Stream Operations
Parallel Streams
For CPU-intensive operations on large datasets, parallel streams automatically split work across multiple cores. Understanding when parallelism helps (and when it hurts) is essential for writing performant stream code.
Creating Parallel Streams
Convert any stream to parallel processing with a single method call.
Create.java
Replay: real traced execution (multi-file project)
import java.util.*;
import java.util.stream.*;
public class Create {
public static void main(String[] args) {
int rangeEnd = 11;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
List<Integer> fromParallelStream = nums.parallelStream()
.map(n -> n)
.collect(Collectors.toList());
System.out.println("parallelStream: " + fromParallelStream);
List<Integer> fromParallel = nums.stream()
.parallel()
.map(n -> n)
.collect(Collectors.toList());
System.out.println("stream().parallel(): " + fromParallel);
boolean isParallel = nums.parallelStream().isParallel();
System.out.println("isParallel: " + isParallel);
}
}
import java.util.*;
import java.util.stream.*;
public class Create {
public static void main(String[] args) {
int rangeEnd = 6;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
List<Integer> fromParallelStream = nums.parallelStream()
.map(n -> n)
.collect(Collectors.toList());
System.out.println("parallelStream: " + fromParallelStream);
List<Integer> fromParallel = nums.stream()
.parallel()
.map(n -> n)
.collect(Collectors.toList());
System.out.println("stream().parallel(): " + fromParallel);
boolean isParallel = nums.parallelStream().isParallel();
System.out.println("isParallel: " + isParallel);
}
}
import java.util.*;
import java.util.stream.*;
public class Create {
public static void main(String[] args) {
int rangeEnd = 16;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
List<Integer> fromParallelStream = nums.parallelStream()
.map(n -> n)
.collect(Collectors.toList());
System.out.println("parallelStream: " + fromParallelStream);
List<Integer> fromParallel = nums.stream()
.parallel()
.map(n -> n)
.collect(Collectors.toList());
System.out.println("stream().parallel(): " + fromParallel);
boolean isParallel = nums.parallelStream().isParallel();
System.out.println("isParallel: " + isParallel);
}
}
rangeEnd ← 11, nums ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], fromParallelStream ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
4public class Create {5 public static void main(String[] args) {6 int rangeEnd→ 11 = 11; //@rangeEnd=11, 6, 167 List<Integer> nums→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] = IntStream.range(1, rangeEnd11).boxed().toList();89 List<Integer> fromParallelStream→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] = nums.parallelStream()10 .map(n -> n)11 .collect(Collectors.toList());12 System.out.println("parallelStream: " + fromParallelStream[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);1314 List<Integer> fromParallel→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] = nums.stream()15 .parallel()16 .map(n -> n)17 .collect(Collectors.toList());18 System.out.println("stream().parallel(): " + fromParallel[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);1920 boolean isParallel→ true = nums.parallelStream().isParallel();21 System.out.println("isParallel: " + isParalleltrue);outputparallelStream: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] stream().parallel(): [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] isParallel: true
rangeEnd ← 6, nums ← [1, 2, 3, 4, 5], fromParallelStream ← [1, 2, 3, 4, 5]
4public class Create {5 public static void main(String[] args) {6 int rangeEnd→ 6 = 6;7 List<Integer> nums→ [1, 2, 3, 4, 5] = IntStream.range(1, rangeEnd6).boxed().toList();89 List<Integer> fromParallelStream→ [1, 2, 3, 4, 5] = nums.parallelStream()10 .map(n -> n)11 .collect(Collectors.toList());12 System.out.println("parallelStream: " + fromParallelStream[1, 2, 3, 4, 5]);1314 List<Integer> fromParallel→ [1, 2, 3, 4, 5] = nums.stream()15 .parallel()16 .map(n -> n)17 .collect(Collectors.toList());18 System.out.println("stream().parallel(): " + fromParallel[1, 2, 3, 4, 5]);1920 boolean isParallel→ true = nums.parallelStream().isParallel();21 System.out.println("isParallel: " + isParalleltrue);outputparallelStream: [1, 2, 3, 4, 5] stream().parallel(): [1, 2, 3, 4, 5] isParallel: true
rangeEnd ← 16, nums ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
4public class Create {5 public static void main(String[] args) {6 int rangeEnd→ 16 = 16;7 List<Integer> nums→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] = IntStream.range(1, rangeEnd16).boxed().toList();89 List<Integer> fromParallelStream→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] = nums.parallelStream()10 .map(n -> n)11 .collect(Collectors.toList());12 System.out.println("parallelStream: " + fromParallelStream[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]);1314 List<Integer> fromParallel→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] = nums.stream()15 .parallel()16 .map(n -> n)17 .collect(Collectors.toList());18 System.out.println("stream().parallel(): " + fromParallel[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]);1920 boolean isParallel→ true = nums.parallelStream().isParallel();21 System.out.println("isParallel: " + isParalleltrue);outputparallelStream: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] stream().parallel(): [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] isParallel: true
parallel stream
A stream that processes elements concurrently across multiple threads using the fork/join framework.
spliterator
The mechanism that divides stream data into chunks for parallel processing; some data structures split better than others.
Performance Considerations
Parallel streams have overhead. They help with large datasets and CPU-bound work, but can hurt performance otherwise.
Performance.java
Replay: real traced execution (multi-file project)
import java.util.*;
import java.util.stream.*;
public class Performance {
private static int work(int n) {
int result = 0;
for (int i = 1; i <= 5; i++) {
result += n * i;
}
return result;
}
public static void main(String[] args) {
int rangeEnd = 8;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
List<Integer> sequential = nums.stream()
.map(Performance::work)
.collect(Collectors.toList());
System.out.println("sequential size: " + sequential.size());
List<Integer> parallel = nums.parallelStream()
.map(Performance::work)
.collect(Collectors.toList());
System.out.println("parallel size: " + parallel.size());
boolean sameResults = sequential.equals(parallel);
System.out.println("sameResults: " + sameResults);
}
}
import java.util.*;
import java.util.stream.*;
public class Performance {
private static int work(int n) {
int result = 0;
for (int i = 1; i <= 5; i++) {
result += n * i;
}
return result;
}
public static void main(String[] args) {
int rangeEnd = 5;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
List<Integer> sequential = nums.stream()
.map(Performance::work)
.collect(Collectors.toList());
System.out.println("sequential size: " + sequential.size());
List<Integer> parallel = nums.parallelStream()
.map(Performance::work)
.collect(Collectors.toList());
System.out.println("parallel size: " + parallel.size());
boolean sameResults = sequential.equals(parallel);
System.out.println("sameResults: " + sameResults);
}
}
import java.util.*;
import java.util.stream.*;
public class Performance {
private static int work(int n) {
int result = 0;
for (int i = 1; i <= 5; i++) {
result += n * i;
}
return result;
}
public static void main(String[] args) {
int rangeEnd = 11;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
List<Integer> sequential = nums.stream()
.map(Performance::work)
.collect(Collectors.toList());
System.out.println("sequential size: " + sequential.size());
List<Integer> parallel = nums.parallelStream()
.map(Performance::work)
.collect(Collectors.toList());
System.out.println("parallel size: " + parallel.size());
boolean sameResults = sequential.equals(parallel);
System.out.println("sameResults: " + sameResults);
}
}
rangeEnd ← 8, nums ← [1, 2, 3, 4, 5, 6, 7]
13public static void main(String[] args) {14 int rangeEnd→ 8 = 8; //@rangeEnd=8, 5, 1115 List<Integer> nums→ [1, 2, 3, 4, 5, 6, 7] = IntStream.range(1, rangeEnd8).boxed().toList();1617 List<Integer> sequential = nums.stream()18 .map(Performance::work)19 .collect(Collectors.toList());20 System.out.println("sequential size: " + sequential.size());result ← 0
pass 1 of 144public class Performance {5 private static int work(int n1) {6 int result→ 0 = 0;7 for (int i = 1; i <= 5; i++) {14 passes — pass 1 is the card above pass nresult1 1 0 2 2 0 3 3 0 4 4 0 5 5 0 6 6 0 7 7 0 8 5 0 9 7 — ⋯ 3 more passes ⋯ 13 2 0 14 1 0 result ← 1
pass 1 of 706int result = 0;7for (int i1 = 1; i <= 5; i++) {8 result→ 1 += n1 * i1;9}70 passes — pass 1 is the card above pass inresult1 1 1 0 → 1 2 2 1 1 → 3 3 3 1 3 → 6 4 4 1 6 → 10 5 5 1 10 → 15 6 1 2 0 → 2 7 2 2 2 → 6 8 3 2 6 → 12 9 4 2 12 → 20 ⋯ 59 more passes ⋯ 69 4 7 42 → 70 70 5 7 70 → 105 return result;
9 }10 return result15;11}return result;
9 }10 return result30;11}return result;
9 }10 return result45;11}return result;
9 }10 return result60;11}return result;
9 }10 return result75;11}return result;
9 }10 return result90;11}return result;
9 }10 return result105;11}sequential ← [15, 30, 45, 60, 75, 90, 105]
17List<Integer> sequential→ [15, 30, 45, 60, 75, 90, 105] = nums.stream()18 .map(Performance::work)19 .collect(Collectors.toList());20System.out.println("sequential size: " + sequential.size());2122List<Integer> parallel = nums.parallelStream()23 .map(Performance::work)24 .collect(Collectors.toList());25System.out.println("parallel size: " + parallel.size());outputsequential size: 7return result;
9 }10 return result75;11}return result;
9 }10 return result60;11}return result;
9 }10 return result45;11}return result;
9 }10 return result90;11}return result;
9 }10 return result30;11}return result;
9 }10 return result15;11}return result;
9 }10 return result105;11}parallel ← [15, 30, 45, 60, 75, 90, 105], sameResults ← true
22 List<Integer> parallel→ [15, 30, 45, 60, 75, 90, 105] = nums.parallelStream()23 .map(Performance::work)24 .collect(Collectors.toList());25 System.out.println("parallel size: " + parallel.size());2627 boolean sameResults→ true = sequential.equals(parallel[15, 30, 45, 60, 75, 90, 105]);28 System.out.println("sameResults: " + sameResultstrue);29}outputparallel size: 7 sameResults: true
rangeEnd ← 5, nums ← [1, 2, 3, 4]
13public static void main(String[] args) {14 int rangeEnd→ 5 = 5;15 List<Integer> nums→ [1, 2, 3, 4] = IntStream.range(1, rangeEnd5).boxed().toList();1617 List<Integer> sequential = nums.stream()18 .map(Performance::work)19 .collect(Collectors.toList());20 System.out.println("sequential size: " + sequential.size());result ← 0
pass 1 of 84public class Performance {5 private static int work(int n1) {6 int result→ 0 = 0;7 for (int i = 1; i <= 5; i++) {All 8 passes — pass 1 is the card above pass nresult1 1 0 2 2 0 3 3 0 4 4 0 5 3 0 6 4 0 7 1 — 8 2 0 result ← 1
pass 1 of 406int result = 0;7for (int i1 = 1; i <= 5; i++) {8 result→ 1 += n1 * i1;9}40 passes — pass 1 is the card above pass resultni1 0 → 1 1 1 2 1 → 3 1 2 3 3 → 6 1 3 4 6 → 10 1 4 5 10 → 15 1 5 6 0 → 2 2 1 7 2 → 6 2 2 8 6 → 12 2 3 9 12 → 20 2 4 ⋯ 29 more passes ⋯ 39 20 → 30 2 5 40 10 → 15 1 5 return result;
9 }10 return result15;11}return result;
9 }10 return result30;11}return result;
9 }10 return result45;11}return result;
9 }10 return result60;11}sequential ← [15, 30, 45, 60]
17List<Integer> sequential→ [15, 30, 45, 60] = nums.stream()18 .map(Performance::work)19 .collect(Collectors.toList());20System.out.println("sequential size: " + sequential.size());2122List<Integer> parallel = nums.parallelStream()23 .map(Performance::work)24 .collect(Collectors.toList());25System.out.println("parallel size: " + parallel.size());outputsequential size: 4return result;
9 }10 return result45;11}return result;
9 }10 return result30;11}result ← 60
7for (int i = 1; i <= 5; i++) {8 result→ 60 += n1 * i4;9}return result;
9 }10 return result60;11}result ← 10
7for (int i = 1; i <= 5; i++) {8 result→ 10 += n1 * i4;9}return result;
9 }10 return result15;11}parallel ← [15, 30, 45, 60], sameResults ← true
22 List<Integer> parallel→ [15, 30, 45, 60] = nums.parallelStream()23 .map(Performance::work)24 .collect(Collectors.toList());25 System.out.println("parallel size: " + parallel.size());2627 boolean sameResults→ true = sequential.equals(parallel[15, 30, 45, 60]);28 System.out.println("sameResults: " + sameResultstrue);29}outputparallel size: 4 sameResults: true
rangeEnd ← 11, nums ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
13public static void main(String[] args) {14 int rangeEnd→ 11 = 11;15 List<Integer> nums→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] = IntStream.range(1, rangeEnd11).boxed().toList();1617 List<Integer> sequential = nums.stream()18 .map(Performance::work)19 .collect(Collectors.toList());20 System.out.println("sequential size: " + sequential.size());result ← 0
pass 1 of 204public class Performance {5 private static int work(int n1) {6 int result→ 0 = 0;7 for (int i = 1; i <= 5; i++) {20 passes — pass 1 is the card above pass resultni1 0 1 — 2 0 2 — 3 0 3 — 4 0 4 — 5 0 5 — 6 0 6 — 7 0 7 — 8 0 8 — 9 0 9 — ⋯ 9 more passes ⋯ 19 0 8 — 20 0 10 — result ← 1
pass 1 of 1006int result = 0;7for (int i1 = 1; i <= 5; i++) {8 result→ 1 += n1 * i1;9}100 passes — pass 1 is the card above pass resultni1 0 → 1 1 1 2 1 → 3 1 2 3 3 → 6 1 3 4 6 → 10 1 4 5 10 → 15 1 5 6 0 → 2 2 1 7 2 → 6 2 2 8 6 → 12 2 3 9 12 → 20 2 4 ⋯ 89 more passes ⋯ 99 54 → 90 9 4 100 90 → 135 9 5 return result;
9 }10 return result15;11}return result;
9 }10 return result30;11}return result;
9 }10 return result45;11}return result;
9 }10 return result60;11}return result;
9 }10 return result75;11}return result;
9 }10 return result90;11}return result;
9 }10 return result105;11}return result;
9 }10 return result120;11}return result;
9 }10 return result135;11}return result;
9 }10 return result150;11}sequential ← [15, 30, 45, 60, 75, 90, 105, 120, 135, 150]
17List<Integer> sequential→ [15, 30, 45, 60, 75, 90, 105, 120, 135, 150] = nums.stream()18 .map(Performance::work)19 .collect(Collectors.toList());20System.out.println("sequential size: " + sequential.size());2122List<Integer> parallel = nums.parallelStream()23 .map(Performance::work)24 .collect(Collectors.toList());25System.out.println("parallel size: " + parallel.size());outputsequential size: 10return result;
9 }10 return result30;11}return result;
9 }10 return result15;11}return result;
9 }10 return result105;11}result ← 50
7for (int i = 1; i <= 5; i++) {8 result→ 50 += n5 * i4;9}return result;
9 }10 return result75;11}return result;
9 }10 return result45;11}return result;
9 }10 return result90;11}return result;
9 }10 return result120;11}return result;
9 }10 return result150;11}return result;
9 }10 return result135;11}return result;
9 }10 return result60;11}parallel ← [15, 30, 45, 60, 75, 90, 105, 120, 135, 150], sameResults ← true
22 List<Integer> parallel→ [15, 30, 45, 60, 75, 90, 105, 120, 135, 150] = nums.parallelStream()23 .map(Performance::work)24 .collect(Collectors.toList());25 System.out.println("parallel size: " + parallel.size());2627 boolean sameResults→ true = sequential.equals(parallel[15, 30, 45, 60, 75, 90, 105, 120, 135, 150]);28 System.out.println("sameResults: " + sameResultstrue);29}outputparallel size: 10 sameResults: true
Ordering in Parallel Streams
Parallel streams may process elements out of order. Use forEachOrdered when order matters for output.
Order.java
Replay: real traced execution (multi-file project)
import java.util.*;
import java.util.stream.*;
public class Order {
public static void main(String[] args) {
int rangeEnd = 11;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
List<Integer> collected = nums.parallelStream()
.map(n -> n)
.collect(Collectors.toList());
System.out.println("collect: " + collected);
System.out.print("forEachOrdered: ");
nums.parallelStream().forEachOrdered(n -> System.out.print(n + " "));
System.out.println();
List<Integer> result = nums.parallelStream()
.map(n -> n * 2)
.collect(Collectors.toList());
System.out.println("collect: " + result);
}
}
import java.util.*;
import java.util.stream.*;
public class Order {
public static void main(String[] args) {
int rangeEnd = 6;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
List<Integer> collected = nums.parallelStream()
.map(n -> n)
.collect(Collectors.toList());
System.out.println("collect: " + collected);
System.out.print("forEachOrdered: ");
nums.parallelStream().forEachOrdered(n -> System.out.print(n + " "));
System.out.println();
List<Integer> result = nums.parallelStream()
.map(n -> n * 2)
.collect(Collectors.toList());
System.out.println("collect: " + result);
}
}
import java.util.*;
import java.util.stream.*;
public class Order {
public static void main(String[] args) {
int rangeEnd = 16;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
List<Integer> collected = nums.parallelStream()
.map(n -> n)
.collect(Collectors.toList());
System.out.println("collect: " + collected);
System.out.print("forEachOrdered: ");
nums.parallelStream().forEachOrdered(n -> System.out.print(n + " "));
System.out.println();
List<Integer> result = nums.parallelStream()
.map(n -> n * 2)
.collect(Collectors.toList());
System.out.println("collect: " + result);
}
}
rangeEnd ← 11, nums ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], collected ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
4public class Order {5 public static void main(String[] args) {6 int rangeEnd→ 11 = 11; //@rangeEnd=11, 6, 167 List<Integer> nums→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] = IntStream.range(1, rangeEnd11).boxed().toList();89 List<Integer> collected→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] = nums.parallelStream()10 .map(n -> n)11 .collect(Collectors.toList());12 System.out.println("collect: " + collected[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);1314 System.out.print("forEachOrdered: ");15 nums.parallelStream().forEachOrdered(n -> System.out.print(n + " "));16 System.out.println();1718 List<Integer> result→ [2, 4, 6, 8, 10, 12, 14, 16, 18, 20] = nums.parallelStream()19 .map(n -> n * 2)20 .collect(Collectors.toList());21 System.out.println("collect: " + result[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]);outputcollect: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] forEachOrdered: collect: [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
rangeEnd ← 6, nums ← [1, 2, 3, 4, 5], collected ← [1, 2, 3, 4, 5]
4public class Order {5 public static void main(String[] args) {6 int rangeEnd→ 6 = 6;7 List<Integer> nums→ [1, 2, 3, 4, 5] = IntStream.range(1, rangeEnd6).boxed().toList();89 List<Integer> collected→ [1, 2, 3, 4, 5] = nums.parallelStream()10 .map(n -> n)11 .collect(Collectors.toList());12 System.out.println("collect: " + collected[1, 2, 3, 4, 5]);1314 System.out.print("forEachOrdered: ");15 nums.parallelStream().forEachOrdered(n -> System.out.print(n + " "));16 System.out.println();1718 List<Integer> result→ [2, 4, 6, 8, 10] = nums.parallelStream()19 .map(n -> n * 2)20 .collect(Collectors.toList());21 System.out.println("collect: " + result[2, 4, 6, 8, 10]);outputcollect: [1, 2, 3, 4, 5] forEachOrdered: collect: [2, 4, 6, 8, 10]
rangeEnd ← 16, nums ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
4public class Order {5 public static void main(String[] args) {6 int rangeEnd→ 16 = 16;7 List<Integer> nums→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] = IntStream.range(1, rangeEnd16).boxed().toList();89 List<Integer> collected→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] = nums.parallelStream()10 .map(n -> n)11 .collect(Collectors.toList());12 System.out.println("collect: " + collected[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]);1314 System.out.print("forEachOrdered: ");15 nums.parallelStream().forEachOrdered(n -> System.out.print(n + " "));16 System.out.println();1718 List<Integer> result→ [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30] = nums.parallelStream()19 .map(n -> n * 2)20 .collect(Collectors.toList());21 System.out.println("collect: " + result[2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30]);outputcollect: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] forEachOrdered: collect: [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30]
encounter order
The defined sequence of elements in a stream; parallel operations may process out of order but can preserve order in results.
Avoiding Shared Mutable State
Shared mutable state in parallel streams causes race conditions. Keep operations stateless.
SharedState.java
Replay: real traced execution (multi-file project)
import java.util.*;
import java.util.stream.*;
public class SharedState {
public static void main(String[] args) {
int rangeEnd = 101;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
List<Integer> sideEffectResult = Collections.synchronizedList(new ArrayList<>());
nums.parallelStream()
.filter(n -> n % 2 == 0)
.forEach(sideEffectResult::add);
Collections.sort(sideEffectResult);
System.out.println("sideEffectResult size: " + sideEffectResult.size());
List<Integer> correctResult = nums.parallelStream()
.filter(n -> n % 2 == 0)
.collect(Collectors.toList());
System.out.println("correctResult size: " + correctResult.size());
}
}
import java.util.*;
import java.util.stream.*;
public class SharedState {
public static void main(String[] args) {
int rangeEnd = 21;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
List<Integer> sideEffectResult = Collections.synchronizedList(new ArrayList<>());
nums.parallelStream()
.filter(n -> n % 2 == 0)
.forEach(sideEffectResult::add);
Collections.sort(sideEffectResult);
System.out.println("sideEffectResult size: " + sideEffectResult.size());
List<Integer> correctResult = nums.parallelStream()
.filter(n -> n % 2 == 0)
.collect(Collectors.toList());
System.out.println("correctResult size: " + correctResult.size());
}
}
import java.util.*;
import java.util.stream.*;
public class SharedState {
public static void main(String[] args) {
int rangeEnd = 501;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
List<Integer> sideEffectResult = Collections.synchronizedList(new ArrayList<>());
nums.parallelStream()
.filter(n -> n % 2 == 0)
.forEach(sideEffectResult::add);
Collections.sort(sideEffectResult);
System.out.println("sideEffectResult size: " + sideEffectResult.size());
List<Integer> correctResult = nums.parallelStream()
.filter(n -> n % 2 == 0)
.collect(Collectors.toList());
System.out.println("correctResult size: " + correctResult.size());
}
}
rangeEnd ← 101, nums ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100]
4public class SharedState {5 public static void main(String[] args) {6 int rangeEnd→ 101 = 101; //@rangeEnd=101, 21, 5017 List<Integer> nums→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100] = IntStream.range(1, rangeEnd101).boxed().toList();89 List<Integer> sideEffectResult→ [] = Collections.synchronizedList(new ArrayList<>());10 nums.parallelStream()11 .filter(n -> n % 2 == 0)12 .forEach(sideEffectResult::add);13 Collections.sort(sideEffectResult→ [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100]);14 System.out.println("sideEffectResult size: " + sideEffectResult.size());1516 List<Integer> correctResult→ [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100] = nums.parallelStream()17 .filter(n -> n % 2 == 0)18 .collect(Collectors.toList());19 System.out.println("correctResult size: " + correctResult.size());outputsideEffectResult size: 50 correctResult size: 50
rangeEnd ← 21, nums ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
4public class SharedState {5 public static void main(String[] args) {6 int rangeEnd→ 21 = 21;7 List<Integer> nums→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20] = IntStream.range(1, rangeEnd21).boxed().toList();89 List<Integer> sideEffectResult→ [] = Collections.synchronizedList(new ArrayList<>());10 nums.parallelStream()11 .filter(n -> n % 2 == 0)12 .forEach(sideEffectResult::add);13 Collections.sort(sideEffectResult→ [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]);14 System.out.println("sideEffectResult size: " + sideEffectResult.size());1516 List<Integer> correctResult→ [2, 4, 6, 8, 10, 12, 14, 16, 18, 20] = nums.parallelStream()17 .filter(n -> n % 2 == 0)18 .collect(Collectors.toList());19 System.out.println("correctResult size: " + correctResult.size());outputsideEffectResult size: 10 correctResult size: 10
rangeEnd ← 501, nums ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 475, 476, 477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491, 492, 493, 494, 495, 496, 497, 498, 499, 500]
4public class SharedState {5 public static void main(String[] args) {6 int rangeEnd→ 501 = 501;7 List<Integer> nums→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 475, 476, 477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491, 492, 493, 494, 495, 496, 497, 498, 499, 500] = IntStream.range(1, rangeEnd501).boxed().toList();89 List<Integer> sideEffectResult→ [] = Collections.synchronizedList(new ArrayList<>());10 nums.parallelStream()11 .filter(n -> n % 2 == 0)12 .forEach(sideEffectResult::add);13 Collections.sort(sideEffectResult→ [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100, 102, 104, 106, 108, 110, 112, 114, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134, 136, 138, 140, 142, 144, 146, 148, 150, 152, 154, 156, 158, 160, 162, 164, 166, 168, 170, 172, 174, 176, 178, 180, 182, 184, 186, 188, 190, 192, 194, 196, 198, 200, 202, 204, 206, 208, 210, 212, 214, 216, 218, 220, 222, 224, 226, 228, 230, 232, 234, 236, 238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258, 260, 262, 264, 266, 268, 270, 272, 274, 276, 278, 280, 282, 284, 286, 288, 290, 292, 294, 296, 298, 300, 302, 304, 306, 308, 310, 312, 314, 316, 318, 320, 322, 324, 326, 328, 330, 332, 334, 336, 338, 340, 342, 344, 346, 348, 350, 352, 354, 356, 358, 360, 362, 364, 366, 368, 370, 372, 374, 376, 378, 380, 382, 384, 386, 388, 390, 392, 394, 396, 398, 400, 402, 404, 406, 408, 410, 412, 414, 416, 418, 420, 422, 424, 426, 428, 430, 432, 434, 436, 438, 440, 442, 444, 446, 448, 450, 452, 454, 456, 458, 460, 462, 464, 466, 468, 470, 472, 474, 476, 478, 480, 482, 484, 486, 488, 490, 492, 494, 496, 498, 500]);14 System.out.println("sideEffectResult size: " + sideEffectResult.size());1516 List<Integer> correctResult→ [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100, 102, 104, 106, 108, 110, 112, 114, 116, 118, 120, 122, 124, 126, 128, 130, 132, 134, 136, 138, 140, 142, 144, 146, 148, 150, 152, 154, 156, 158, 160, 162, 164, 166, 168, 170, 172, 174, 176, 178, 180, 182, 184, 186, 188, 190, 192, 194, 196, 198, 200, 202, 204, 206, 208, 210, 212, 214, 216, 218, 220, 222, 224, 226, 228, 230, 232, 234, 236, 238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258, 260, 262, 264, 266, 268, 270, 272, 274, 276, 278, 280, 282, 284, 286, 288, 290, 292, 294, 296, 298, 300, 302, 304, 306, 308, 310, 312, 314, 316, 318, 320, 322, 324, 326, 328, 330, 332, 334, 336, 338, 340, 342, 344, 346, 348, 350, 352, 354, 356, 358, 360, 362, 364, 366, 368, 370, 372, 374, 376, 378, 380, 382, 384, 386, 388, 390, 392, 394, 396, 398, 400, 402, 404, 406, 408, 410, 412, 414, 416, 418, 420, 422, 424, 426, 428, 430, 432, 434, 436, 438, 440, 442, 444, 446, 448, 450, 452, 454, 456, 458, 460, 462, 464, 466, 468, 470, 472, 474, 476, 478, 480, 482, 484, 486, 488, 490, 492, 494, 496, 498, 500] = nums.parallelStream()17 .filter(n -> n % 2 == 0)18 .collect(Collectors.toList());19 System.out.println("correctResult size: " + correctResult.size());outputsideEffectResult size: 250 correctResult size: 250
thread safety
Code that behaves correctly when accessed by multiple threads simultaneously; parallel streams require stateless operations.
Combiners for Parallel Reduce
When reducing in parallel, provide a combiner to merge partial results from different threads.
Combiners.java
Replay: real traced execution (multi-file project)
import java.util.*;
import java.util.stream.*;
public class Combiners {
public static void main(String[] args) {
int rangeEnd = 101;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
int sum = nums.parallelStream()
.reduce(0, Integer::sum, Integer::sum);
System.out.println("sum: " + sum);
List<Integer> evens = nums.parallelStream()
.filter(n -> n % 2 == 0)
.collect(
ArrayList::new,
ArrayList::add,
ArrayList::addAll // combiner
);
System.out.println("evens count: " + evens.size());
Map<Integer, List<Integer>> byMod = nums.parallelStream()
.collect(Collectors.groupingBy(n -> n % 5, TreeMap::new, Collectors.toList()));
System.out.println("byMod: " + byMod.keySet());
}
}
import java.util.*;
import java.util.stream.*;
public class Combiners {
public static void main(String[] args) {
int rangeEnd = 11;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
int sum = nums.parallelStream()
.reduce(0, Integer::sum, Integer::sum);
System.out.println("sum: " + sum);
List<Integer> evens = nums.parallelStream()
.filter(n -> n % 2 == 0)
.collect(
ArrayList::new,
ArrayList::add,
ArrayList::addAll // combiner
);
System.out.println("evens count: " + evens.size());
Map<Integer, List<Integer>> byMod = nums.parallelStream()
.collect(Collectors.groupingBy(n -> n % 5, TreeMap::new, Collectors.toList()));
System.out.println("byMod: " + byMod.keySet());
}
}
import java.util.*;
import java.util.stream.*;
public class Combiners {
public static void main(String[] args) {
int rangeEnd = 51;
List<Integer> nums = IntStream.range(1, rangeEnd).boxed().toList();
int sum = nums.parallelStream()
.reduce(0, Integer::sum, Integer::sum);
System.out.println("sum: " + sum);
List<Integer> evens = nums.parallelStream()
.filter(n -> n % 2 == 0)
.collect(
ArrayList::new,
ArrayList::add,
ArrayList::addAll // combiner
);
System.out.println("evens count: " + evens.size());
Map<Integer, List<Integer>> byMod = nums.parallelStream()
.collect(Collectors.groupingBy(n -> n % 5, TreeMap::new, Collectors.toList()));
System.out.println("byMod: " + byMod.keySet());
}
}
rangeEnd ← 101, nums ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100]
4public class Combiners {5 public static void main(String[] args) {6 int rangeEnd→ 101 = 101; //@rangeEnd=101, 11, 517 List<Integer> nums→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100] = IntStream.range(1, rangeEnd101).boxed().toList();89 int sum→ 5050 = nums.parallelStream()10 .reduce(0, Integer::sum, Integer::sum);11 System.out.println("sum: " + sum5050);1213 List<Integer> evens→ [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100] = nums.parallelStream()14 .filter(n -> n % 2 == 0)15 .collect(16 ArrayList::new,17 ArrayList::add,18 ArrayList::addAll // combiner19 );20 System.out.println("evens count: " + evens.size());2122 Map<Integer, List<Integer>> byMod→ {0=[5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100], 1=[1, 6, 11, 16, 21, 26, 31, 36, 41, 46, 51, 56, 61, 66, 71, 76, 81, 86, 91, 96], 2=[2, 7, 12, 17, 22, 27, 32, 37, 42, 47, 52, 57, 62, 67, 72, 77, 82, 87, 92, 97], 3=[3, 8, 13, 18, 23, 28, 33, 38, 43, 48, 53, 58, 63, 68, 73, 78, 83, 88, 93, 98], 4=[4, 9, 14, 19, 24, 29, 34, 39, 44, 49, 54, 59, 64, 69, 74, 79, 84, 89, 94, 99]} = nums.parallelStream()23 .collect(Collectors.groupingBy(n -> n % 5, TreeMap::new, Collectors.toList()));24 System.out.println("byMod: " + byMod.keySet());outputsum: 5050 evens count: 50 byMod: [0, 1, 2, 3, 4]
rangeEnd ← 11, nums ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], sum ← 55
4public class Combiners {5 public static void main(String[] args) {6 int rangeEnd→ 11 = 11;7 List<Integer> nums→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] = IntStream.range(1, rangeEnd11).boxed().toList();89 int sum→ 55 = nums.parallelStream()10 .reduce(0, Integer::sum, Integer::sum);11 System.out.println("sum: " + sum55);1213 List<Integer> evens→ [2, 4, 6, 8, 10] = nums.parallelStream()14 .filter(n -> n % 2 == 0)15 .collect(16 ArrayList::new,17 ArrayList::add,18 ArrayList::addAll // combiner19 );20 System.out.println("evens count: " + evens.size());2122 Map<Integer, List<Integer>> byMod→ {0=[5, 10], 1=[1, 6], 2=[2, 7], 3=[3, 8], 4=[4, 9]} = nums.parallelStream()23 .collect(Collectors.groupingBy(n -> n % 5, TreeMap::new, Collectors.toList()));24 System.out.println("byMod: " + byMod.keySet());outputsum: 55 evens count: 5 byMod: [0, 1, 2, 3, 4]
rangeEnd ← 51, nums ← [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50]
4public class Combiners {5 public static void main(String[] args) {6 int rangeEnd→ 51 = 51;7 List<Integer> nums→ [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50] = IntStream.range(1, rangeEnd51).boxed().toList();89 int sum→ 1275 = nums.parallelStream()10 .reduce(0, Integer::sum, Integer::sum);11 System.out.println("sum: " + sum1275);1213 List<Integer> evens→ [2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50] = nums.parallelStream()14 .filter(n -> n % 2 == 0)15 .collect(16 ArrayList::new,17 ArrayList::add,18 ArrayList::addAll // combiner19 );20 System.out.println("evens count: " + evens.size());2122 Map<Integer, List<Integer>> byMod→ {0=[5, 10, 15, 20, 25, 30, 35, 40, 45, 50], 1=[1, 6, 11, 16, 21, 26, 31, 36, 41, 46], 2=[2, 7, 12, 17, 22, 27, 32, 37, 42, 47], 3=[3, 8, 13, 18, 23, 28, 33, 38, 43, 48], 4=[4, 9, 14, 19, 24, 29, 34, 39, 44, 49]} = nums.parallelStream()23 .collect(Collectors.groupingBy(n -> n % 5, TreeMap::new, Collectors.toList()));24 System.out.println("byMod: " + byMod.keySet());outputsum: 1275 evens count: 25 byMod: [0, 1, 2, 3, 4]
Exercise: Practical.java
Compare sequential vs parallel performance for computing statistics on a large dataset