证明加载类的动作仅发生一次

public class Ex23 extends B{
    //3 (i,j,k ==1
    static int k = printInit("Ex23 k initializer");
    Ex23(){
        System.out.println("Ex23 constructor");
    }
    public static void main(String args[]){
        //4
        System.out.println("hi");
        //
        System.out.println(new A());
        System.out.println(C.a);
        D d = new D();
    }
}

class D{
    D(){
        //10
        System.out.println("D constructor");
    }
}
class C{
    //7 (k == 1
    static int k = printInit("k initializer");
    static A a = new A();
    static int printInit(String s){
        System.out.println(s);
        return 1;
    }
}
class B extends A{
    //2
    static int j = printInit("j initializer");
    B(){
        System.out.println("B constructor");
    }
}

class A{
    //1
    static int i = printInit("i initializer");
    static int printInit(String s){
        System.out.println(s);
        return 1;
    }
    A(){
        //5
        //8
        System.out.println("A constructor()");
    }

    @Override
    //6
    //9
    public String toString(){
        return "A toString";
    }
}

控制台输出顺序为

i initializer
j initializer
Ex23 k initializer
hi
A constructor()
A toString
k initializer
A constructor()
A toString
D constructor

由此可见，类的加载仅在该类的第一个实例的创建或者对static成员的访问时才发生。