Java 如何将Map转为数组、List、Set的方法

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.

2. Map Values to Array

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

@Test
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.

3. Map Values to List

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

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

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

And using Guava:

@Test
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:

@Test
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.