31 - Covariant Return - Java 1.6

They use several time a covariant return in the master exams... Be careful...

I find the explanations here interesting

Example :

1) Java 1.1 and after (this will compile) - It is NOT a covariant return !!!


package covariantreturn;

public class A {
    A method() {
        return this;
    }
}

package covariantreturn;

public class B extends A {

    A method() {

        return this;

    }

}

package covariantreturn;

public class C extends B {

      A method() {

        return this;

    }

}

2a) Java 1.5 and after (will not compile in Java < 1.5 !!!) - Covariant Return !

You return a subtype of A in B

You return a subtype of B in C (can be B) 

package covariantreturn;

public class A {

    A method() {

        return this;

    }

}

package covariantreturn;

public class B extends A {

    B method() {

        return this;

    }

}

package covariantreturn;

public class C extends B {

      B method() {

        return this;

    }

}

2b) 

You return a subtype of A in B

You return a subtype of B in C (can be C) 

package covariantreturn;

public class A {

    A method() {

        return this;

    }

}

package covariantreturn;

public class B extends A {

    B method() {

        return this;

    }

}

package covariantreturn;

public class C extends B {

      C method() {

        return this;

    }

}