.empty
70// Practical examples71System.out.println("\nPractical:");7273Optional<Integer> port→ Optional.empty = Optional.empty();74int serverPort→ 8080 = port.orElse(8080);75System.out.println(" Server port: " + serverPort8080);7677Optional<String> env→ Optional.empty = Optional.ofNullable(null);78String environment = env.orElseGet(() -> {79 System.out.println(" Reading from config...");80 return "development";81});82System.out.println(" Environment: " + environment);output Practical: Server port: 8080() ->
77Optional<String> env = Optional.ofNullable(null);78String environment = env.orElseGet(() -> {79 System.out.println(" Reading from config...");80 return "development";81});output Reading from config...environment ← development, apiKey ← Optional.empty
77Optional<String> env = Optional.ofNullable(null);78String environment→ development = env.orElseGet(() -> {79 System.out.println(" Reading from config...");80 return "development";81});82System.out.println(" Environment: " + environmentdevelopment);8384Optional<String> apiKey→ Optional.empty = Optional.empty();85try {output Environment: developmentcatch (RuntimeException e)
88 );89} catch (RuntimeException ejava.lang.RuntimeException: API key not configured) {90 System.out.println(" " + e.getMessage());91}output API key not configured
orElse() provides default. orElseGet() computes default lazily.
orElse
Get value if present, otherwise use default: `opt.orElse("default")`.
Transform with map
Apply function to Optional's value.
MapFlatmap.java
Replay: real traced execution (multi-file project)
// map and flatMap transformations
// Concept: transformation - converting Optional values
import java.util.Optional;
public class MapFlatmap {
public static void main(String[] args) {
// map - transform value if present
Optional<String> name = Optional.of("alice");
Optional<String> upper = name.map(s -> s.toUpperCase());
System.out.println("Original: " + name);
System.out.println("Uppercase: " + upper);
Optional<Integer> length = name.map(s -> s.length());
System.out.println("Length: " + length);
// map on empty Optional
Optional<String> empty = Optional.empty();
Optional<String> result = empty.map(s -> s.toUpperCase());
System.out.println("\nEmpty mapped: " + result);
// Chain multiple maps
System.out.println("\nChained maps:");
Optional<String> processed = Optional.of(" hello ")
.map(s -> s.trim())
.map(s -> s.toUpperCase())
.map(s -> s + "!");
System.out.println("Result: " + processed.orElse(""));
// flatMap - when function returns Optional
Optional<String> userId = Optional.of("123");
Optional<User> user = userId.flatMap(id -> findUser(id));
System.out.println("\nUser: " + user);
Optional<String> email = userId
.flatMap(id -> findUser(id))
.map(u -> u.email);
System.out.println("Email: " + email);
// map vs flatMap
System.out.println("\nmap vs flatMap:");
// map wraps result in Optional
Optional<Optional<User>> nested = userId.map(id -> findUser(id));
System.out.println("map result: " + nested);
// flatMap doesn't wrap
Optional<User> flat = userId.flatMap(id -> findUser(id));
System.out.println("flatMap result: " + flat);
// Practical transformation pipeline
System.out.println("\nPipeline:");
Optional<String> input = Optional.of("42");
Optional<String> output = input
.map(s -> Integer.parseInt(s))
.map(n -> n * 2)
.map(n -> "Result: " + n);
output.ifPresent(System.out::println);
// Safe navigation
Optional<Integer> price = Optional.of("Product123")
.flatMap(id -> findProduct(id))
.map(p -> p.price);
System.out.println("\nProduct price: " + price.orElse(0));
}
static Optional<User> findUser(String id) {
if (id.equals("123")) {
return Optional.of(new User("Alice", "alice@example.com"));
}
return Optional.empty();
}
static Optional<Product> findProduct(String id) {
if (id.equals("Product123")) {
return Optional.of(new Product("Laptop", 999));
}
return Optional.empty();
}
static class User {
String name;
String email;
User(String name, String email) {
this.name = name;
this.email = email;
}
public String toString() {
return "User(" + name + ", " + email + ")";
}
}
static class Product {
String name;
int price;
Product(String name, int price) {
this.name = name;
this.price = price;
}
}
}
// map and flatMap transformations
// Concept: transformation - converting Optional values
import java.util.Optional;
public class MapFlatmap {
public static void main(String[] args) {
// map - transform value if present
Optional<String> name = Optional.of("bob");
Optional<String> upper = name.map(s -> s.toUpperCase());
System.out.println("Original: " + name);
System.out.println("Uppercase: " + upper);
Optional<Integer> length = name.map(s -> s.length());
System.out.println("Length: " + length);
// map on empty Optional
Optional<String> empty = Optional.empty();
Optional<String> result = empty.map(s -> s.toUpperCase());
System.out.println("\nEmpty mapped: " + result);
// Chain multiple maps
System.out.println("\nChained maps:");
Optional<String> processed = Optional.of(" hello ")
.map(s -> s.trim())
.map(s -> s.toUpperCase())
.map(s -> s + "!");
System.out.println("Result: " + processed.orElse(""));
// flatMap - when function returns Optional
Optional<String> userId = Optional.of("123");
Optional<User> user = userId.flatMap(id -> findUser(id));
System.out.println("\nUser: " + user);
Optional<String> email = userId
.flatMap(id -> findUser(id))
.map(u -> u.email);
System.out.println("Email: " + email);
// map vs flatMap
System.out.println("\nmap vs flatMap:");
// map wraps result in Optional
Optional<Optional<User>> nested = userId.map(id -> findUser(id));
System.out.println("map result: " + nested);
// flatMap doesn't wrap
Optional<User> flat = userId.flatMap(id -> findUser(id));
System.out.println("flatMap result: " + flat);
// Practical transformation pipeline
System.out.println("\nPipeline:");
Optional<String> input = Optional.of("42");
Optional<String> output = input
.map(s -> Integer.parseInt(s))
.map(n -> n * 2)
.map(n -> "Result: " + n);
output.ifPresent(System.out::println);
// Safe navigation
Optional<Integer> price = Optional.of("Product123")
.flatMap(id -> findProduct(id))
.map(p -> p.price);
System.out.println("\nProduct price: " + price.orElse(0));
}
static Optional<User> findUser(String id) {
if (id.equals("123")) {
return Optional.of(new User("Alice", "alice@example.com"));
}
return Optional.empty();
}
static Optional<Product> findProduct(String id) {
if (id.equals("Product123")) {
return Optional.of(new Product("Laptop", 999));
}
return Optional.empty();
}
static class User {
String name;
String email;
User(String name, String email) {
this.name = name;
this.email = email;
}
public String toString() {
return "User(" + name + ", " + email + ")";
}
}
static class Product {
String name;
int price;
Product(String name, int price) {
this.name = name;
this.price = price;
}
}
}
// map and flatMap transformations
// Concept: transformation - converting Optional values
import java.util.Optional;
public class MapFlatmap {
public static void main(String[] args) {
// map - transform value if present
Optional<String> name = Optional.empty();
Optional<String> upper = name.map(s -> s.toUpperCase());
System.out.println("Original: " + name);
System.out.println("Uppercase: " + upper);
Optional<Integer> length = name.map(s -> s.length());
System.out.println("Length: " + length);
// map on empty Optional
Optional<String> empty = Optional.empty();
Optional<String> result = empty.map(s -> s.toUpperCase());
System.out.println("\nEmpty mapped: " + result);
// Chain multiple maps
System.out.println("\nChained maps:");
Optional<String> processed = Optional.of(" hello ")
.map(s -> s.trim())
.map(s -> s.toUpperCase())
.map(s -> s + "!");
System.out.println("Result: " + processed.orElse(""));
// flatMap - when function returns Optional
Optional<String> userId = Optional.of("123");
Optional<User> user = userId.flatMap(id -> findUser(id));
System.out.println("\nUser: " + user);
Optional<String> email = userId
.flatMap(id -> findUser(id))
.map(u -> u.email);
System.out.println("Email: " + email);
// map vs flatMap
System.out.println("\nmap vs flatMap:");
// map wraps result in Optional
Optional<Optional<User>> nested = userId.map(id -> findUser(id));
System.out.println("map result: " + nested);
// flatMap doesn't wrap
Optional<User> flat = userId.flatMap(id -> findUser(id));
System.out.println("flatMap result: " + flat);
// Practical transformation pipeline
System.out.println("\nPipeline:");
Optional<String> input = Optional.of("42");
Optional<String> output = input
.map(s -> Integer.parseInt(s))
.map(n -> n * 2)
.map(n -> "Result: " + n);
output.ifPresent(System.out::println);
// Safe navigation
Optional<Integer> price = Optional.of("Product123")
.flatMap(id -> findProduct(id))
.map(p -> p.price);
System.out.println("\nProduct price: " + price.orElse(0));
}
static Optional<User> findUser(String id) {
if (id.equals("123")) {
return Optional.of(new User("Alice", "alice@example.com"));
}
return Optional.empty();
}
static Optional<Product> findProduct(String id) {
if (id.equals("Product123")) {
return Optional.of(new Product("Laptop", 999));
}
return Optional.empty();
}
static class User {
String name;
String email;
User(String name, String email) {
this.name = name;
this.email = email;
}
public String toString() {
return "User(" + name + ", " + email + ")";
}
}
static class Product {
String name;
int price;
Product(String name, int price) {
this.name = name;
this.price = price;
}
}
}
name ← Optional[alice], upper ← Optional[ALICE], length ← Optional[5]
6public class MapFlatmap {7 public static void main(String[] args) {8 // map - transform value if present9 Optional<String> name→ Optional[alice] = Optional.of("alice");10 //@name=Optional.of("alice"), Optional.of("bob"), Optional.empty()11 12 Optional<String> upper→ Optional[ALICE] = name.map(s -> s.toUpperCase());13 System.out.println("Original: " + nameOptional[alice]);14 System.out.println("Uppercase: " + upperOptional[ALICE]);15 16 Optional<Integer> length→ Optional[5] = name.map(s -> s.length());17 System.out.println("Length: " + lengthOptional[5]);18 19 // map on empty Optional20 Optional<String> empty→ Optional.empty = Optional.empty();21 Optional<String> result→ Optional.empty = empty.map(s -> s.toUpperCase());22 System.out.println("\nEmpty mapped: " + resultOptional.empty);23 24 //@help h125 // map(Function) transforms value, returns Optional<T>26 // flatMap(Function) when function returns Optional27 // map chains transformations safely28 //@end29 30 // Chain multiple maps31 System.out.println("\nChained maps:");32 33 Optional<String> processed→ Optional[HELLO!] = Optional.of(" hello ")34 .map(s -> s.trim())35 .map(s -> s.toUpperCase())36 .map(s -> s + "!");37 38 System.out.println("Result: " + processed.orElse(""));39 40 // flatMap - when function returns Optional41 Optional<String> userId→ Optional[123] = Optional.of("123");42 43 Optional<User> user = userId.flatMap(id -> findUser(id));44 System.out.println("\nUser: " + user);outputOriginal: Optional[alice] Uppercase: Optional[ALICE] Length: Optional[5] Empty mapped: Optional.empty Chained maps: Result: HELLO!static Optional<User> findUser(String id)
pass 1 of 483static Optional<User> findUser(String id123) {84 if (id.equals("123")) {All 4 passes — pass 1 is the card above pass namethis.namethis.priceprice1 — — — — 2 — — — — 3 — — — — 4 Laptop Laptop 999 999 → Optional[999] if (id.equals("123"))
pass 1 of 483static Optional<User> findUser(String id) {84 if (id.equals("123")) {85 return Optional.of(new User("Alice", "alice@example.com"));86 }All 4 passes — pass 1 is the card above pass idnamethis.namethis.priceprice1 — — — — — 2 — — — — — 3 — — — — — 4 Product123 Laptop Laptop 999 999 → Optional[999] this.name ← Alice, this.email ← alice@example.com, user ← Optional[User(Alice, alice@example.com)]
pass 1 of 443 Optional<User> user→ Optional[User(Alice, alice@example.com)] = userId.flatMap(id -> findUser(id));44 System.out.println("\nUser: " + userOptional[User(Alice, alice@example.com)]);45 46 Optional<String> email = userId47 .flatMap(id -> findUser(id))48 .map(u -> u.email);49 System.out.println("Email: " + email);50 51 // map vs flatMap52 System.out.println("\nmap vs flatMap:");53 54 // map wraps result in Optional55 Optional<Optional<User>> nested = userId.map(id -> findUser(id));56 System.out.println("map result: " + nested);57 58 // flatMap doesn't wrap59 Optional<User> flat = userId.flatMap(id -> findUser(id));60 System.out.println("flatMap result: " + flat);61 62 // Practical transformation pipeline63 System.out.println("\nPipeline:");64 65 Optional<String> input = Optional.of("42");66 67 Optional<String> output = input68 .map(s -> Integer.parseInt(s))69 .map(n -> n * 2)70 .map(n -> "Result: " + n);71 72 output.ifPresent(System.out::println);73 74 // Safe navigation75 Optional<Integer> price = Optional.of("Product123")76 .flatMap(id -> findProduct(id))77 .map(p -> p.price);78 79 System.out.println("\nProduct price: " + price.orElse(0));80 81}8283static Optional<User> findUser(String id) {84 if (id.equals("123")) {85 return Optional.of(new User("Alice", "alice@example.com"));86 }87 return Optional.empty();88}8990static Optional<Product> findProduct(String id) {91 if (id.equals("Product123")) {92 return Optional.of(new Product("Laptop", 999));93 }94 return Optional.empty();95}9697static class User {98 String name;99 String email;100 User(String nameAlice, String emailalice@example.com) {101 this.name→ Alice = nameAlice;102 this.email→ alice@example.com = emailalice@example.com;103 }output User: Optional[User(Alice, alice@example.com)]All 4 passes — pass 1 is the card above pass idthis.namethis.emailuseremailnestedflatinputoutputthis.priceprice1 — Alice alice@example.com Optional[User(Alice, alice@example.com)] alice@example.com — — — — — — 2 — Alice alice@example.com — alice@example.com → Optional[alice@example.com] — — — — — — 3 — Alice alice@example.com — alice@example.com Optional[Optional[User(Alice, alice@example.com)]] — — — — — 4 Product123 Alice alice@example.com — alice@example.com — Optional[User(Alice, alice@example.com)] Optional[42] Optional[Result: 84] 999 999 → Optional[999] static Optional<Product> findProduct(String id)
90static Optional<Product> findProduct(String idProduct123) {91 if (id.equals("Product123")) {this.name ← Laptop, this.price ← 999, price ← Optional[999]
74 // Safe navigation75 Optional<Integer> price→ Optional[999] = Optional.of("Product123")76 .flatMap(id -> findProduct(id))77 .map(p -> p.price);78 79 System.out.println("\nProduct price: " + price.orElse(0));80 81}8283static Optional<User> findUser(String id) {84 if (id.equals("123")) {85 return Optional.of(new User("Alice", "alice@example.com"));86 }87 return Optional.empty();88}8990static Optional<Product> findProduct(String id) {91 if (id.equals("Product123")) {92 return Optional.of(new Product("Laptop", 999));93 }94 return Optional.empty();95}9697static class User {98 String name;99 String email;100 User(String name, String email) {101 this.name = name;102 this.email = email;103 }104 public String toString() {105 return "User(" + name + ", " + email + ")";106 }107}108109static class Product {110 String name;111 int price;112 Product(String nameLaptop, int price999) {113 this.name→ Laptop = nameLaptop;114 this.price→ 999 = price999;115 }output Product price: 999
name ← Optional[bob], upper ← Optional[BOB], length ← Optional[3]
6public class MapFlatmap {7 public static void main(String[] args) {8 // map - transform value if present9 Optional<String> name→ Optional[bob] = Optional.of("bob");10 11 Optional<String> upper→ Optional[BOB] = name.map(s -> s.toUpperCase());12 System.out.println("Original: " + nameOptional[bob]);13 System.out.println("Uppercase: " + upperOptional[BOB]);14 15 Optional<Integer> length→ Optional[3] = name.map(s -> s.length());16 System.out.println("Length: " + lengthOptional[3]);17 18 // map on empty Optional19 Optional<String> empty→ Optional.empty = Optional.empty();20 Optional<String> result→ Optional.empty = empty.map(s -> s.toUpperCase());21 System.out.println("\nEmpty mapped: " + resultOptional.empty);22 23 24 // Chain multiple maps25 System.out.println("\nChained maps:");26 27 Optional<String> processed→ Optional[HELLO!] = Optional.of(" hello ")28 .map(s -> s.trim())29 .map(s -> s.toUpperCase())30 .map(s -> s + "!");31 32 System.out.println("Result: " + processed.orElse(""));33 34 // flatMap - when function returns Optional35 Optional<String> userId→ Optional[123] = Optional.of("123");36 37 Optional<User> user = userId.flatMap(id -> findUser(id));38 System.out.println("\nUser: " + user);outputOriginal: Optional[bob] Uppercase: Optional[BOB] Length: Optional[3] Empty mapped: Optional.empty Chained maps: Result: HELLO!static Optional<User> findUser(String id)
pass 1 of 477static Optional<User> findUser(String id123) {78 if (id.equals("123")) {All 4 passes — pass 1 is the card above pass namethis.namethis.priceprice1 — — — — 2 — — — — 3 — — — — 4 Laptop Laptop 999 999 → Optional[999] if (id.equals("123"))
pass 1 of 477static Optional<User> findUser(String id) {78 if (id.equals("123")) {79 return Optional.of(new User("Alice", "alice@example.com"));80 }All 4 passes — pass 1 is the card above pass idnamethis.namethis.priceprice1 — — — — — 2 — — — — — 3 — — — — — 4 Product123 Laptop Laptop 999 999 → Optional[999] this.name ← Alice, this.email ← alice@example.com, user ← Optional[User(Alice, alice@example.com)]
pass 1 of 437 Optional<User> user→ Optional[User(Alice, alice@example.com)] = userId.flatMap(id -> findUser(id));38 System.out.println("\nUser: " + userOptional[User(Alice, alice@example.com)]);39 40 Optional<String> email = userId41 .flatMap(id -> findUser(id))42 .map(u -> u.email);43 System.out.println("Email: " + email);44 45 // map vs flatMap46 System.out.println("\nmap vs flatMap:");47 48 // map wraps result in Optional49 Optional<Optional<User>> nested = userId.map(id -> findUser(id));50 System.out.println("map result: " + nested);51 52 // flatMap doesn't wrap53 Optional<User> flat = userId.flatMap(id -> findUser(id));54 System.out.println("flatMap result: " + flat);55 56 // Practical transformation pipeline57 System.out.println("\nPipeline:");58 59 Optional<String> input = Optional.of("42");60 61 Optional<String> output = input62 .map(s -> Integer.parseInt(s))63 .map(n -> n * 2)64 .map(n -> "Result: " + n);65 66 output.ifPresent(System.out::println);67 68 // Safe navigation69 Optional<Integer> price = Optional.of("Product123")70 .flatMap(id -> findProduct(id))71 .map(p -> p.price);72 73 System.out.println("\nProduct price: " + price.orElse(0));74 75}7677static Optional<User> findUser(String id) {78 if (id.equals("123")) {79 return Optional.of(new User("Alice", "alice@example.com"));80 }81 return Optional.empty();82}8384static Optional<Product> findProduct(String id) {85 if (id.equals("Product123")) {86 return Optional.of(new Product("Laptop", 999));87 }88 return Optional.empty();89}9091static class User {92 String name;93 String email;94 User(String nameAlice, String emailalice@example.com) {95 this.name→ Alice = nameAlice;96 this.email→ alice@example.com = emailalice@example.com;97 }output User: Optional[User(Alice, alice@example.com)]All 4 passes — pass 1 is the card above pass idthis.namethis.emailuseremailnestedflatinputoutputthis.priceprice1 — Alice alice@example.com Optional[User(Alice, alice@example.com)] alice@example.com — — — — — — 2 — Alice alice@example.com — alice@example.com → Optional[alice@example.com] — — — — — — 3 — Alice alice@example.com — alice@example.com Optional[Optional[User(Alice, alice@example.com)]] — — — — — 4 Product123 Alice alice@example.com — alice@example.com — Optional[User(Alice, alice@example.com)] Optional[42] Optional[Result: 84] 999 999 → Optional[999] static Optional<Product> findProduct(String id)
84static Optional<Product> findProduct(String idProduct123) {85 if (id.equals("Product123")) {this.name ← Laptop, this.price ← 999, price ← Optional[999]
68 // Safe navigation69 Optional<Integer> price→ Optional[999] = Optional.of("Product123")70 .flatMap(id -> findProduct(id))71 .map(p -> p.price);72 73 System.out.println("\nProduct price: " + price.orElse(0));74 75}7677static Optional<User> findUser(String id) {78 if (id.equals("123")) {79 return Optional.of(new User("Alice", "alice@example.com"));80 }81 return Optional.empty();82}8384static Optional<Product> findProduct(String id) {85 if (id.equals("Product123")) {86 return Optional.of(new Product("Laptop", 999));87 }88 return Optional.empty();89}9091static class User {92 String name;93 String email;94 User(String name, String email) {95 this.name = name;96 this.email = email;97 }98 public String toString() {99 return "User(" + name + ", " + email + ")";100 }101}102103static class Product {104 String name;105 int price;106 Product(String nameLaptop, int price999) {107 this.name→ Laptop = nameLaptop;108 this.price→ 999 = price999;109 }output Product price: 999
name ← Optional.empty, upper ← Optional.empty, length ← Optional.empty
6public class MapFlatmap {7 public static void main(String[] args) {8 // map - transform value if present9 Optional<String> name→ Optional.empty = Optional.empty();10 11 Optional<String> upper→ Optional.empty = name.map(s -> s.toUpperCase());12 System.out.println("Original: " + nameOptional.empty);13 System.out.println("Uppercase: " + upperOptional.empty);14 15 Optional<Integer> length→ Optional.empty = name.map(s -> s.length());16 System.out.println("Length: " + lengthOptional.empty);17 18 // map on empty Optional19 Optional<String> empty→ Optional.empty = Optional.empty();20 Optional<String> result→ Optional.empty = empty.map(s -> s.toUpperCase());21 System.out.println("\nEmpty mapped: " + resultOptional.empty);22 23 24 // Chain multiple maps25 System.out.println("\nChained maps:");26 27 Optional<String> processed→ Optional[HELLO!] = Optional.of(" hello ")28 .map(s -> s.trim())29 .map(s -> s.toUpperCase())30 .map(s -> s + "!");31 32 System.out.println("Result: " + processed.orElse(""));33 34 // flatMap - when function returns Optional35 Optional<String> userId→ Optional[123] = Optional.of("123");36 37 Optional<User> user = userId.flatMap(id -> findUser(id));38 System.out.println("\nUser: " + user);outputOriginal: Optional.empty Uppercase: Optional.empty Length: Optional.empty Empty mapped: Optional.empty Chained maps: Result: HELLO!static Optional<User> findUser(String id)
pass 1 of 477static Optional<User> findUser(String id123) {78 if (id.equals("123")) {All 4 passes — pass 1 is the card above pass namethis.namethis.priceprice1 — — — — 2 — — — — 3 — — — — 4 Laptop Laptop 999 999 → Optional[999] if (id.equals("123"))
pass 1 of 477static Optional<User> findUser(String id) {78 if (id.equals("123")) {79 return Optional.of(new User("Alice", "alice@example.com"));80 }All 4 passes — pass 1 is the card above pass idnamethis.namethis.priceprice1 — — — — — 2 — — — — — 3 — — — — — 4 Product123 Laptop Laptop 999 999 → Optional[999] this.name ← Alice, this.email ← alice@example.com, user ← Optional[User(Alice, alice@example.com)]
pass 1 of 437 Optional<User> user→ Optional[User(Alice, alice@example.com)] = userId.flatMap(id -> findUser(id));38 System.out.println("\nUser: " + userOptional[User(Alice, alice@example.com)]);39 40 Optional<String> email = userId41 .flatMap(id -> findUser(id))42 .map(u -> u.email);43 System.out.println("Email: " + email);44 45 // map vs flatMap46 System.out.println("\nmap vs flatMap:");47 48 // map wraps result in Optional49 Optional<Optional<User>> nested = userId.map(id -> findUser(id));50 System.out.println("map result: " + nested);51 52 // flatMap doesn't wrap53 Optional<User> flat = userId.flatMap(id -> findUser(id));54 System.out.println("flatMap result: " + flat);55 56 // Practical transformation pipeline57 System.out.println("\nPipeline:");58 59 Optional<String> input = Optional.of("42");60 61 Optional<String> output = input62 .map(s -> Integer.parseInt(s))63 .map(n -> n * 2)64 .map(n -> "Result: " + n);65 66 output.ifPresent(System.out::println);67 68 // Safe navigation69 Optional<Integer> price = Optional.of("Product123")70 .flatMap(id -> findProduct(id))71 .map(p -> p.price);72 73 System.out.println("\nProduct price: " + price.orElse(0));74 75}7677static Optional<User> findUser(String id) {78 if (id.equals("123")) {79 return Optional.of(new User("Alice", "alice@example.com"));80 }81 return Optional.empty();82}8384static Optional<Product> findProduct(String id) {85 if (id.equals("Product123")) {86 return Optional.of(new Product("Laptop", 999));87 }88 return Optional.empty();89}9091static class User {92 String name;93 String email;94 User(String nameAlice, String emailalice@example.com) {95 this.name→ Alice = nameAlice;96 this.email→ alice@example.com = emailalice@example.com;97 }output User: Optional[User(Alice, alice@example.com)]All 4 passes — pass 1 is the card above pass idthis.namethis.emailuseremailnestedflatinputoutputthis.priceprice1 — Alice alice@example.com Optional[User(Alice, alice@example.com)] alice@example.com — — — — — — 2 — Alice alice@example.com — alice@example.com → Optional[alice@example.com] — — — — — — 3 — Alice alice@example.com — alice@example.com Optional[Optional[User(Alice, alice@example.com)]] — — — — — 4 Product123 Alice alice@example.com — alice@example.com — Optional[User(Alice, alice@example.com)] Optional[42] Optional[Result: 84] 999 999 → Optional[999] static Optional<Product> findProduct(String id)
84static Optional<Product> findProduct(String idProduct123) {85 if (id.equals("Product123")) {this.name ← Laptop, this.price ← 999, price ← Optional[999]
68 // Safe navigation69 Optional<Integer> price→ Optional[999] = Optional.of("Product123")70 .flatMap(id -> findProduct(id))71 .map(p -> p.price);72 73 System.out.println("\nProduct price: " + price.orElse(0));74 75}7677static Optional<User> findUser(String id) {78 if (id.equals("123")) {79 return Optional.of(new User("Alice", "alice@example.com"));80 }81 return Optional.empty();82}8384static Optional<Product> findProduct(String id) {85 if (id.equals("Product123")) {86 return Optional.of(new Product("Laptop", 999));87 }88 return Optional.empty();89}9091static class User {92 String name;93 String email;94 User(String name, String email) {95 this.name = name;96 this.email = email;97 }98 public String toString() {99 return "User(" + name + ", " + email + ")";100 }101}102103static class Product {104 String name;105 int price;106 Product(String nameLaptop, int price999) {107 this.name→ Laptop = nameLaptop;108 this.price→ 999 = price999;109 }output Product price: 999
map() transforms value if present. flatMap() for nested Optionals.
map
Transform Optional: `opt.map(String::toUpperCase)`. Returns Optional.
Optional return types
Use Optional for methods that might not return a value.
ReturnTypes.java
Replay: real traced execution (multi-file project)
// Optional in return types
// Concept: pattern - using Optional for method returns
import java.util.Optional;
import java.util.HashMap;
import java.util.Map;
public class ReturnTypes {
// Repository with Optional returns
static class UserRepository {
private Map<Integer, User> users = new HashMap<>();
UserRepository() {
users.put(1, new User(1, "Alice", "alice@example.com"));
users.put(2, new User(2, "Bob", "bob@example.com"));
}
// Return Optional instead of null
Optional<User> findById(int id) {
return Optional.ofNullable(users.get(id));
}
Optional<User> findByEmail(String email) {
return users.values().stream()
.filter(u -> u.email.equals(email))
.findFirst();
}
}
static class User {
int id;
String name;
String email;
User(int id, String name, String email) {
this.id = id;
this.name = name;
this.email = email;
}
public String toString() {
return "User(" + id + ", " + name + ")";
}
}
public static void main(String[] args) {
UserRepository repo = new UserRepository();
// Call methods returning Optional
System.out.println("Finding users:");
Optional<User> user1 = repo.findById(1);
user1.ifPresent(u -> System.out.println(" Found: " + u));
Optional<User> user99 = repo.findById(99);
user99.ifPresentOrElse(
u -> System.out.println(" Found: " + u),
() -> System.out.println(" User 99 not found")
);
// Chain Optional operations
System.out.println("\nChained operations:");
String userName = repo.findById(1)
.map(u -> u.name)
.orElse("Unknown");
System.out.println(" User 1 name: " + userName);
String email = repo.findById(99)
.map(u -> u.email)
.orElse("no-email@example.com");
System.out.println(" User 99 email: " + email);
// Find by email
System.out.println("\nFind by email:");
repo.findByEmail("alice@example.com")
.ifPresent(u -> System.out.println(" Found: " + u));
repo.findByEmail("unknown@example.com")
.ifPresent(u -> System.out.println(" Won't print"));
// Configuration service
System.out.println("\nConfiguration:");
ConfigService config = new ConfigService();
int port = config.getPort().orElse(8080);
System.out.println(" Port: " + port);
String host = config.getHost().orElse("localhost");
System.out.println(" Host: " + host);
config.getApiKey().ifPresentOrElse(
key -> System.out.println(" API key: " + key),
() -> System.out.println(" WARNING: No API key configured")
);
// Error handling
System.out.println("\nWith error handling:");
try {
User user = repo.findById(99)
.orElseThrow(() -> new RuntimeException("User not found"));
} catch (RuntimeException e) {
System.out.println(" Error: " + e.getMessage());
}
// Transform to different type
Optional<String> welcomeMsg = repo.findById(2)
.map(u -> "Welcome, " + u.name + "!");
System.out.println("\n" + welcomeMsg.orElse("Welcome, Guest!"));
}
static class ConfigService {
Optional<Integer> getPort() {
return Optional.of(9090);
}
Optional<String> getHost() {
return Optional.empty(); // Not configured
}
Optional<String> getApiKey() {
return Optional.empty(); // Not configured
}
}
}
public static void main(String[] args)
46public static void main(String[] args) {47 UserRepository repo = new UserRepository();this.id ← 1, this.name ← Alice, this.email ← alice@example.com
pass 1 of 213 UserRepository() {14 users.put(1, new User(1, "Alice", "alice@example.com"));15 users.put(2, new User(2, "Bob", "bob@example.com"));16 }17 18 // Return Optional instead of null19 Optional<User> findById(int id) {20 return Optional.ofNullable(users.get(id));21 }22 23 Optional<User> findByEmail(String email) {24 return users.values().stream()25 .filter(u -> u.email.equals(email))26 .findFirst();27 }28}2930static class User {31 int id;32 String name;33 String email;34 35 User(int id1, String nameAlice, String emailalice@example.com) {36 this.id→ 1 = id1;37 this.name→ Alice = nameAlice;38 this.email→ alice@example.com = emailalice@example.com;39 }this.id ← 2, this.name ← Bob, this.email ← bob@example.com
pass 2 of 214 users.put(1, new User(1, "Alice", "alice@example.com"));15 users.put(2, new User(2, "Bob", "bob@example.com"));16 }17 18 // Return Optional instead of null19 Optional<User> findById(int id) {20 return Optional.ofNullable(users.get(id));21 }22 23 Optional<User> findByEmail(String email) {24 return users.values().stream()25 .filter(u -> u.email.equals(email))26 .findFirst();27 }28}2930static class User {31 int id;32 String name;33 String email;34 35 User(int id2, String nameBob, String emailbob@example.com) {36 this.id→ 2 = id2;37 this.name→ Bob = nameBob;38 this.email→ bob@example.com = emailbob@example.com;39 }repo ← ⟨ReturnTypes$UserRepository A⟩
46public static void main(String[] args) {47 UserRepository repo→ ⟨ReturnTypes$UserRepository A⟩ = new UserRepository();48 49 // Call methods returning Optional50 System.out.println("Finding users:");51 52 Optional<User> user1 = repo.findById(1);53 user1.ifPresent(u -> System.out.println(" Found: " + u));outputFinding users:Optional<User> findById(int id)
pass 1 of 618// Return Optional instead of null19Optional<User> findById(int id1) {20 return Optional.ofNullable(users.get(id1));21}All 6 passes — pass 1 is the card above pass ide1 1 — 2 99 — 3 1 — 4 99 — 5 99 java.lang.RuntimeException: User not found 6 2 — user1 ← Optional[User(1, Alice)]
52Optional<User> user1→ Optional[User(1, Alice)] = repo.findById(1);53user1.ifPresent(u -> System.out.println(" Found: " + u));5455Optional<User> user99 = repo.findById(99);56user99.ifPresentOrElse(user99 ← Optional.empty
55Optional<User> user99→ Optional.empty = repo.findById(99);56user99.ifPresentOrElse(57 u -> System.out.println(" Found: " + u),58 () -> System.out.println(" User 99 not found")59);6061//@help h162// Return Optional<T> instead of returning null63// Caller must explicitly handle absence64// Makes API contract clear65//@end6667// Chain Optional operations68System.out.println("\nChained operations:");6970String userName = repo.findById(1)71 .map(u -> u.name)72 .orElse("Unknown");73System.out.println(" User 1 name: " + userName);output Chained operations:userName ← Alice
70String userName→ Alice = repo.findById(1)71 .map(u -> u.name)72 .orElse("Unknown");73System.out.println(" User 1 name: " + userNameAlice);7475String email = repo.findById(99)76 .map(u -> u.email)77 .orElse("no-email@example.com");78System.out.println(" User 99 email: " + email);output User 1 name: Aliceemail ← no-email@example.com
75String email→ no-email@example.com = repo.findById(99)76 .map(u -> u.email)77 .orElse("no-email@example.com");78System.out.println(" User 99 email: " + emailno-email@example.com);7980// Find by email81System.out.println("\nFind by email:");8283repo.findByEmail("alice@example.com")84 .ifPresent(u -> System.out.println(" Found: " + u));output User 99 email: no-email@example.com Find by email:Optional<User> findByEmail(String email)
pass 1 of 223Optional<User> findByEmail(String emailalice@example.com) {24 return users.values().stream()25 .filter(u -> u.email.equals(email))26 .findFirst();27}repo.findByEmail("alice@example.com")
83repo.findByEmail("alice@example.com")84 .ifPresent(u -> System.out.println(" Found: " + u));8586repo.findByEmail("unknown@example.com")87 .ifPresent(u -> System.out.println(" Won't print"));Optional<User> findByEmail(String email)
pass 2 of 223Optional<User> findByEmail(String emailunknown@example.com) {24 return users.values().stream()25 .filter(u -> u.email.equals(email))26 .findFirst();27}config ← ⟨ReturnTypes$ConfigService B⟩
86repo.findByEmail("unknown@example.com")87 .ifPresent(u -> System.out.println(" Won't print"));8889// Configuration service90System.out.println("\nConfiguration:");9192ConfigService config→ ⟨ReturnTypes$ConfigService B⟩ = new ConfigService();9394int port = config.getPort().orElse(8080);95System.out.println(" Port: " + port);output Configuration:port ← 9090
94int port→ 9090 = config.getPort().orElse(8080);95System.out.println(" Port: " + port9090);9697String host = config.getHost().orElse("localhost");98System.out.println(" Host: " + host);output Port: 9090host ← localhost
97String host→ localhost = config.getHost().orElse("localhost");98System.out.println(" Host: " + hostlocalhost);99100config.getApiKey().ifPresentOrElse(101 key -> System.out.println(" API key: " + key),102 () -> System.out.println(" WARNING: No API key configured")103);output Host: localhostconfig.getApiKey().ifPresentOrElse(
100config.getApiKey().ifPresentOrElse(101 key -> System.out.println(" API key: " + key),102 () -> System.out.println(" WARNING: No API key configured")103);104105// Error handling106System.out.println("\nWith error handling:");output With error handling:catch (RuntimeException e)
110 .orElseThrow(() -> new RuntimeException("User not found"));111} catch (RuntimeException ejava.lang.RuntimeException: User not found) {112 System.out.println(" Error: " + e.getMessage());113}output Error: User not foundOptional<String> welcomeMsg = repo.findById(2)
115// Transform to different type116Optional<String> welcomeMsg = repo.findById(2)117 .map(u -> "Welcome, " + u.name + "!");welcomeMsg ← Optional[Welcome, Bob!]
115 // Transform to different type116 Optional<String> welcomeMsg→ Optional[Welcome, Bob!] = repo.findById(2)117 .map(u -> "Welcome, " + u.name + "!");118 119 System.out.println("\n" + welcomeMsg.orElse("Welcome, Guest!"));120}output Welcome, Bob!
Return Optional<T> instead of nullable T. Caller knows to check.
Exercise: Practical.java
Refactor a service layer to use Optional