Accessing parent class static field using child class name doesn't load the child class?

Question

class A {
    static int super_var = 1;
    static {
        System.out.println("super");
    }
}

class B extends A {
    static int sub_var = 2;
    static {
        System.out.println("sub");
    }    
}
public class Demo{
    public static void main(String []args){
        System.out.println(B.super_var);
    }
}

outputs are :

super
1

this means that the child class not going to load or any other thing? how is it works?

I don't understand your question. What output did you expect? — Rohit Jain, Mar 22 '14 at 19:37
@RohitJain I suppose the static initialiser for `B` was expected to be called. — Boris the Spider, Mar 22 '14 at 19:38

score 12 · Accepted Answer · answered Mar 22 '14 at 19:47

When you access the static fields of a super class on subclass reference, only the class that declares the field will be loaded and initialized, in this case it is A. This is specified in JLS §12.4.1 - When Initialization Occurs:

A reference to a static field (§8.3.1.1) causes initialization of only the class or interface that actually declares it, even though it might be referred to through the name of a subclass, a subinterface, or a class that implements an interface.

Emphasis mine.

So in your code, class B would not even be initialized, and hence its static block would not be executed.

score 0 · Answer 2 · edited May 23 '17 at 10:31

Checkout the answer to this question: In what order do static initializer blocks in Java run?

The static block gets called only when a class is accessed (either creating an instance or accessing a member field or static method). However, you access a member of class A only, so there is no reason for class B to be initialized yet. The static initializer of B will be called as soon as you access a member from that class (either a field or static method, or create an instance from class B).

The reason is that class B doesn't need to be initialized until you access one of its members. Because A doesn't know about B (and cannot access it) there's really no reason for B to initialize at that stage.

You will find out that when you access B.sub_var the static initializer of B will be executed.

Accessing parent class static field using child class name doesn't load the child class?

2 Answers2

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