java - 交换变量时奇怪的 java for-each 行为

Question

通过以下代码，我期望四个元素，但只得到三个。显然我正在做一些疯狂愚蠢的事情。

import java.util.HashSet;

import java.util.Set;

public class MyTest {
    public static void main(String[] args) {
        String[] myStrings = new String[] { "a", "b" };

        Set<String> set = new HashSet<String>();
        for (String x : myStrings) {
            for (String y : myStrings) {
                String temp = x;
                x = y;
                y = temp;

                set.add(x + y);
            }
        }

        System.out.println(set);
    }
}

结果:

[ba, aa, ab]

Answer 1

集合不允许重复，并且您要添加“ba”两次。如果您的循环控制令人困惑，但这是简化的跟踪，并扩展了右值。

x = "a"

y = "a"
temp = "a"
x = "a"
y = "a"
add("aa")

y = "b"
temp = "a"
x = "b"
y = "a"
add("ba")

x = "b"

y = "a"
temp = "b"
x = "a"
y = "b"
add("ab")

y = "b"
temp = "a"
x = "b"
y = "a"
add("ba")