{"id":328969,"date":"2023-10-02T19:59:33","date_gmt":"2023-10-02T19:59:33","guid":{"rendered":"https:\/\/www.baeldung.com\/java-converting-hashmap-to-an-arraylist"},"modified":"2023-10-02T19:59:33","modified_gmt":"2023-10-02T19:59:33","slug":"converting-hashmap-to-an-arraylist-in-java","status":"publish","type":"post","link":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/2023\/10\/02\/converting-hashmap-to-an-arraylist-in-java\/","title":{"rendered":"Converting HashMap to an ArrayList in Java"},"content":{"rendered":"

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1. Overview<\/h2>\n
<\/div>\n

In this short tutorial, we’ll shed light on how to convert HashMap<\/em><\/a> values into an A<\/em>rrayList<\/em><\/a> in Java.<\/p>\n

First, we’ll explain how to do it using core Java methods. Then, we will demonstrate how to tackle our central puzzle using external libraries such as Guava.<\/p>\n

2. Converting Using Core Java<\/h2>\n
<\/div>\n

Converting a HashMap<\/em> to an ArrayList<\/em> is a common task. In this section, we\u2019ll cover different ways to do this using Java classes and methods.<\/p>\n

2.1. Using the ArrayList<\/em> Constructor<\/h3>\n
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ArrayList<\/em> constructor<\/a> provides the most common and easiest way to convert a HashMap<\/em> to an ArrayList<\/em>.<\/p>\n

The basic idea here is to pass the values of the HashMap<\/em> as a parameter to the ArrayList<\/em> constructor<\/strong>:<\/p>\n

ArrayList<String> convertUsingConstructor(HashMap<Integer, String> hashMap) {\r\n    if (hashMap == null) {\r\n        return null;\r\n    }\r\n    return new ArrayList<String>(hashMap.values());\r\n}<\/code><\/pre>\n
As we can see, we started by checking if our HashMap<\/em> is null<\/em> to make our method null-safe. Then, we used the values()<\/em> method, which returns a collection view of the values contained in the given HashMap<\/em>.<\/p>\n
Now, let’s confirm this using a test case:<\/p>\n
public class HashMapToArrayListConverterUtilsUnitTest {\r\n    private HashMap<Integer, String> hashMap;\r\n    @Before\r\n    public void beforeEach() {\r\n        hashMap = new HashMap<>();\r\n        hashMap.put(1, "AAA");\r\n        hashMap.put(2, "BBB");\r\n        hashMap.put(3, "CCC");\r\n        hashMap.put(4, "DDD");\r\n    }\r\n    @Test\r\n    public void givenAHashMap_whenConvertUsingConstructor_thenReturnArrayList() {\r\n        ArrayList<String> myList = HashMapToArrayListConverterUtils.convertUsingConstructor(hashMap);\r\n        assertThat(hashMap.values(), containsInAnyOrder(myList.toArray()));\r\n    }\r\n    \/\/ ...\r\n}<\/code><\/pre>\n
As expected, the test passed with success.<\/p>\n
2.2. Using the addAll()<\/em> Method<\/h3>\n
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The addAll()<\/em><\/a> method is another great option to consider if we want to convert a HashMap<\/em> to an ArrayList<\/em>.<\/p>\n
As the name implies, this method allows to add all of the elements in the specified collection to the end of the list<\/strong>.<\/p>\n
Now, let’s exemplify the use of the addAll()<\/em> method:<\/p>\n
ArrayList<String> convertUsingAddAllMethod(HashMap<Integer, String> hashMap) {\r\n    if (hashMap == null) {\r\n        return null;\r\n    }\r\n    ArrayList<String> arrayList = new ArrayList<String>(hashMap.size());\r\n    arrayList.addAll(hashMap.values());\r\n    return arrayList;\r\n}\r\n<\/code><\/pre>\n
As shown above, we first initialized the ArrayList<\/em> with the same size as the passed HashMap<\/em>. Then, we called the addAll()<\/em> method to append all the values of the HashMap<\/em>.<\/p>\n
Again, let’s add a new test case to verify that everything works as expected:<\/p>\n
@Test\r\npublic void givenAHashMap_whenConvertUsingAddAllMethod_thenReturnArrayList() {\r\n    ArrayList<String> myList = HashMapToArrayListConverterUtils.convertUsingAddAllMethod(hashMap);\r\n    assertThat(hashMap.values(), containsInAnyOrder(myList.toArray()));\r\n}<\/code><\/pre>\n
Unsurprisingly, the test case passes with success.<\/p>\n
2.3. Using the Stream API<\/h3>\n
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Java 8 comes with a lot of new features and enhancements. Among these features, we find the Stream API<\/a>.<\/p>\n
So, let’s illustrate how to use the stream API to convert a HashMap<\/em> to an ArrayList<\/em> using a practical example:<\/p>\n
ArrayList<String> convertUsingStreamApi(HashMap<Integer, String> hashMap) {\r\n    if (hashMap == null) {\r\n        return null;\r\n    }\r\n    return hashMap.values()\r\n      .stream()\r\n      .collect(Collectors.toCollection(ArrayList::new));\r\n}<\/code><\/pre>\n
In a nutshell, we created a Stream<\/em> from the values of the given HashMap<\/em>. Then, we used the collect()<\/em> method with the Collectors<\/em><\/a> class to create a new ArrayList<\/em> holding the elements of our Stream<\/em><\/strong>.<\/p>\n
As always, let’s confirm our method using a new test case:<\/p>\n
@Test\r\npublic void givenAHashMap_whenConvertUsingStreamApi_thenReturnArrayList() {\r\n    ArrayList<String> myList = HashMapToArrayListConverterUtils.convertUsingStreamApi(hashMap);\r\n    assertThat(hashMap.values(), containsInAnyOrder(myList.toArray()));\r\n}<\/code><\/pre>\n
2.4. Using a Traditional Loop<\/h3>\n
<\/div>\n
Alternatively, we\u00a0can\u00a0use\u00a0a\u00a0traditional\u00a0for<\/em>\u00a0loop to achieve the same objective:<\/p>\n
ArrayList<String> convertUsingForLoop(HashMap<Integer, String> hashMap) {\r\n    if (hashMap == null) {\r\n        return null;\r\n    }\r\n    ArrayList<String> arrayList = new ArrayList<String>(hashMap.size());\r\n    for (Map.Entry<Integer, String> entry : hashMap.entrySet()) {\r\n        arrayList.add(entry.getValue());\r\n    }\r\n    return arrayList;\r\n}<\/code><\/pre>\n
As we can see, we iterated through the entries of the HashMap<\/em>. Furthermore, we used the entry.getValue()<\/em> on each Entry<\/em> to get its value. Then, we used the add()<\/em> method to add the returned value to the ArrayList<\/em>.<\/p>\n
Lastly, let’s test our method using another test case:<\/p>\n
@Test\r\npublic void givenAHashMap_whenConvertUsingForLoop_thenReturnArrayList() {\r\n    ArrayList<String> myList = HashMapToArrayListConverterUtils.convertUsingForLoop(hashMap);\r\n    assertThat(hashMap.values(), containsInAnyOrder(myList.toArray()));\r\n}<\/code><\/pre>\n
3. Using Guava<\/h2>\n
<\/div>\n
Guava<\/a> offers a rich set of utility classes that simplify common programming tasks such as collection manipulation.<\/p>\n
First, we need to add the Guava dependency<\/a> to the pom.xml<\/em> file:<\/p>\n
<dependency>\r\n    <groupId>com.google.guava<\/groupId>\r\n    <artifactId>guava<\/artifactId>\r\n    <version>31.0.1-jre<\/version>\r\n<\/dependency><\/code><\/pre>\n
Guava Library provides the Lists<\/em>\u00a0utility class<\/a> mainly to work with lists. So, let\u2019s see it in practice:<\/p>\n
public ArrayList<String> convertUsingGuava(HashMap<Integer, String> hashMap) {\r\n    if (hashMap == null) {\r\n        return null;\r\n    }\r\n    EntryTransformer<Integer, String, String> entryMapTransformer = (key, value) -> value;\r\n    return Lists.newArrayList(Maps.transformEntries(hashMap, entryMapTransformer)\r\n      .values());\r\n}<\/code><\/pre>\n
The Lists<\/em> class comes with the newArrayList()<\/em> method, which creates a new ArrayList<\/em> based on the given elements<\/strong>.<\/p>\n
As shown above, we used a custom EntryTransformer<\/em><\/a> to get the value from the key-value pair. Then, we passed our transformer to the Maps.transformEntries()<\/em> method alongside the HashMap<\/em>.<\/p>\n
That way, we tell Guava to create a new ArrayList<\/em> from the values of the HashMap<\/em><\/strong>.<\/p>\n
Finally, we’ll add a test case for our method:<\/p>\n
@Test\r\npublic void givenAHashMap_whenConvertUsingGuava_thenReturnArrayList() {\r\n    ArrayList<String> myList = HashMapToArrayListConverterUtils.convertUsingGuava(hashMap);\r\n    assertThat(hashMap.values(), containsInAnyOrder(myList.toArray()));\r\n}<\/code><\/pre>\n
4. Conclusion<\/h2>\n
<\/div>\n
In this article, we explored different ways to convert a HashMap<\/em> to an ArrayList<\/em> in Java.<\/p>\n
We looked at some ways to do this using the core Java. Then, we demonstrated how to use third-party libraries to accomplish the same thing.<\/p>\n
As always, the code used in this article can be found over on GitHub<\/a>.<\/p>\n
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<\/p>","protected":false},"excerpt":{"rendered":"
\"\"<\/p>\n
Explore several ways to convert a HashMap to an ArrayList in Java.<\/p>\n
<\/a>\u00a0<\/a>\u00a0<\/a>\u00a0<\/a>\u00a0<\/a>\u00a0<\/a>\u00a0<\/a>\u00a0<\/div>\n<\/div>","protected":false},"author":792,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"slim_seo":{"title":"Converting HashMap to an ArrayList in Java - ITTeacherITFreelance.hk","description":"Explore several ways to convert a HashMap to an ArrayList in Java. \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0"},"footnotes":""},"categories":[6,1307],"tags":[10674],"_links":{"self":[{"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/posts\/328969"}],"collection":[{"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/users\/792"}],"replies":[{"embeddable":true,"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/comments?post=328969"}],"version-history":[{"count":1,"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/posts\/328969\/revisions"}],"predecessor-version":[{"id":328970,"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/posts\/328969\/revisions\/328970"}],"wp:attachment":[{"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/media?parent=328969"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/categories?post=328969"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/itteacheritfreelance.hk\/wordpress\/index.php\/wp-json\/wp\/v2\/tags?post=328969"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}