Convert a Map to an Array, List or Set in Java – 在Java中把地图转换成数组、列表或集合

最后修改: 2014年 6月 2日


1. Overview


This short article will show how to convert the values of a Map to an Array, a List or a Set using plain Java as well as a quick Guava based example.


This article is part of the “Java – Back to Basic” series here on Baeldung.

本文是Baeldung网站上“Java – Back to Basic “系列的一部分。

2. Map Values to Array


First, let’s look at converting the values of the Map into an array, using plain java:


public void givenUsingCoreJava_whenMapValuesConvertedToArray_thenCorrect() {
    Map<Integer, String> sourceMap = createMap();

    Collection<String> values = sourceMap.values();
    String[] targetArray = values.toArray(new String[0]);

Note, that toArray(new T[0]) is the preferred way to use the method over the toArray(new T[size]). As Aleksey Shipilëv proves in his blog post, it seems faster, safer, and cleaner.

注意,toArray(new T[0])是比toArray(new T[size])更好的使用方法。正如Aleksey Shipilëv在他的博客文章中所证明的那样,它似乎更快、更安全、更干净。

3. Map Values to List


Next, let’s convert the values of a Map to a List – using plain Java:


public void givenUsingCoreJava_whenMapValuesConvertedToList_thenCorrect() {
    Map<Integer, String> sourceMap = createMap();

    List<String> targetList = new ArrayList<>(sourceMap.values());

And using Guava:


public void givenUsingGuava_whenMapValuesConvertedToList_thenCorrect() {
    Map<Integer, String> sourceMap = createMap();

    List<String> targetList = Lists.newArrayList(sourceMap.values());

4. Map Values to Set


Finally, let’s convert the values of the Map to a Set, using plain java:


public void givenUsingCoreJava_whenMapValuesConvertedToS_thenCorrect() {
    Map<Integer, String> sourceMap = createMap();

    Set<String> targetSet = new HashSet<>(sourceMap.values());

5. Conclusion


As you can see, all conversions can be done with a single line, using only the Java standard collections library.


The implementation of all these examples and code snippets can be found over on GitHub project – this is a Maven-based project, so it should be easy to import and run as it is.

所有这些例子和代码片段的实现都可以在GitHub项目上找到 – 这是一个基于Maven的项目,所以应该很容易导入并按原样运行。