1. 代码
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public class A {
public static int num01 = B.num02;
public static A instance01_a = new A("A的对象"); // 打印输出二：执行构造函数 A
public static final int finalInt = (int) (Math.random()*100);
public static B instance02_b = new B("B的对象"); // 打印输出三：执行构造函数 B
public static final String finalStr = "finalStr";
public static final Integer finalInteger = new Integer(10);
public static int num02 = 1;
public static C instance03_c = null;
public A(String from){
System.out.println("------------begin A::A------------");
System.out.println("A::A, from = "+from);
System.out.println("A::A, A.num01 = "+A.num01);
System.out.println("A::A, B.num02 = "+B.num02);
System.out.println("A::A, B.instance_a = "+B.instance_a);
System.out.println("------------end A::A------------");
}
public static void main(String[] args) {
System.out.println("main, A.num01 = "+ A.num01);
System.out.println("main, B.num01 = "+B.num01);
System.out.println("main, C.instance = "+C.instance_a); // 打印输出四：构造函数 A
}
}
class B{
public static int num01 = A.num02;
public static A instance_a = new A("A的对象"); // 打印输出一：执行构造函数 A
public static int num02 = 1;
public B(String from){
System.out.println("------------start B::B------------");
System.out.println("B::B, from = "+from);
System.out.println("B::B, B.num02 = "+num02);
System.out.println("B::B, A.num02 = "+A.num02);
System.out.println("B::B, A.instance02_b = "+A.instance02_b);
System.out.println("B::B, A.instance01_a = " +A.instance01_a);
System.out.println("B::B, A.finalInt = "+A.finalInt);
System.out.println("B::B, A.finalStr = "+A.finalStr);
System.out.println("B::B, A.finalInteger = "+A.finalInteger);
System.out.println("------------end B::B------------");
}
}
class C{
public static final A instance_a = new A("A的对象");
}
2. 打印结果：
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------------begin A::A------------
A::A, from = A的对象
A::A, A.num01 = 0
A::A, B.num02 = 0
A::A, B.instance_a = null
------------end A::A------------
------------begin A::A------------
A::A, from = A的对象
A::A, A.num01 = 1
A::A, B.num02 = 1
A::A, B.instance_a = [email protected]
------------end A::A------------
------------start B::B------------
B::B, from = B的对象
B::B, B.num02 = 1
B::B, A.num02 = 0
B::B, A.instance02_b = null
B::B, A.instance01_a = [email protected]
B::B, A.finalInt = 97
B::B, A.finalStr = finalStr
B::B, A.finalInteger = null
------------end B::B------------
main, A.num01 = 1
main, B.num01 = 0
------------begin A::A------------
A::A, from = A的对象
A::A, A.num01 = 1
A::A, B.num02 = 1
A::A, B.instance_a = [email protected]
------------end A::A------------
main, C.instance = [email protected]
3.初始化过程分析：（只分析打印的第一个构造函数中）
（1）只有主动请求一个类时，该类才会初始化。初始化仅包括静态变量、函数等静态部分
（2）继承关系时先初始化父类，再初始化子类
（3）静态变量会按照声明顺序，依次声明并设置该类型的默认值，（此时不赋值为初始化的值）
（4）声明完毕后，再按照声明顺序依次设置为初始化的值，如果没有初始化则跳过
（5）当初始化到A.num01 = B.num02时，暂停初始化A.num01，设置当前类为B，跳转到步骤3并执行
（6）当初始化到 B.instance_a = new A("A的对象")时，暂停初始化B.instance_a, 实例化A并赋值给B.instance_a
（7）（接上）实例化A的过程中，需要B.num02, B.num02的当前值为默认值0。 【B.instance_a的值，但是后者还未初始化并处于暂停状态】； B.instance_a也是如此，当前值为null。