如何交换 Integer 的值？

示例1

import sun.misc.Unsafe;

import java.lang.reflect.Field;

public class SwapExample {

    private static Unsafe unsafe;

    static {
        try {
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            unsafe = (Unsafe) field.get(null);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        Integer a = new Integer(3);
        Integer b = new Integer(4);
        System.out.println("a=" + a + ",&a=" + addressOf(a));
        System.out.println("b=" + b + ",&b=" + addressOf(b));
        swap(a, b);
        System.out.println("a=" + a + ",&a=" + addressOf(a));
        System.out.println("b=" + b + ",&b=" + addressOf(b));
    }

    private static void swap(Integer a, Integer b) {
        Integer tmp = a;
        a = b;
        b = tmp;
        System.out.println("[a=" + a + ",&a=" + addressOf(a));
        System.out.println("[b=" + b + ",&b=" + addressOf(b));
    }


    public static long addressOf(Object o) {

        Object[] array = new Object[]{o};

        long baseOffset = unsafe.arrayBaseOffset(Object[].class);
        int addressSize = unsafe.addressSize();
        long objectAddress;
        switch (addressSize) {
            case 4:
                objectAddress = unsafe.getInt(array, baseOffset);
                break;
            case 8:
                objectAddress = unsafe.getLong(array, baseOffset);
                break;
            default:
                throw new Error("unsupported address size: " + addressSize);
        }
        return (objectAddress);
    }

}

结果：

a=3,&a=3983889842
b=4,&b=3983889844
[a=4,&a=3983889844
[b=3,&b=3983889842
a=3,&a=3983889842
b=4,&b=3983889844

swap 方法中的地址对应的值是没有变化的，只是形参的地址交换了。但是实参的地址没有交换，这个类似值传递。

如果我们在swap中试图改变&a的值看看是否成功

很明显没有修改成功，只是新建了一个对象，这个类似 String

如果一定要修改&a地址中的值呢？

反射：

于是交换的值的代码：

WHY?

Integer tmp = a; 这一步是tmp和a指向同一个地址，我们改了这个地址的值，所以tmp的值也变了。