Difference in java generics

Question

please , I want to know the difference between writing

public class Something<T extends Comparable<T>> {// }

and

public class Something<T extends Comparable> {// }

and how would that affect the code

Answer 1

The difference is that in the first case the type parameter T must be comparable to itself whereas in the second case T can be comparable to anything. Generally, when a class C is made comparable it is declared to implement Comparable<C> anyway. Nevertheless, here's an example of when the first wouldn't work but the second would:

class C1<T extends Comparable<T>> {  // first case
}

class C2<T extends Comparable> {  // second case
}

class A {  // some super class
}

class B extends A implements Comparable<A> {  // comparable to super class
    @Override
    public int compareTo(A o) {
        return 0;
    }
}

Now:

new C1<B>();  // error
new C2<B>();  // works

In general, you should never use the second approach; try to stay away from raw types whenever possible. Also note that an even better option for the second approach would be

public class Something<T extends Comparable<? super T>> { /*...*/ }

Using this with C1 would allow the new C1<B>() line above to compile as well.