Question

Java is object oriented programming language.Inheritance is one of most important features.
We use encapsulation to hide object states.In the below program i should have used private access modifier for age and counter but to test this inheritance i have used public.

1. Why inheritance does not work in case of direct object state access
   (primitive types or object reference). SOP 3 output is different from expected.
2. Since compiler does not give any error in line SOP 13 and SOP 14. Why Father class details are printed not child class.

.

public class Father {

    public int age = 50;
    /*
     * Counter keeps track of total no of instances created so far.
     */
    public static int counter = 0;

    public Father(){
        super();
        synchronized (Father.class) {
            ++Father.counter;   
        }
    }

    public int getAge(){
        return this.age;
    }

    public static int getStaticCount(){
        return Father.counter;
    }

}

public class Child extends Father {

    public int age = 25;
    public static int counter = 0;

    public Child(){
        super();
        synchronized (Child.class) {
            ++Child.counter;    
        }
    }

    public int getAge(){
        return this.age;
    }

    public static int getStaticCount(){
        return Child.counter;
    }

    public static void main(String args[]){

        Father father = new Father();
        Father child = new Child();
        Child realChild = new Child();

        System.out.println("Expecting Father Class details to be printed");
        System.out.println("SOP 1 : Father Age : "+father.age);     //prints 50 as expected.
        System.out.println("SOP 2 : Father Age : "+father.getAge());//prints 50 as expected.

        System.out.println("Expecting Child Class details to be printed");
        /*
         * Why inheritance does not work in case of direct integer access.
         */
        System.out.println("SOP 3 : Child Age : "+child.age); //prints 50 ?? , Father Age . Why ?
        System.out.println("SOP 4 : Child Age : "+child.getAge());//prints 25 as expected.

        System.out.println("Expecting Child Class details to be printed");
        System.out.println("SOP 5 : Child Age : "+realChild.age); //prints 25 as expected.
        System.out.println("SOP 6 : Child Age : "+realChild.getAge());//prints 25 as expected.

        /*
         *Total No of static Count : proper way of accessing static field using Class Name. 
         */
        System.out.println("SOP 7 : Father Instance Count : Using Class Reference :"+Father.counter);
        System.out.println("SOP 8 : Father Instance Count : Using Class Reference :"+Father.getStaticCount());

        /*
         * Incorrect Way to use static. Since Compiler allows this lets see output.
         */

        System.out.println("SOP 9 : Father Instance Count : Using Object Reference :"+father.counter); //prints 3 as expected.
        System.out.println("SOP 10 : Father Instance Count : Using Object Reference :"+father.getStaticCount());//prints 3 as expected.

        /*
         *Total No of static Count : proper way of accessing static field using Class Name.  
         */
        System.out.println("SOP 11 : Child Instance Count : Using Class Reference :"+Child.counter); // output is 2 as expected
        System.out.println("SOP 12 : Child Instance Count : Using Class Reference :"+Child.getStaticCount()); // output is 2 as expected

        /*
         * Incorrect Way to use static.Since Compiler allows this lets see output.
         * This invokes function of parent class. Why ? Inheritance does not work for static fields.
         */
        System.out.println("SOP 13 : child Instance Count : Using Object Reference :"+child.counter); // output is 3 but expected is 2 .          why ? 
        System.out.println("SOP 14 : child Instance Count : Using Object Reference :"+child.getStaticCount()); // output is 3 but expected is 2 .  why ?

        /*
         * Incorrect Way to use static.Since Compiler allows this lets see output.
         * This invokes function of parent class. Why ?
         */
        System.out.println("SOP 15 : child Instance Count : Using Object Reference :"+realChild.counter); // output is 2 as expected
        System.out.println("SOP 16 : child Instance Count : Using Object Reference :"+realChild.getStaticCount()); // output is 2 as expected
    }

}

My question is why inheritance only works for instance methods. Why output is different for SOP 3, SOP 13 and SOP 14.

Answer 1

The reason is that polymorphism does not apply to instance variables. What you are doing is variable shadowing.

In SOP 3 and 4 the variable with the same name in the child class shadows the variable in the base class. And since this is resolved at compile time the value of the static type is chosen.

In SOP 13 and SOP 14 this happens for the same reason. At the scope of the method the child’s class hiding variable is unknown.

From JSL:

The scope of a declaration of a member m declared in or inherited by a
class type C (§8.1.6) is the entire body of C, including any nested
type declarations.