为什么像 List/Map/Set.of(...) 或 Arrays.asList(...) return 这样的方法是一个不可变列表？

Question

返回不可变列表而不是可变列表背后的原因是什么？

Answer 1

性能

For optimal performance, the immutable collections store a data set that never changes. However, you may be able to take advantage of the performance and space-saving benefits even if your data is subject to change. These collections may provide better performance than the mutable collections, even if your data changes occasionally.

List#of are static factory methods which provide a convenient way to create immutable lists. In other words, it's a convenience method to create immutable lists. Prior to Java-9, this was possible through separate APIs like Collections#unmodifiableList.

如果你想得到一个可变列表，你可以用这个不可变列表作为参数实例化一个ArrayList。

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> mutableList = new ArrayList<>(List.of("A", "B", "C"));
        mutableList.add("D");
        System.out.println(mutableList);
    }
}

输出：

[A, B, C, D]

为什么像 List/Map/Set.of(...) 或 Arrays.asList(...) return 这样的方法是一个不可变列表？

Why do methods like List/Map/Set.of(...) or Arrays.asList(...) return an immutable list?

java

list

java-9

性能