BAEL-6783: Converting HashMap to an ArrayList in Java (#14814)

core-java-modules/core-java-collections-conversions-3/README.md

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+## Java Collections Cookbooks and Examples
+
+This module contains articles about conversions among Collection types in Java.
+
+### Relevant Articles:

core-java-modules/core-java-collections-conversions-3/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-collections-conversions-3</artifactId>
+    <name>core-java-collections-conversions-3</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <artifactId>core-java-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+    </parent>
+
+    <build>
+        <finalName>core-java-collections-conversions-3</finalName>
+        <resources>
+            <resource>
+                <directory>src/main/resources</directory>
+                <filtering>true</filtering>
+            </resource>
+        </resources>
+    </build>
+
+</project>

core-java-modules/core-java-collections-conversions-3/src/main/java/com/baeldung/hashmaptoarraylist/HashMapToArrayListConverterUtils.java

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+package com.baeldung.hashmaptoarraylist;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+import com.google.common.collect.Lists;
+import com.google.common.collect.Maps;
+import com.google.common.collect.Maps.EntryTransformer;
+
+public class HashMapToArrayListConverterUtils {
+
+    static ArrayList<String> convertUsingConstructor(HashMap<Integer, String> hashMap) {
+        if (hashMap == null) {
+            return null;
+        }
+        return new ArrayList<String>(hashMap.values());
+    }
+
+    static ArrayList<String> convertUsingAddAllMethod(HashMap<Integer, String> hashMap) {
+        if (hashMap == null) {
+            return null;
+        }
+
+        ArrayList<String> arrayList = new ArrayList<String>(hashMap.size());
+        arrayList.addAll(hashMap.values());
+
+        return arrayList;
+    }
+
+    static ArrayList<String> convertUsingStreamApi(HashMap<Integer, String> hashMap) {
+        if (hashMap == null) {
+            return null;
+        }
+
+        return hashMap.values()
+            .stream()
+            .collect(Collectors.toCollection(ArrayList::new));
+    }
+
+    static ArrayList<String> convertUsingForLoop(HashMap<Integer, String> hashMap) {
+        if (hashMap == null) {
+            return null;
+        }
+
+        ArrayList<String> arrayList = new ArrayList<String>(hashMap.size());
+        for (Map.Entry<Integer, String> entry : hashMap.entrySet()) {
+            arrayList.add(entry.getValue());
+        }
+
+        return arrayList;
+    }
+
+    static public ArrayList<String> convertUsingGuava(HashMap<Integer, String> hashMap) {
+        if (hashMap == null) {
+            return null;
+        }
+
+        EntryTransformer<Integer, String, String> entryMapTransformer = (key, value) -> value;
+
+        return Lists.newArrayList(Maps.transformEntries(hashMap, entryMapTransformer)
+            .values());
+    }
+
+}

core-java-modules/core-java-collections-conversions-3/src/test/java/com/baeldung/hashmaptoarraylist/HashMapToArrayListConverterUtilsUnitTest.java

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+package com.baeldung.hashmaptoarraylist;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.containsInAnyOrder;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+
+import org.junit.Before;
+import org.junit.Test;
+
+public class HashMapToArrayListConverterUtilsUnitTest {
+
+    private HashMap<Integer, String> hashMap;
+
+    @Before
+    public void beforeEach() {
+        hashMap = new HashMap<>();
+        hashMap.put(1, "AAA");
+        hashMap.put(2, "BBB");
+        hashMap.put(3, "CCC");
+        hashMap.put(4, "DDD");
+    }
+
+    @Test
+    public void givenAHashMap_whenConvertUsingConstructor_thenReturnArrayList() {
+        ArrayList<String> myList = HashMapToArrayListConverterUtils.convertUsingConstructor(hashMap);
+
+        assertThat(hashMap.values(), containsInAnyOrder(myList.toArray()));
+    }
+
+    @Test
+    public void givenAHashMap_whenConvertUsingAddAllMethod_thenReturnArrayList() {
+        ArrayList<String> myList = HashMapToArrayListConverterUtils.convertUsingAddAllMethod(hashMap);
+
+        assertThat(hashMap.values(), containsInAnyOrder(myList.toArray()));
+    }
+
+    @Test
+    public void givenAHashMap_whenConvertUsingForLoop_thenReturnArrayList() {
+        ArrayList<String> myList = HashMapToArrayListConverterUtils.convertUsingForLoop(hashMap);
+
+        assertThat(hashMap.values(), containsInAnyOrder(myList.toArray()));
+    }
+
+    @Test
+    public void givenAHashMap_whenConvertUsingStreamApi_thenReturnArrayList() {
+        ArrayList<String> myList = HashMapToArrayListConverterUtils.convertUsingStreamApi(hashMap);
+
+        assertThat(hashMap.values(), containsInAnyOrder(myList.toArray()));
+    }
+
+    @Test
+    public void givenAHashMap_whenConvertUsingGuava_thenReturnArrayList() {
+        ArrayList<String> myList = HashMapToArrayListConverterUtils.convertUsingGuava(hashMap);
+
+        assertThat(hashMap.values(), containsInAnyOrder(myList.toArray()));
+    }
+
+}