在Java中，在超类中创建的某些类的对象是否可以在子类中使用

Question

class A
{

    class B b;

    B b = new b();
}

class B extends A

{

    b.function();

}

Here can B use the same object created in A?

Answer 1

Following is your program:

class C { 
    public String cvariable; 
    public void cfunction(){ 
        System.out.println("string");   
    }
} 

class A { 
    public C c1; 
    public void funtiona(){ 
        c1 = new C(); 
    } 
} 

public class B extends A { 
    public void functionb(){ 
        c1.cfunction(); 
    } 
    public static void main(String args[]){ 
        B b = new B(); 
        b.functionb(); 
    } 
} 

It is correctly throwing null pointer exception. It proceed as follows:

• In the main method you call functionb()
• In functionb() you call cfunction() with c1, but c1 is just an variable of type C(as not initialized yet) which contains null. So getting null pointer exception.

Answer 2

See the following program, It will throw java.lang.StackOverflowError

class B{
    A a = new A();
    public B(){
        System.out.println(a.hashCode());
    }
}

public class A extends B{
    public void show(){
        a.hashCode();
    }   

    public static void main(String[] args){
        new A().show();
    }
}

This is because program goes in the infinite loop, As before creating a child class object it calls the parent class constructor and in parent class for hash code it again calls the child class constructor. so an infinite loop