java 面试题分析

题: 定义两个 Integer 对象传递给一个方法, 方法执行后两个 Integer 的值需要互相转换

public static void practise(MyConsumer<Integer> consumer) {
    Integer value1 = 1;
    Integer value2 = 2;
    System.out.println("start: value1=" + value1 + ", value2=" + value2);
    consumer.accept(value1, value2);
    System.out.println("end: value1=" + value1 + ", value2=" + value2);
}

@FunctionalInterface
public interface MyConsumer<T> {
    void accept (T value1, T value2);
}

编程方式一: 传值与传引用的区别

practise((value1, value2) -> {
    Integer temp = value1;
    value1 = value2;
    value2 = temp;
    return;
});

输入结果:
    start: value1=1, value2=2
    end: value1=1, value2=2

编程方式二: Integer 在 -127 ~ 128 之间的缓存

practise((value1, value2) -> {
    try {
        Field declaredField = Integer.class.getDeclaredField("value");
        declaredField.setAccessible(true);

        Integer intValue = value1.intValue();
        declaredField.set(value1, value2);
        declaredField.set(value2, intValue);
    } catch (Exception e) {}
    return;
});

输出结果: 
    start: value1=1, value2=2
    end: value1=2, value2=2

编程方式三: 清空缓存

practise((value1, value2) -> {
    try {
        Field declaredField = Integer.class.getDeclaredField("value");
        declaredField.setAccessible(true);

        Integer intValue = new Integer(value1.intValue());
        declaredField.set(value1, value2);
        declaredField.set(value2, intValue);
    } catch (Exception e) {}
    return;
});

输出结果: 
    start: value1=1, value2=2
    end: value1=2, value2=1