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Guide to Stream.reduce() (#6425) 

* Initial Commit

* Update Application.java
This commit is contained in:
Alejandro Gervasio 2019-02-27 10:46:07 -03:00 committed by KevinGilmore
parent 0c5293da99
commit b293765466
4 changed files with 226 additions and 0 deletions
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70
java-streams/src/main/java/com/baeldung/reduce/application/Application.java Normal file
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package com.baeldung.reduce.application;
import com.baeldung.reduce.entities.User;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.annotations.BenchmarkMode;
import org.openjdk.jmh.annotations.Fork;
import org.openjdk.jmh.annotations.Mode;
import org.openjdk.jmh.annotations.Warmup;
public class Application {
public static void main(String[] args) throws Exception {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
int result1 = numbers.stream().reduce(0, (subtotal, element) -> subtotal + element);
System.out.println(result1);
int result2 = numbers.stream().reduce(0, Integer::sum);
System.out.println(result2);
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
String result3 = letters.stream().reduce("", (partialString, element) -> partialString + element);
System.out.println(result3);
String result4 = letters.stream().reduce("", String::concat);
System.out.println(result4);
String result5 = letters.stream().reduce("", (partialString, element) -> partialString.toUpperCase() + element.toUpperCase());
System.out.println(result5);
List<User> users = Arrays.asList(new User("John", 30), new User("Julie", 35));
int result6 = users.stream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
System.out.println(result6);
String result7 = letters.parallelStream().reduce("", String::concat);
System.out.println(result7);
int result8 = users.parallelStream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
System.out.println(result8);
org.openjdk.jmh.Main.main(args);
}
@Benchmark
@Fork(value = 1, warmups = 2)
@Warmup(iterations = 2)
@BenchmarkMode(Mode.AverageTime)
public void executeReduceOnParallelizedStream() {
List<User> userList = new ArrayList<>();
for (int i = 0; i <= 1000000; i++) {
userList.add(new User("John" + i, i));
}
userList.parallelStream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
}
@Benchmark
@Fork(value = 1, warmups = 2)
@Warmup(iterations = 2)
@BenchmarkMode(Mode.AverageTime)
public void executeReduceOnSequentialStream() {
List<User> userList = new ArrayList<>();
for (int i = 0; i <= 1000000; i++) {
userList.add(new User("John" + i, i));
}
userList.stream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
}
}

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java-streams/src/main/java/com/baeldung/reduce/entities/User.java Normal file
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package com.baeldung.reduce.entities;
public class User {
private final String name;
private final int age;
public User(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
@Override
public String toString() {
return "User{" + "name=" + name + ", age=" + age + '}';
}
}

52
java-streams/src/main/java/com/baeldung/reduce/utilities/NumberUtils.java Normal file
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package com.baeldung.reduce.utilities;
import java.util.List;
import java.util.function.BiFunction;
import java.util.logging.Level;
import java.util.logging.Logger;
public abstract class NumberUtils {
private static final Logger LOGGER = Logger.getLogger(NumberUtils.class.getName());
public static int divideListElements(List<Integer> values, Integer divider) {
return values.stream()
.reduce(0, (a, b) -> {
try {
return a / divider + b / divider;
} catch (ArithmeticException e) {
LOGGER.log(Level.INFO, "Arithmetic Exception: Division by Zero");
}
return 0;
});
}
public static int divideListElementsWithExtractedTryCatchBlock(List<Integer> values, int divider) {
return values.stream().reduce(0, (a, b) -> divide(a, divider) + divide(b, divider));
}
public static int divideListElementsWithApplyFunctionMethod(List<Integer> values, int divider) {
BiFunction<Integer, Integer, Integer> division = (a, b) -> a / b;
return values.stream().reduce(0, (a, b) -> applyFunction(division, a, divider) + applyFunction(division, b, divider));
}
private static int divide(int value, int factor) {
int result = 0;
try {
result = value / factor;
} catch (ArithmeticException e) {
LOGGER.log(Level.INFO, "Arithmetic Exception: Division by Zero");
}
return result;
}
private static int applyFunction(BiFunction<Integer, Integer, Integer> function, int a, int b) {
try {
return function.apply(a, b);
}
catch(Exception e) {
LOGGER.log(Level.INFO, "Exception occurred!");
}
return 0;
}
}

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java-streams/src/test/java/com/baeldung/reduce/tests/StreamReduceUnitTest.java Normal file
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package com.baeldung.reduce.tests;
import com.baeldung.reduce.entities.User;
import com.baeldung.reduce.utilities.NumberUtils;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import static org.assertj.core.api.Assertions.assertThat;
import org.junit.Test;
public class StreamReduceUnitTest {
@Test
public void givenIntegerList_whenReduceWithSumAccumulatorLambda_thenCorrect() {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
int result = numbers.stream().reduce(0, (a, b) -> a + b);
assertThat(result).isEqualTo(21);
}
@Test
public void givenIntegerList_whenReduceWithSumAccumulatorMethodReference_thenCorrect() {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
int result = numbers.stream().reduce(0, Integer::sum);
assertThat(result).isEqualTo(21);
}
@Test
public void givenStringList_whenReduceWithConcatenatorAccumulatorLambda_thenCorrect() {
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
String result = letters.stream().reduce("", (a, b) -> a + b);
assertThat(result).isEqualTo("abcde");
}
@Test
public void givenStringList_whenReduceWithConcatenatorAccumulatorMethodReference_thenCorrect() {
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
String result = letters.stream().reduce("", String::concat);
assertThat(result).isEqualTo("abcde");
}
@Test
public void givenStringList_whenReduceWithUppercaseConcatenatorAccumulator_thenCorrect() {
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
String result = letters.stream().reduce("", (a, b) -> a.toUpperCase() + b.toUpperCase());
assertThat(result).isEqualTo("ABCDE");
}
@Test
public void givenUserList_whenReduceWithAgeAccumulatorAndSumCombiner_thenCorrect() {
List<User> users = Arrays.asList(new User("John", 30), new User("Julie", 35));
int result = users.stream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum);
assertThat(result).isEqualTo(65);
}
@Test
public void givenStringList_whenReduceWithParallelStream_thenCorrect() {
List<String> letters = Arrays.asList("a", "b", "c", "d", "e");
String result = letters.parallelStream().reduce("", String::concat);
assertThat(result).isEqualTo("abcde");
}
@Test
public void givenNumberUtilsClass_whenCalledDivideListElements_thenCorrect() {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
assertThat(NumberUtils.divideListElements(numbers, 1)).isEqualTo(21);
}
@Test
public void givenNumberUtilsClass_whenCalledDivideListElementsWithExtractedTryCatchBlock_thenCorrect() {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
assertThat(NumberUtils.divideListElementsWithExtractedTryCatchBlock(numbers, 1)).isEqualTo(21);
}
@Test
public void givenStream_whneCalleddivideListElementsWithApplyFunctionMethod_thenCorrect() {
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
assertThat(NumberUtils.divideListElementsWithApplyFunctionMethod(numbers, 1)).isEqualTo(21);
}
}