04.接口初始化规则与类加载器准备阶段和初始化阶段的意义

接口初始化规则

当一个接口在初始化时，并不要求其父接口都完成了初始化

只有在真正使用父接口的时候 (如引用接口中所定义的常量时)，才会初始化

public class MyTest5 {
    public static void main(String[] args) {
        System.out.println(MyChild5.b);
    }
}

interface MyParent5 {
    public static int a = 5;
}

interface MyChild5 extends MyParent5 {
    public static int b = 6;
}

类加载器准备阶段

准备阶段会给

1. counter1 赋初值 0
2. singleton 赋初值 null
3. counter2 赋初值 0

构造方法不会执行

public class MyTest6 {

    public static void main(String[] args) {
        Singleton singleton = Singleton.getInstance();

        System.out.println("counter1: " + Singleton.counter1);
        System.out.println("counter2: " + Singleton.counter2);
    }
}

class Singleton {
    public static int counter1 = 1;

    private static Singleton singleton = new Singleton();

    private Singleton() {
        counter1++;
        counter2++;
        System.out.println(counter1);
        System.out.println(counter2);
    }

    public static int counter2 = 0;

    public static Singleton getInstance() {
        return singleton;
    }
}

输出为

2
1
counter1: 2
counter2: 0

类加载器初始化阶段

初始化阶段开始进行真正赋值

1. counter1 显式的赋值为 1
2. singleton 的值会导致构造方法的执行，导致 counter1++ 变为 2，counter2++ 变为 1
3. counter2 = 0，会导致 counter2 的值变为 0