why not NullPointerException

[pooja jain]

class A {
    void m1() {
        System.out.println("Called");
    }
}
 
public class B extends A {
    A a = null;
    void m2() {
        a = new A();
    }
 
    void m3() {
        a.m1();  
    }
 
    public static void main(String[] s) {
         B b = new B();
         b.m2();
         b.m3();
    }    
}
 
output : Called
 

My doubt is , why it is not throwing null pointer exception . the object we have asigned to a reference got create in a method ( m2 ) so after terminating this method , that object should also get end ( eligible for GC ) and a reference should again point to null & then NullPointerException .

please help .

Thanks .

[Jay Pawar]

The object a is a member of class B and not local reference of class A.

[Jeff Bosch]

If you want to get rid of the object referenced by a, then you need to set a = null. Now the object is freed for garbage collection. Had you declared a in addition to defining it within method m3, then it would have local scope and would die at the end of the method execution.

[pooja jain]

Originally posted by Jay Pawar:
The object a is a member of class B and not local reference of class A.

a is not object . a is pointing to an object & that object I am creating in a method scope . so after method , how it can be live ?

[Jeff Jetton]

Originally posted by pooja jain:

a is not object . a is pointing to an object & that object I am creating in a method scope . so after method , how it can be live ?

Yes, technically, a is not the object. But it is a reference to an object. That reference is declared outside the method.

So even though you've created an object inside the method, you've assigned it to a reference that was created outside the method. As long as that reference is in scope, the object to which it points will not be eligible for garbage collection.

- Jeff

[Jay Pawar]

Oops my mistake you are right a is not object it is reference.Need to get my mind out of C++ world. and the answer to your question is well explained by Jeff.

[Mike Zhong]

should do it like this:

void m2() {
A a = new A();
}