From ca4440d7925b295813e26dee7583f468f9ca617f Mon Sep 17 00:00:00 2001 From: Alejandro Gervasio Date: Sat, 23 Feb 2019 23:20:37 -0300 Subject: [PATCH] Guide to Stream.reduce() (#6372) * Initial Commit * Update StreamReduceUnitTest.java * Update StreamReduceUnitTest.java * Update StreamReduceUnitTest.java --- .../streamreduce/application/Application.java | 62 +++++++++ .../baeldung/streamreduce/entities/User.java | 25 ++++ .../streamreduce/utilities/NumberUtils.java | 52 ++++++++ .../tests/StreamReduceUnitTest.java | 126 ++++++++++++++++++ 4 files changed, 265 insertions(+) create mode 100644 core-java-8/src/main/java/com/baeldung/streamreduce/application/Application.java create mode 100644 core-java-8/src/main/java/com/baeldung/streamreduce/entities/User.java create mode 100644 core-java-8/src/main/java/com/baeldung/streamreduce/utilities/NumberUtils.java create mode 100644 core-java-8/src/test/java/com/baeldung/streamreduce/tests/StreamReduceUnitTest.java diff --git a/core-java-8/src/main/java/com/baeldung/streamreduce/application/Application.java b/core-java-8/src/main/java/com/baeldung/streamreduce/application/Application.java new file mode 100644 index 0000000000..0b1dd952dc --- /dev/null +++ b/core-java-8/src/main/java/com/baeldung/streamreduce/application/Application.java @@ -0,0 +1,62 @@ +package com.baeldung.streamreduce.application; + +import com.baeldung.streamreduce.entities.User; +import com.baeldung.streamreduce.utilities.NumberUtils; +import java.util.ArrayList; +import java.util.Arrays; +import java.util.List; + +public class Application { + + public static void main(String[] args) { + List numbers = Arrays.asList(1, 2, 3, 4, 5, 6); + int result1 = numbers.stream().reduce(0, (a, b) -> a + b); + System.out.println(result1); + + int result2 = numbers.stream().reduce(0, Integer::sum); + System.out.println(result2); + + List letters = Arrays.asList("a", "b", "c", "d", "e"); + String result3 = letters.stream().reduce("", (a, b) -> a + b); + System.out.println(result3); + + String result4 = letters.stream().reduce("", String::concat); + System.out.println(result4); + + String result5 = letters.stream().reduce("", (a, b) -> a.toUpperCase() + b.toUpperCase()); + System.out.println(result5); + + List 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); + + List userList = new ArrayList<>(); + for (int i = 0; i <= 1000000; i++) { + userList.add(new User("John" + i, i)); + } + + long t1 = System.currentTimeMillis(); + int result9 = userList.stream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum); + long t2 = System.currentTimeMillis(); + System.out.println(result9); + System.out.println("Sequential stream time: " + (t2 - t1) + "ms"); + + long t3 = System.currentTimeMillis(); + int result10 = userList.parallelStream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum); + long t4 = System.currentTimeMillis(); + System.out.println(result10); + System.out.println("Parallel stream time: " + (t4 - t3) + "ms"); + + int result11 = NumberUtils.divideListElements(numbers, 1); + System.out.println(result11); + + int result12 = NumberUtils.divideListElementsWithExtractedTryCatchBlock(numbers, 0); + System.out.println(result12); + } +} diff --git a/core-java-8/src/main/java/com/baeldung/streamreduce/entities/User.java b/core-java-8/src/main/java/com/baeldung/streamreduce/entities/User.java new file mode 100644 index 0000000000..bc13a8cde6 --- /dev/null +++ b/core-java-8/src/main/java/com/baeldung/streamreduce/entities/User.java @@ -0,0 +1,25 @@ +package com.baeldung.streamreduce.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 + '}'; + } +} diff --git a/core-java-8/src/main/java/com/baeldung/streamreduce/utilities/NumberUtils.java b/core-java-8/src/main/java/com/baeldung/streamreduce/utilities/NumberUtils.java new file mode 100644 index 0000000000..7a6a85e6c4 --- /dev/null +++ b/core-java-8/src/main/java/com/baeldung/streamreduce/utilities/NumberUtils.java @@ -0,0 +1,52 @@ +package com.baeldung.streamreduce.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 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 values, int divider) { + return values.stream().reduce(0, (a, b) -> divide(a, divider) + divide(b, divider)); + } + + public static int divideListElementsWithApplyFunctionMethod(List values, int divider) { + BiFunction 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 function, int a, int b) { + try { + return function.apply(a, b); + } + catch(Exception e) { + LOGGER.log(Level.INFO, "Exception occurred!"); + } + return 0; + } +} diff --git a/core-java-8/src/test/java/com/baeldung/streamreduce/tests/StreamReduceUnitTest.java b/core-java-8/src/test/java/com/baeldung/streamreduce/tests/StreamReduceUnitTest.java new file mode 100644 index 0000000000..7a50592164 --- /dev/null +++ b/core-java-8/src/test/java/com/baeldung/streamreduce/tests/StreamReduceUnitTest.java @@ -0,0 +1,126 @@ +package com.baeldung.streamreduce.tests; + +import com.baeldung.streamreduce.entities.User; +import com.baeldung.streamreduce.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 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 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 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 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 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 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 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 numbers = Arrays.asList(1, 2, 3, 4, 5, 6); + + assertThat(NumberUtils.divideListElements(numbers, 1)).isEqualTo(21); + } + + @Test + public void givenNumberUtilsClass_whenCalledDivideListElementsWithExtractedTryCatchBlock_thenCorrect() { + List numbers = Arrays.asList(1, 2, 3, 4, 5, 6); + + assertThat(NumberUtils.divideListElementsWithExtractedTryCatchBlock(numbers, 1)).isEqualTo(21); + } + + @Test + public void givenNumberUtilsClass_whenCalledDivideListElementsWithExtractedTryCatchBlockAndListContainsZero_thenCorrect() { + List numbers = Arrays.asList(0, 1, 2, 3, 4, 5, 6); + + assertThat(NumberUtils.divideListElementsWithExtractedTryCatchBlock(numbers, 1)).isEqualTo(21); + } + + @Test + public void givenNumberUtilsClass_whenCalledDivideListElementsWithExtractedTryCatchBlockAndDividerIsZero_thenCorrect() { + List numbers = Arrays.asList(1, 2, 3, 4, 5, 6); + + assertThat(NumberUtils.divideListElementsWithExtractedTryCatchBlock(numbers, 0)).isEqualTo(0); + } + + @Test + public void givenStream_whneCalleddivideListElementsWithApplyFunctionMethod_thenCorrect() { + List numbers = Arrays.asList(1, 2, 3, 4, 5, 6); + + assertThat(NumberUtils.divideListElementsWithApplyFunctionMethod(numbers, 1)).isEqualTo(21); + } + + @Test + public void givenTwoStreams_whenCalledReduceOnParallelizedStream_thenFasterExecutionTime() { + List userList = new ArrayList<>(); + for (int i = 0; i <= 1000000; i++) { + userList.add(new User("John" + i, i)); + } + long currentTime1 = System.currentTimeMillis(); + userList.stream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum); + long sequentialExecutionTime = System.currentTimeMillis() -currentTime1; + long currentTime2 = System.currentTimeMillis(); + userList.parallelStream().reduce(0, (partialAgeResult, user) -> partialAgeResult + user.getAge(), Integer::sum); + long parallelizedExecutionTime = System.currentTimeMillis() - currentTime2; + + assertThat(parallelizedExecutionTime).isLessThan(sequentialExecutionTime); + } +}