具有不同返回类型的多态中的覆盖函数

Question

Take a look into following code and help me understanding why the return type of "mf.display" is Object Type. Though "mf" is of "MyFather" type but still return type of "mf.display()" must be Integer type

class MyFather
{
    Object display()
    {
        System.out.println(1000);
        return 1000;
     }
}


class MySon extends MyFather
{
    @Override
    Integer display()
    {
        System.out.println(500);
        return 500;
    }
}


public class TestInheritance {

    public static void main(String[] args) {
        MyFather mf = new MySon();
        Integer myInt = mf.display();  // Error.Type mismatch cannot convert from Object to  
                                       //   Integer
    }
}

Answer 1

The type myFather declares a method Object display() , so when you write

mf.display()

the type of that expression is Object . It doesn't matter what type of object you assigned to mf . This makes sense because mf could also have been a method parameter:

static void useFather(myFather mf) {
   mf.display();
}

and you could call such a method with any instance, such as

useFather(new myFather());

The code must not be allowed to assume it has an instance of the subclass.

Answer 2

For clarity of discussion I should state that , mySon class (which should be named MySon) is overriding the display() of myFather class (Again should be named MyFather) because as per the Java norms , for overriding a method

The return type must be the same as, or a subtype of, the return type declared
in the original overridden method in the superclass

Now having stated that lets continue on our discussion ,

The only way to access an object is through a reference variable, and there are a few key things to remember about references.

1. A reference variable's type determines the methods that can be invoked on the object the variable is referencing.( This one is important for you to understand )
2. A reference variable can be of only one type, and once declared, that type can never be changed (although the object it references can change).
3. A reference is a variable, so it can be reassigned to other objects, (unless the reference is declared final ).
4. A reference variable can refer to any object of the same type as the declared reference, or—this is the big one—it can refer to any subtype of the declared type! ( This is the thing which you have done by writing myFather mf = new mySon(); )
5. A reference variable can be declared as a class type or an interface type. If the variable is declared as an interface type, it can reference any object of any class that implements the interface.

I think you should refer some standard Java book on this subject if you want to learn more about overriding and overloading.

Cheers!