qna

--- Java Stack Memory vs Heap Memory ---

Stack Memory

• Holds the values of the primitive type variables. (int, float, double, char, boolean, etc.),
• Reference variables are stored in the Stack.

Heap Memory

• Stores the non-primitive objects,
• String Constant Pool also resides in the Heap Memory.

Image Source: Scalar

---

--- Find output of this program ---

class A{

    public void m1(){
        System.out.println("A.m1()");
    }

    public void m2(){ // <---------------------------
        System.out.println("A.m2()");
    }
}

class B extends A{

    public void m2(){
        System.out.println("B.m2()");
    }
}

class OverridingTest{
    public static void main(String[] args) {
        A a = new B();
        a.m1(); // A.m1()
        a.m2(); // B.m2() // <--------------
    }
}

• If class-A doesn't contain method m2(), then compile time error. But if class-A contains method m2(), then m2() of class-B will be called. Because variable a is referring to object of class-B,
• But why:
• The error occurred because of Java's static type checking,
• The class of the reference variable determines which instance methods can be called on it and since class-A doesn't have method m2(), the compiler will complain,
• But when we are adding m2() in class-A, compiler error will be gone. Now in runtime it will call m2() of class-B since variable a is referring to an object of class-B due to polymorphism rule.