Covariant return type

Covariant return type Means

Hi friend

We have return type in our methods, but what is covariant return type?

Let’s have an example.

Suppose that you have a class hierarchy in which Object is a superclass of java.lang.Number, which is in turn a superclass of ImaginaryNumber.

The class hierarchy for ImaginaryNumber

Object -> java.lang.Number -> ImaginaryNumber

Now suppose that you have a method declared to return a Number:

public Number returnANumber()
{
    …
}

The returnANumber method can return an ImaginaryNumber but not an Object. ImaginaryNumber is a Number because it’s subclass of Number. However, an Object is not necessarily a Number — it could be a String or another type.You can override a method and define it to return a subclass of the original method, like this

public ImaginaryNumber returnANumber()
{
    …
}

Above means that we can return same or subclass type as the return type but not superclass type and this is called “covariant return type” .

Following is the .java file of code with explanation..

import java.lang.Number;
class ImaginaryNumber extends Number {@Override
    public int intValue()
    {
        throw new UnsupportedOperationException(“Not supported yet.”); //To change body of generated methods, choose Tools | Templates.
    }
    @Override
    public long longValue()
    {
        throw new UnsupportedOperationException(“Not supported yet.”); //To change body of generated methods, choose Tools | Templates.
    }
    @Override
    public float floatValue()
    {
        throw new UnsupportedOperationException(“Not supported yet.”); //To change body of generated methods, choose Tools | Templates.
    }
    @Override
    public double doubleValue()
    {
        throw new UnsupportedOperationException(“Not supported yet.”); //To change body of generated methods, choose Tools | Templates.
    }
}
class Myclass extends ImaginaryNumber
{
    // no error because we can return same as return type
    public ImaginaryNumber returnANumber()
    {
        ImaginaryNumber n1 = null;
        return n1;
    }
    // ERROR, we cannot return super class type as Number is the super class of ImaginaryNumber class
    // public ImaginaryNumber returnANumber1(){
    // Number n1= null;
    // return n1;
    //
    //}
    // ERROR, we cannot return super class type as Object is the super class of ImaginaryNumber class
    // public ImaginaryNumber returnANumber1(){
    // Object n1= null;
    // return n1;
    //
    //}
    // no error because we can return same as return type
    public Number returnANumber2()
    {
        Number n1 = null;
        return n1;
    }
    // no error because we can return subclass type as return type
    public Number returnANumber3()
    {
        ImaginaryNumber n1 = null;
        return n1;
    }
    // ERROR, we cannot return super class type as Object is the super class of Number class
    // public Number returnANumber4(){
    // Object n1=null;
    // return n1;
    //
    //}
    public static void main(String[] args) {}
}