Method overriding in Java. Why is this program not calling the sub class method?

Question

Why is this program not calling the sub class method? What is the concept behind it? I'm totally confused with this. Below is the code.

Super class

public class TV {

     public void checkType(TV b) {
          System.out.println("Its a TV");
     }
}

Child class

public class LedTv extends TV {

    public void checkType(LedTv b) {
         System.out.println("Its a LED TV");
    }

}

Test case to get the result

public class TestTV {

    public static void main(String argss[]) {
         TV a = new TV();
         TV b = new LedTv();
         a.checkType(a);
         b.checkType(b);

    }
}

Both of the checkType method prints

Its a TV
Its a TV

because it´s not overriden, it´s overloaded. You are passing a variable of the type `TV` to the method, hence it´s calling `checkType(TV b)`. — SomeJavaGuy, Nov 02 '15 at 10:43
But why is the second checkType() method printing **Its a TV** — BhajjiMaga, Nov 02 '15 at 10:49
Because the variable `b` is of type `TV`, not `LedTV` (even though it references to such an object, but that doesn't matter here). — Tom, Nov 02 '15 at 11:03

score 3 · Answer 1 · answered Nov 02 '15 at 10:45

Methods get overriden during inheritance when they have the same signature. According to the docs signature depends on:

the method name
the number of the arguments
the type of the arguments

In your case these methods

public void checkType(TV b)
public void checkType(LedTv b)

Clearly have different signatures as the type of the argument is different. What you get is called method overloading

score 3 · Accepted Answer · edited May 23 '17 at 12:10

public class LedTv extends TV {
    @Override //always put this here
    public void checkType() {//no need for argument
        System.out.println("Its a LED TV");
    }

}

As you can see from the above example I added an annotation @Override. While this isn't necessary it forces the compiler to check and make sure you are actually overriding something. If you are not you will get an error, which helps solve bugs. The reason I got rid of the argument that you are passing to check type is because it's redundant. you can always use the this keyword to point to the object you are currently inside of.

I realise I forgot to explain what overriding and overloading is so I think this example will show you.

public class Main{

  public static void main(String[] args){
     new Main();
  }

  Main(){
     new SuperClass().method();
     new SuperClass().method(101010);
     new Subclass().method();
     new Subclass().method(595959);
  }

  class SuperClass{

     SuperClass(){
        System.out.println(this.getClass().getSimpleName());
     }

     public void method(){
        System.out.println("method() from SuperClass -> Calling method(int x)");
        method(0);
     }
     public void method(int x){
        System.out.print("method() from SuperClass " + x + "\n\n");
     }
  }
  class Subclass extends SuperClass{
     @Override
     public void method(int x){
        System.out.print("method() from SubClass " + x + "\n\n");
     }
  }

}

Output

SuperClass
method() from SuperClass -> Calling method(int x)
method() from SuperClass 0

SuperClass
method() from SuperClass 101010

Subclass
method() from SuperClass -> Calling method(int x)
method() from SubClass 0

Subclass
method() from SubClass 595959

As you can see in SuperClass method() is overloaded with an int as method(int x) and method() is overridden in Subclass. In the output you can clearly see whats going on. What's the difference between overloading a method and overriding it in Java?

So you are telling the parameters has no role in which method needs to be called? — BhajjiMaga, Nov 02 '15 at 11:03
@BhajjiMaga He's saying that you don't pass in the object, since that is the context anyway (`this`). Look at your call `a.checkType(a)`, where you're giving `a` twice. It should be `a.checkType()` with code from this answer. — Andreas, Nov 02 '15 at 11:10
@BhajjiMaga There is no need. https://docs.oracle.com/javase/tutorial/java/javaOO/thiskey.html "this" keyword points to itself. maybe you can find other resources on it. I added an example to show you how overloading and overriding works. — WalterM, Nov 02 '15 at 11:36

score 0 · Answer 3 · answered Nov 02 '15 at 10:43

0

overriding method must have the same types for arguments. If you want to override method in subclass it must have the same arguments only return type can be different but from the same hierarchy as in superclass method

answered Nov 02 '15 at 10:43

burovmarley

644
3
8

score 0 · Answer 4 · answered Nov 02 '15 at 10:44

void checkType(TV) is a different function prototype to void checkType(LedTv).

Therefore you are not overriding, but rather you are overloading.

For overriding, the function name and parameter types must be identical, and the return type related. The exact definition of relatedness is beyond the scope of this question.

score 0 · Answer 5 · answered Nov 02 '15 at 11:01

You are doing method overloading. For method overriding both the method signature of the parent and child class should be same. So in the child class instead of this

public void checkType(LedTv b) {
     System.out.println("Its a LED TV");
}

type this and check -

public void checkType(TV b) {
     System.out.println("Its a LED TV");
}

The Law · Answer 6 · 2015-11-02T10:56:08.587

Because both of your objects are of type TV.

TV a = new TV();
TV b = new LedTV();

is basically the same object base type. In case of creating b, all you do is basically telling you want superclass instance with the sub-class functionality as well. And you are overloading the methods, not overriding them. You tell the program if parameter is LedTV, print out this, if parameter is TV, print out this. You are outsourcing the if-else logic to the program itself, simply. What you do is

if(TV == TV) print TV
else if (TV == LedTV) print LedTV

But the problem is both objects are of type TV in your code, so the superclass method is always used and the first if is true, and second one is not even evaluated.

Change b to this:

LedTv b = new LedTv();
b.checkType(b);

It will now work correctly, because b is of type LedTV..

Method overriding in Java. Why is this program not calling the sub class method?

6 Answers6