String Processing
String Builder
When building strings in loops or through multiple concatenations, performance can suffer dramatically. StringBuilder provides a mutable buffer that grows efficiently, making it useful for generated text such as reports, CSV output, HTML, or SQL.
Why StringBuilder?
String concatenation creates new String objects. StringBuilder keeps one mutable buffer and appends into it.
Performance.java
Replay: real traced execution (multi-file project)
public class Performance {
public static void main(String[] args) {
int count = 3;
String names = "";
for (int i = 1; i <= count; i++) {
names = names + "Name" + i;
if (i < count) {
names = names + ", ";
}
}
System.out.println(names);
StringBuilder builder = new StringBuilder();
for (int i = 1; i <= count; i++) {
builder.append("Name").append(i);
if (i < count) {
builder.append(", ");
}
}
System.out.println(builder.toString());
}
}
public class Performance {
public static void main(String[] args) {
int count = 5;
String names = "";
for (int i = 1; i <= count; i++) {
names = names + "Name" + i;
if (i < count) {
names = names + ", ";
}
}
System.out.println(names);
StringBuilder builder = new StringBuilder();
for (int i = 1; i <= count; i++) {
builder.append("Name").append(i);
if (i < count) {
builder.append(", ");
}
}
System.out.println(builder.toString());
}
}
count ← 3, names ← (empty)
1public class Performance {2 public static void main(String[] args) {3 int count→ 3 = 3; //@count=3, 545 String names→ (empty) = "";6 for (int i = 1; i <= count; i++) {names ← Name1
pass 1 of 35String names = "";6for (int i1 = 1; i <= count3; i++) {7 names→ Name1 = names + "Name" + i1;8 if (i < count) {All 3 passes — pass 1 is the card above pass inames1 1 (empty) → Name1 2 2 Name1, → Name1, Name2 3 3 Name1, Name2, → Name1, Name2, Name3 names ← Name1,
pass 1 of 27names = names + "Name" + i;8if (i1 < count3) {9 names→ Name1, = names + ", ";10}names ← Name1, Name2,
pass 2 of 27names = names + "Name" + i;8if (i2 < count3) {9 names→ Name1, Name2, = names + ", ";10}builder ← (empty)
11}12System.out.println(namesName1, Name2, Name3);1314StringBuilder builder→ (empty) = new StringBuilder();15for (int i = 1; i <= count; i++) {outputName1, Name2, Name3for (int i = 1; i <= count; i++)
pass 1 of 314StringBuilder builder = new StringBuilder();15for (int i1 = 1; i <= count3; i++) {16 builder.append("Name").append(i1);17 if (i < count) {All 3 passes — pass 1 is the card above pass i1 1 2 2 3 3 if (i < count)
pass 1 of 216builder.append("Name").append(i);17if (i1 < count3) {18 builder.append(", ");19}if (i < count)
pass 2 of 216builder.append("Name").append(i);17if (i2 < count3) {18 builder.append(", ");19}System.out.println(builder.toString());
20 }21 System.out.println(builder.toString());22}outputName1, Name2, Name3
count ← 5, names ← (empty)
1public class Performance {2 public static void main(String[] args) {3 int count→ 5 = 5;45 String names→ (empty) = "";6 for (int i = 1; i <= count; i++) {names ← Name1
pass 1 of 55String names = "";6for (int i1 = 1; i <= count5; i++) {7 names→ Name1 = names + "Name" + i1;8 if (i < count) {All 5 passes — pass 1 is the card above pass inames1 1 (empty) → Name1 2 2 Name1, → Name1, Name2 3 3 Name1, Name2, → Name1, Name2, Name3 4 4 Name1, Name2, Name3, → Name1, Name2, Name3, Name4 5 5 Name1, Name2, Name3, Name4, → Name1, Name2, Name3, Name4, Name5 names ← Name1,
pass 1 of 47names = names + "Name" + i;8if (i1 < count5) {9 names→ Name1, = names + ", ";10}All 4 passes — pass 1 is the card above pass inames1 1 Name1 → Name1, 2 2 Name1, Name2 → Name1, Name2, 3 3 Name1, Name2, Name3 → Name1, Name2, Name3, 4 4 Name1, Name2, Name3, Name4 → Name1, Name2, Name3, Name4, builder ← (empty)
11}12System.out.println(namesName1, Name2, Name3, Name4, Name5);1314StringBuilder builder→ (empty) = new StringBuilder();15for (int i = 1; i <= count; i++) {outputName1, Name2, Name3, Name4, Name5for (int i = 1; i <= count; i++)
pass 1 of 514StringBuilder builder = new StringBuilder();15for (int i1 = 1; i <= count5; i++) {16 builder.append("Name").append(i1);17 if (i < count) {All 5 passes — pass 1 is the card above pass i1 1 2 2 3 3 4 4 5 5 if (i < count)
pass 1 of 416builder.append("Name").append(i);17if (i1 < count5) {18 builder.append(", ");19}All 4 passes — pass 1 is the card above pass i1 1 2 2 3 3 4 4 System.out.println(builder.toString());
20 }21 System.out.println(builder.toString());22}outputName1, Name2, Name3, Name4, Name5
mutability
StringBuilder can be changed after creation, unlike String, which creates a new object for every modification.
Basic Operations
Use append to add values to the end of a builder.
Basics.java
Replay: real traced execution (multi-file project)
// StringBuilder basics
public class Basics {
public static void main(String[] args) {
// Create empty StringBuilder
String first = "Hello";
String second = "World";
StringBuilder sb1 = new StringBuilder();
sb1.append(first);
sb1.append(" ");
sb1.append(second);
System.out.println(sb1.toString());
// Create with initial string
StringBuilder sb2 = new StringBuilder("Java");
System.out.println(sb2);
// Create with initial capacity
StringBuilder sb3 = new StringBuilder(50);
sb3.append("Efficient");
System.out.println("Length: " + sb3.length());
System.out.println("Capacity: " + sb3.capacity());
// Append different types
StringBuilder sb4 = new StringBuilder();
sb4.append("Age: ");
sb4.append(25);
sb4.append(", Score: ");
sb4.append(95.5);
sb4.append(", Active: ");
sb4.append(true);
System.out.println(sb4);
}
}
// StringBuilder basics
public class Basics {
public static void main(String[] args) {
// Create empty StringBuilder
String first = "Goodbye";
String second = "World";
StringBuilder sb1 = new StringBuilder();
sb1.append(first);
sb1.append(" ");
sb1.append(second);
System.out.println(sb1.toString());
// Create with initial string
StringBuilder sb2 = new StringBuilder("Java");
System.out.println(sb2);
// Create with initial capacity
StringBuilder sb3 = new StringBuilder(50);
sb3.append("Efficient");
System.out.println("Length: " + sb3.length());
System.out.println("Capacity: " + sb3.capacity());
// Append different types
StringBuilder sb4 = new StringBuilder();
sb4.append("Age: ");
sb4.append(25);
sb4.append(", Score: ");
sb4.append(95.5);
sb4.append(", Active: ");
sb4.append(true);
System.out.println(sb4);
}
}
// StringBuilder basics
public class Basics {
public static void main(String[] args) {
// Create empty StringBuilder
String first = "Hello";
String second = "Java";
StringBuilder sb1 = new StringBuilder();
sb1.append(first);
sb1.append(" ");
sb1.append(second);
System.out.println(sb1.toString());
// Create with initial string
StringBuilder sb2 = new StringBuilder("Java");
System.out.println(sb2);
// Create with initial capacity
StringBuilder sb3 = new StringBuilder(50);
sb3.append("Efficient");
System.out.println("Length: " + sb3.length());
System.out.println("Capacity: " + sb3.capacity());
// Append different types
StringBuilder sb4 = new StringBuilder();
sb4.append("Age: ");
sb4.append(25);
sb4.append(", Score: ");
sb4.append(95.5);
sb4.append(", Active: ");
sb4.append(true);
System.out.println(sb4);
}
}
first ← Hello, second ← World, sb1 ← (empty), sb2 ← Java, sb3 ← (empty)
5public static void main(String[] args) {6 // Create empty StringBuilder7 String first→ Hello = "Hello"; //@first="Hello", "Goodbye"8 String second→ World = "World"; //@second="World", "Java"9 StringBuilder sb1→ (empty) = new StringBuilder();10 sb1.append(firstHello);11 sb1.append(" ");12 sb1.append(secondWorld);13 System.out.println(sb1.toString());1415 // Create with initial string16 StringBuilder sb2→ Java = new StringBuilder("Java");17 System.out.println(sb2Java);1819 // Create with initial capacity20 StringBuilder sb3→ (empty) = new StringBuilder(50);21 sb3.append("Efficient");22 System.out.println("Length: " + sb3.length());23 System.out.println("Capacity: " + sb3.capacity());2425 // Append different types26 StringBuilder sb4→ (empty) = new StringBuilder();27 sb4.append("Age: ");28 sb4.append(25);29 sb4.append(", Score: ");30 sb4.append(95.5);31 sb4.append(", Active: ");32 sb4.append(true);33 System.out.println(sb4Age: 25, Score: 95.5, Active: true);34}outputHello World Java Length: 9 Capacity: 50 Age: 25, Score: 95.5, Active: true
first ← Goodbye, second ← World, sb1 ← (empty), sb2 ← Java, sb3 ← (empty)
5public static void main(String[] args) {6 // Create empty StringBuilder7 String first→ Goodbye = "Goodbye";8 String second→ World = "World";9 StringBuilder sb1→ (empty) = new StringBuilder();10 sb1.append(firstGoodbye);11 sb1.append(" ");12 sb1.append(secondWorld);13 System.out.println(sb1.toString());1415 // Create with initial string16 StringBuilder sb2→ Java = new StringBuilder("Java");17 System.out.println(sb2Java);1819 // Create with initial capacity20 StringBuilder sb3→ (empty) = new StringBuilder(50);21 sb3.append("Efficient");22 System.out.println("Length: " + sb3.length());23 System.out.println("Capacity: " + sb3.capacity());2425 // Append different types26 StringBuilder sb4→ (empty) = new StringBuilder();27 sb4.append("Age: ");28 sb4.append(25);29 sb4.append(", Score: ");30 sb4.append(95.5);31 sb4.append(", Active: ");32 sb4.append(true);33 System.out.println(sb4Age: 25, Score: 95.5, Active: true);34}outputGoodbye World Java Length: 9 Capacity: 50 Age: 25, Score: 95.5, Active: true
first ← Hello, second ← Java, sb1 ← (empty), sb2 ← Java, sb3 ← (empty)
5public static void main(String[] args) {6 // Create empty StringBuilder7 String first→ Hello = "Hello";8 String second→ Java = "Java";9 StringBuilder sb1→ (empty) = new StringBuilder();10 sb1.append(firstHello);11 sb1.append(" ");12 sb1.append(secondJava);13 System.out.println(sb1.toString());1415 // Create with initial string16 StringBuilder sb2→ Java = new StringBuilder("Java");17 System.out.println(sb2Java);1819 // Create with initial capacity20 StringBuilder sb3→ (empty) = new StringBuilder(50);21 sb3.append("Efficient");22 System.out.println("Length: " + sb3.length());23 System.out.println("Capacity: " + sb3.capacity());2425 // Append different types26 StringBuilder sb4→ (empty) = new StringBuilder();27 sb4.append("Age: ");28 sb4.append(25);29 sb4.append(", Score: ");30 sb4.append(95.5);31 sb4.append(", Active: ");32 sb4.append(true);33 System.out.println(sb4Age: 25, Score: 95.5, Active: true);34}outputHello Java Java Length: 9 Capacity: 50 Age: 25, Score: 95.5, Active: true
append
append adds content to the end of the StringBuilder and returns the same builder.
Insert and Delete
StringBuilder can also modify text in the middle of the buffer.
InsertDelete.java
Replay: real traced execution (multi-file project)
// Insert and delete operations
public class InsertDelete {
public static void main(String[] args) {
// Insert
StringBuilder sb1 = new StringBuilder("Hello World");
sb1.insert(6, "Beautiful ");
System.out.println(sb1); // Hello Beautiful World
// Insert at beginning
StringBuilder sb2 = new StringBuilder("Java");
sb2.insert(0, "I love ");
System.out.println(sb2); // I love Java
// Insert at end (same as append)
StringBuilder sb3 = new StringBuilder("Start");
sb3.insert(sb3.length(), " End");
System.out.println(sb3); // Start End
// Delete range
StringBuilder sb4 = new StringBuilder("Hello Beautiful World");
sb4.delete(6, 16); // Remove "Beautiful "
System.out.println(sb4); // Hello World
// Delete single character
StringBuilder sb5 = new StringBuilder("Helllo");
sb5.deleteCharAt(3); // Remove extra 'l'
System.out.println(sb5); // Hello
// Replace
StringBuilder sb6 = new StringBuilder("Hello World");
sb6.replace(6, 11, "Java");
System.out.println(sb6); // Hello Java
}
}
sb1 ← Hello World, sb2 ← Java, sb3 ← Start, sb4 ← Hello Beautiful World
5public static void main(String[] args) {6 // Insert7 StringBuilder sb1→ Hello World = new StringBuilder("Hello World");8 sb1.insert(6, "Beautiful ");9 System.out.println(sb1Hello Beautiful World); // Hello Beautiful World1011 // Insert at beginning12 StringBuilder sb2→ Java = new StringBuilder("Java");13 sb2.insert(0, "I love ");14 System.out.println(sb2I love Java); // I love Java1516 // Insert at end (same as append)17 StringBuilder sb3→ Start = new StringBuilder("Start");18 sb3.insert(sb3.length(), " End");19 System.out.println(sb3Start End); // Start End2021 // Delete range22 StringBuilder sb4→ Hello Beautiful World = new StringBuilder("Hello Beautiful World");23 sb4.delete(6, 16); // Remove "Beautiful "24 System.out.println(sb4Hello World); // Hello World2526 // Delete single character27 StringBuilder sb5→ Helllo = new StringBuilder("Helllo");28 sb5.deleteCharAt(3); // Remove extra 'l'29 System.out.println(sb5Hello); // Hello3031 // Replace32 StringBuilder sb6→ Hello World = new StringBuilder("Hello World");33 sb6.replace(6, 11, "Java");34 System.out.println(sb6Hello Java); // Hello Java35}outputHello Beautiful World I love Java Start End Hello World Hello Hello Java
Method Chaining
Chaining.java
Replay: real traced execution (multi-file project)
// Method chaining
public class Chaining {
public static void main(String[] args) {
// Method chaining
StringBuilder sb = new StringBuilder();
sb.append("First")
.append(" ")
.append("Second")
.append(" ")
.append("Third");
System.out.println(sb);
// Build a sentence
String sentence = new StringBuilder()
.append("The")
.append(" quick")
.append(" brown")
.append(" fox")
.toString();
System.out.println(sentence);
// Mix different operations
String result = new StringBuilder()
.append("Count: ")
.append(10)
.append(", ")
.append("Value: ")
.append(3.14)
.toString();
System.out.println(result);
// Build CSV line
String csv = new StringBuilder()
.append("John")
.append(",")
.append(30)
.append(",")
.append("Engineer")
.append(",")
.append(75000)
.toString();
System.out.println(csv);
}
}
sb ← (empty), sentence ← The quick brown fox, result ← Count: 10, Value: 3.14
5public static void main(String[] args) {6 // Method chaining7 StringBuilder sb→ (empty) = new StringBuilder();8 sb.append("First")9 .append(" ")10 .append("Second")11 .append(" ")12 .append("Third");13 System.out.println(sbFirst Second Third);1415 // Build a sentence16 String sentence→ The quick brown fox = new StringBuilder()17 .append("The")18 .append(" quick")19 .append(" brown")20 .append(" fox")21 .toString();22 System.out.println(sentenceThe quick brown fox);2324 // Mix different operations25 String result→ Count: 10, Value: 3.14 = new StringBuilder()26 .append("Count: ")27 .append(10)28 .append(", ")29 .append("Value: ")30 .append(3.14)31 .toString();32 System.out.println(resultCount: 10, Value: 3.14);3334 // Build CSV line35 String csv→ John,30,Engineer,75000 = new StringBuilder()36 .append("John")37 .append(",")38 .append(30)39 .append(",")40 .append("Engineer")41 .append(",")42 .append(75000)43 .toString();44 System.out.println(csvJohn,30,Engineer,75000);45}outputFirst Second Third The quick brown fox Count: 10, Value: 3.14 John,30,Engineer,75000
method_chaining
Because append returns the same builder, multiple append calls can be chained into a fluent sequence.
Reverse
StringBuilder includes utility methods for reversing, changing characters, resizing, and extracting substrings.
Reverse.java
Replay: real traced execution (multi-file project)
// Reverse and other operations
public class Reverse {
public static void main(String[] args) {
// Reverse string
StringBuilder sb1 = new StringBuilder("Hello");
sb1.reverse();
System.out.println(sb1); // olleH
// Check palindrome
String word = "racecar";
StringBuilder sb2 = new StringBuilder(word);
boolean isPalindrome = word.equals(sb2.reverse().toString());
System.out.println(word + " is palindrome: " + isPalindrome);
// Set character at index
StringBuilder sb3 = new StringBuilder("Hello");
sb3.setCharAt(0, 'J');
System.out.println(sb3); // Jello
// Set length (truncate or pad)
StringBuilder sb4 = new StringBuilder("Hello");
sb4.setLength(3);
System.out.println(sb4); // Hel
StringBuilder sb5 = new StringBuilder("Hi");
sb5.setLength(5); // Pads with null chars
System.out.println("Length: " + sb5.length());
// Substring
StringBuilder sb6 = new StringBuilder("Hello World");
String sub = sb6.substring(6, 11);
System.out.println(sub); // World
}
}
// Reverse and other operations
public class Reverse {
public static void main(String[] args) {
// Reverse string
StringBuilder sb1 = new StringBuilder("Hello");
sb1.reverse();
System.out.println(sb1); // olleH
// Check palindrome
String word = "level";
StringBuilder sb2 = new StringBuilder(word);
boolean isPalindrome = word.equals(sb2.reverse().toString());
System.out.println(word + " is palindrome: " + isPalindrome);
// Set character at index
StringBuilder sb3 = new StringBuilder("Hello");
sb3.setCharAt(0, 'J');
System.out.println(sb3); // Jello
// Set length (truncate or pad)
StringBuilder sb4 = new StringBuilder("Hello");
sb4.setLength(3);
System.out.println(sb4); // Hel
StringBuilder sb5 = new StringBuilder("Hi");
sb5.setLength(5); // Pads with null chars
System.out.println("Length: " + sb5.length());
// Substring
StringBuilder sb6 = new StringBuilder("Hello World");
String sub = sb6.substring(6, 11);
System.out.println(sub); // World
}
}
// Reverse and other operations
public class Reverse {
public static void main(String[] args) {
// Reverse string
StringBuilder sb1 = new StringBuilder("Hello");
sb1.reverse();
System.out.println(sb1); // olleH
// Check palindrome
String word = "java";
StringBuilder sb2 = new StringBuilder(word);
boolean isPalindrome = word.equals(sb2.reverse().toString());
System.out.println(word + " is palindrome: " + isPalindrome);
// Set character at index
StringBuilder sb3 = new StringBuilder("Hello");
sb3.setCharAt(0, 'J');
System.out.println(sb3); // Jello
// Set length (truncate or pad)
StringBuilder sb4 = new StringBuilder("Hello");
sb4.setLength(3);
System.out.println(sb4); // Hel
StringBuilder sb5 = new StringBuilder("Hi");
sb5.setLength(5); // Pads with null chars
System.out.println("Length: " + sb5.length());
// Substring
StringBuilder sb6 = new StringBuilder("Hello World");
String sub = sb6.substring(6, 11);
System.out.println(sub); // World
}
}
sb1 ← Hello, word ← racecar, sb2 ← racecar, isPalindrome ← true
5public static void main(String[] args) {6 // Reverse string7 StringBuilder sb1→ Hello = new StringBuilder("Hello");8 sb1.reverse();9 System.out.println(sb1olleH); // olleH1011 // Check palindrome12 String word→ racecar = "racecar"; //@word="racecar", "level", "java"13 StringBuilder sb2→ racecar = new StringBuilder(word);14 boolean isPalindrome→ true = word.equals(sb2.reverse().toString());15 System.out.println(wordracecar + " is palindrome: " + isPalindrometrue);1617 // Set character at index18 StringBuilder sb3→ Hello = new StringBuilder("Hello");19 sb3.setCharAt(0, 'J');20 System.out.println(sb3Jello); // Jello2122 // Set length (truncate or pad)23 StringBuilder sb4→ Hello = new StringBuilder("Hello");24 sb4.setLength(3);25 System.out.println(sb4Hel); // Hel2627 StringBuilder sb5→ Hi = new StringBuilder("Hi");28 sb5.setLength(5); // Pads with null chars29 System.out.println("Length: " + sb5.length());3031 // Substring32 StringBuilder sb6→ Hello World = new StringBuilder("Hello World");33 String sub→ World = sb6.substring(6, 11);34 System.out.println(subWorld); // World35}outputolleH racecar is palindrome: true Jello Hel Length: 5 World
sb1 ← Hello, word ← level, sb2 ← level, isPalindrome ← true, sb3 ← Hello
5public static void main(String[] args) {6 // Reverse string7 StringBuilder sb1→ Hello = new StringBuilder("Hello");8 sb1.reverse();9 System.out.println(sb1olleH); // olleH1011 // Check palindrome12 String word→ level = "level";13 StringBuilder sb2→ level = new StringBuilder(word);14 boolean isPalindrome→ true = word.equals(sb2.reverse().toString());15 System.out.println(wordlevel + " is palindrome: " + isPalindrometrue);1617 // Set character at index18 StringBuilder sb3→ Hello = new StringBuilder("Hello");19 sb3.setCharAt(0, 'J');20 System.out.println(sb3Jello); // Jello2122 // Set length (truncate or pad)23 StringBuilder sb4→ Hello = new StringBuilder("Hello");24 sb4.setLength(3);25 System.out.println(sb4Hel); // Hel2627 StringBuilder sb5→ Hi = new StringBuilder("Hi");28 sb5.setLength(5); // Pads with null chars29 System.out.println("Length: " + sb5.length());3031 // Substring32 StringBuilder sb6→ Hello World = new StringBuilder("Hello World");33 String sub→ World = sb6.substring(6, 11);34 System.out.println(subWorld); // World35}outputolleH level is palindrome: true Jello Hel Length: 5 World
sb1 ← Hello, word ← java, sb2 ← java, isPalindrome ← false, sb3 ← Hello
5public static void main(String[] args) {6 // Reverse string7 StringBuilder sb1→ Hello = new StringBuilder("Hello");8 sb1.reverse();9 System.out.println(sb1olleH); // olleH1011 // Check palindrome12 String word→ java = "java";13 StringBuilder sb2→ java = new StringBuilder(word);14 boolean isPalindrome→ false = word.equals(sb2.reverse().toString());15 System.out.println(wordjava + " is palindrome: " + isPalindromefalse);1617 // Set character at index18 StringBuilder sb3→ Hello = new StringBuilder("Hello");19 sb3.setCharAt(0, 'J');20 System.out.println(sb3Jello); // Jello2122 // Set length (truncate or pad)23 StringBuilder sb4→ Hello = new StringBuilder("Hello");24 sb4.setLength(3);25 System.out.println(sb4Hel); // Hel2627 StringBuilder sb5→ Hi = new StringBuilder("Hi");28 sb5.setLength(5); // Pads with null chars29 System.out.println("Length: " + sb5.length());3031 // Substring32 StringBuilder sb6→ Hello World = new StringBuilder("Hello World");33 String sub→ World = sb6.substring(6, 11);34 System.out.println(subWorld); // World35}outputolleH java is palindrome: false Jello Hel Length: 5 World
Practical Use
StringBuilder is useful for building structured text in loops or from many parts.
Practical.java
Replay: real traced execution (multi-file project)
// Practical: HTML builder
public class Practical {
public static String buildTable(String[][] data) {
StringBuilder html = new StringBuilder();
html.append("<table>\n");
// Header row
if (data.length > 0) {
html.append(" <thead>\n <tr>");
for (String header : data[0]) {
html.append("<th>").append(header).append("</th>");
}
html.append("</tr>\n </thead>\n");
}
// Data rows
html.append(" <tbody>\n");
for (int i = 1; i < data.length; i++) {
html.append(" <tr>");
for (String cell : data[i]) {
html.append("<td>").append(cell).append("</td>");
}
html.append("</tr>\n");
}
html.append(" </tbody>\n");
html.append("</table>");
return html.toString();
}
public static String buildSelectQuery(String table, String[] columns, String condition) {
StringBuilder sql = new StringBuilder();
sql.append("SELECT ");
if (columns.length == 0) {
sql.append("*");
} else {
for (int i = 0; i < columns.length; i++) {
if (i > 0) sql.append(", ");
sql.append(columns[i]);
}
}
sql.append(" FROM ").append(table);
if (condition != null && !condition.isEmpty()) {
sql.append(" WHERE ").append(condition);
}
sql.append(";");
return sql.toString();
}
public static void main(String[] args) {
// Build HTML table
String[][] tableData = {
{"Name", "Age", "City"},
{"Alice", "25", "New York"},
{"Bob", "30", "London"},
{"Charlie", "35", "Paris"}
};
System.out.println(buildTable(tableData));
// Build SQL queries
System.out.println("\n" + buildSelectQuery("users", new String[0], null));
System.out.println(buildSelectQuery("users", new String[]{"name", "email"}, null));
System.out.println(buildSelectQuery("users", new String[]{"*"}, "age > 25"));
}
}
public static void main(String[] args)
60public static void main(String[] args) {61 // Build HTML table62 String[][] tableData = {63 {"Name", "Age", "City"},64 {"Alice", "25", "New York"},65 {"Bob", "30", "London"},66 {"Charlie", "35", "Paris"}67 };6869 System.out.println(buildTable(tableData));html ← (empty)
5public static String buildTable(String[][] data) {6 StringBuilder html→ (empty) = new StringBuilder();78 html.append("<table>\n");if (data.length > 0)
10// Header row11if (data.length4 > 0) {12 html.append(" <thead>\n <tr>");13 for (String header : data[0]) {for (String header : data[0])
pass 1 of 312html.append(" <thead>\n <tr>");13for (String headerName : data[0]) {14 html.append("<th>").append(headerName).append("</th>");15}All 3 passes — pass 1 is the card above pass header1 Name 2 Age 3 City html.append("</tr> </thead> ");
15 }16 html.append("</tr>\n </thead>\n");17}html.append(" <tbody> ");
19// Data rows20html.append(" <tbody>\n");21for (int i = 1; i < data.length; i++) {for (int i = 1; i < data.length; i++)
pass 1 of 320html.append(" <tbody>\n");21for (int i1 = 1; i < data.length4; i++) {22 html.append(" <tr>");23 for (String cell : data[i]) {All 3 passes — pass 1 is the card above pass i1 1 2 2 3 3 for (String cell : data[i])
pass 1 of 922html.append(" <tr>");23for (String cellAlice : data[i]) {24 html.append("<td>").append(cellAlice).append("</td>");25}All 9 passes — pass 1 is the card above pass celli1 Alice 1 2 25 1 3 New York 1 4 Bob 2 5 30 2 6 London 2 7 Charlie 3 8 35 3 9 Paris 3 html.append("</tr> ");
25 }26 html.append("</tr>\n");27}html.append("</tr> ");
25 }26 html.append("</tr>\n");27}html.append("</tr> ");
25 }26 html.append("</tr>\n");27}html.append(" </tbody> ");
27 }28 html.append(" </tbody>\n");2930 html.append("</table>");3132 return html.toString();33}System.out.println(buildTable(tableData));
69System.out.println(buildTable(tableData));7071// Build SQL queries72System.out.println("\n" + buildSelectQuery("users", new String[0], null));73System.out.println(buildSelectQuery("users", new String[]{"name", "email"}, null));output<table> <thead> <tr><th>Name</th><th>Age</th><th>City</th></tr> </thead> <tbody> <tr><td>Alice</td><td>25</td><td>New York</td></tr> <tr><td>Bob</td><td>30</td><td>London</td></tr> <tr><td>Charlie</td><td>35</td><td>Paris</td></tr> </tbody> </table>sql ← (empty)
pass 1 of 335public static String buildSelectQuery(String tableusers, String[] columns, String conditionnull) {36 StringBuilder sql→ (empty) = new StringBuilder();3738 sql.append("SELECT ");All 3 passes — pass 1 is the card above pass conditioncolumns.lengthisql1 null 0 — (empty) 2 null — 1 (empty) 3 age > 25 — — (empty) if (columns.length == 0)
40if (columns.length0 == 0) {41 sql.append("*");42} else {sql.append(" FROM ").append(table);
49 sql.append(" FROM ").append(tableusers);5051 if (condition != null && !condition.isEmpty()) {52 sql.append(" WHERE ").append(condition);53 }5455 sql.append(";");5657 return sql.toString();58}System.out.println(" " + buildSelectQuery("users", new String[0], null…
71// Build SQL queries72System.out.println("\n" + buildSelectQuery("users", new String[0], null));73System.out.println(buildSelectQuery("users", new String[]{"name", "email"}, null));74System.out.println(buildSelectQuery("users", new String[]{"*"}, "age > 25"));output SELECT * FROM users;for (int i = 0; i < columns.length; i++)
pass 1 of 342} else {43 for (int i0 = 0; i < columns.length2; i++) {44 if (i > 0) sql.append(", ");45 sql.append(columns[i]name);46 }All 3 passes — pass 1 is the card above pass icolumns.lengthcolumns[i]1 0 2 name 2 1 2 — 3 0 1 * if (i > 0)
43for (int i = 0; i < columns.length; i++) {44 if (i1 > 0) sql.append(", ");45 sql.append(columns[i]);sql.append(columns[i]);
44 if (i > 0) sql.append(", ");45 sql.append(columns[i]email);46}values this step1isql.append(" FROM ").append(table);
49 sql.append(" FROM ").append(tableusers);5051 if (condition != null && !condition.isEmpty()) {52 sql.append(" WHERE ").append(condition);53 }5455 sql.append(";");5657 return sql.toString();58}System.out.println(buildSelectQuery("users", new String[]{"name", "ema…
72 System.out.println("\n" + buildSelectQuery("users", new String[0], null));73 System.out.println(buildSelectQuery("users", new String[]{"name", "email"}, null));74 System.out.println(buildSelectQuery("users", new String[]{"*"}, "age > 25"));75}outputSELECT name, email FROM users;sql.append(" FROM ").append(table);
49sql.append(" FROM ").append(tableusers);if (condition != null && !condition.isEmpty())
51if (conditionage > 25 != null && !condition.isEmpty()) {52 sql.append(" WHERE ").append(conditionage > 25);53}sql.append(";");
55 sql.append(";");5657 return sql.toString();58}System.out.println(buildSelectQuery("users", new String[]{"*"}, "age >…
73 System.out.println(buildSelectQuery("users", new String[]{"name", "email"}, null));74 System.out.println(buildSelectQuery("users", new String[]{"*"}, "age > 25"));75}outputSELECT * FROM users WHERE age > 25;
Exercise: Practical.java
Build a comma-separated list of names from an array, handling the last element without a trailing comma