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BAEL-9176 Code and Unit Tests for Converting Nested Loops to Streams in Java (#18372)
- Added a new streams-7 module. - Implements the solution showcasing three use-cases: looping through all the elements, lopping through elements and using a filter, looping through elements and short-circuiting. - Provides unit tests for each of the implemented use-cases.
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‎core-java-modules/core-java-streams-7/README.md

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### Relevant Articles:
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‎core-java-modules/core-java-streams-7/pom.xml

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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<artifactId>core-java-streams-7</artifactId>
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<packaging>jar</packaging>
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<name>core-java-streams-7</name>
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<parent>
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<groupId>com.baeldung.core-java-modules</groupId>
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<artifactId>core-java-modules</artifactId>
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<version>0.0.1-SNAPSHOT</version>
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</parent>
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<build>
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<finalName>core-java-streams-7</finalName>
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<resources>
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<resource>
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<directory>../core-java-streams-7/src/main</directory>
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<filtering>true</filtering>
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</resource>
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</resources>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-compiler-plugin</artifactId>
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<configuration>
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<source>${maven.compiler.source}</source>
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<target>${maven.compiler.target}</target>
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<compilerArgument>-parameters</compilerArgument>
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</configuration>
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</plugin>
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</plugins>
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</build>
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<properties>
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<maven.compiler.source>12</maven.compiler.source>
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<maven.compiler.target>12</maven.compiler.target>
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<vavr.version>0.10.2</vavr.version>
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<assertj-core.version>3.23.1</assertj-core.version>
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<commons-lang3.version>3.12.0</commons-lang3.version>
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</properties>
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</project>

‎core-java-modules/core-java-streams-7/src/main/java/com/baeldung/streams/NestedLoopsToStreamsConverter.java

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package com.baeldung.streams;
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import java.util.List;
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import java.util.ArrayList;
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import java.util.Optional;
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import java.util.stream.Collectors;
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public class NestedLoopsToStreamsConverter {
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public static List<int[]> getAllPairsImperative(List<Integer> list1, List<Integer> list2) {
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List<int[]> pairs = new ArrayList<>();
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for (Integer num1 : list1) {
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for (Integer num2 : list2) {
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pairs.add(new int[] { num1, num2 });
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}
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}
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return pairs;
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}
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public static List<int[]> getAllPairsStream(List<Integer> list1, List<Integer> list2) {
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return list1.stream()
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.flatMap(num1 -> list2.stream().map(num2 -> new int[] { num1, num2 }))
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.collect(Collectors.toList());
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}
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public static List<int[]> getFilteredPairsImperative(List<Integer> list1, List<Integer> list2) {
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List<int[]> pairs = new ArrayList<>();
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for (Integer num1 : list1) {
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for (Integer num2 : list2) {
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if (num1 + num2 > 7) {
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pairs.add(new int[] { num1, num2 });
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}
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}
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}
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return pairs;
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}
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public static List<int[]> getFilteredPairsStream(List<Integer> list1, List<Integer> list2) {
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return list1.stream()
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.flatMap(num1 -> list2.stream().map(num2 -> new int[] { num1, num2 }))
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.filter(pair -> pair[0] + pair[1] > 7)
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.collect(Collectors.toList());
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}
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public static Optional<int[]> getFirstMatchingPairImperative(List<Integer> list1, List<Integer> list2) {
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for (Integer num1 : list1) {
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for (Integer num2 : list2) {
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if (num1 + num2 > 7) {
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return Optional.of(new int[] { num1, num2 });
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}
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}
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}
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return Optional.empty();
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}
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public static Optional<int[]> getFirstMatchingPairStream(List<Integer> list1, List<Integer> list2) {
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return list1.stream()
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.flatMap(num1 -> list2.stream().map(num2 -> new int[] { num1, num2 }))
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.filter(pair -> pair[0] + pair[1] > 7)
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.findFirst();
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}
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}

‎core-java-modules/core-java-streams-7/src/test/java/com/baeldung/streams/NestedLoopsToStreamsConverterUnitTest.java

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package com.baeldung.streams;
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import static org.junit.jupiter.api.Assertions.assertArrayEquals;
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import static org.junit.Assert.assertEquals;
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import java.util.Arrays;
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import java.util.List;
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import java.util.Optional;
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import org.junit.Test;
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public class NestedLoopsToStreamsConverterUnitTest {
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@Test
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public void givenTwoLists_whenGeneratingAllPairs_thenResultsShouldMatch() {
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List<Integer> list1 = Arrays.asList(1, 2, 3);
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List<Integer> list2 = Arrays.asList(4, 5, 6);
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List<int[]> imperativeResult = NestedLoopsToStreamsConverter.getAllPairsImperative(list1, list2);
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List<int[]> streamResult = NestedLoopsToStreamsConverter.getAllPairsStream(list1, list2);
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assertEquals(imperativeResult.size(), streamResult.size());
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for (int i = 0; i < imperativeResult.size(); i++) {
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assertArrayEquals(imperativeResult.get(i), streamResult.get(i));
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}
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}
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@Test
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public void givenTwoLists_whenFilteringPairsBySum_thenResultsShouldMatch() {
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List<Integer> list1 = Arrays.asList(1, 2, 3);
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List<Integer> list2 = Arrays.asList(4, 5, 6);
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List<int[]> imperativeResult = NestedLoopsToStreamsConverter.getFilteredPairsImperative(list1, list2);
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List<int[]> streamResult = NestedLoopsToStreamsConverter.getFilteredPairsStream(list1, list2);
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assertEquals(imperativeResult.size(), streamResult.size());
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for (int i = 0; i < imperativeResult.size(); i++) {
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assertArrayEquals(imperativeResult.get(i), streamResult.get(i));
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}
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}
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@Test
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public void givenTwoLists_whenFindingFirstMatchingPair_thenResultsShouldMatch() {
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List<Integer> list1 = Arrays.asList(1, 2, 3);
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List<Integer> list2 = Arrays.asList(4, 5, 6);
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Optional<int[]> imperativeResult = NestedLoopsToStreamsConverter.getFirstMatchingPairImperative(list1, list2);
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Optional<int[]> streamResult = NestedLoopsToStreamsConverter.getFirstMatchingPairStream(list1, list2);
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assertEquals(imperativeResult.isPresent(), streamResult.isPresent());
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imperativeResult.ifPresent(pair -> assertArrayEquals(pair, streamResult.get()));
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}
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}

‎core-java-modules/pom.xml

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<module>core-java-streams-4</module>
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<module>core-java-streams-5</module>
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<module>core-java-streams-6</module>
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<module>core-java-streams-7</module>
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<module>core-java-streams-collect</module>
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<module>core-java-streams-maps</module>
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<module>core-java-string-operations-3</module>

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