Return subclass from generic abstract class java

Question

So, I have this abstract class that represents an Input type object -

public abstract class Input<E>

And I have two classes that extends it, one is ButtonInput and the other is TextInput. Both extend Input so it doesn't really matter. I'll use TextInput just to explain.
This is the TextInput class defention:

public class TextInput extends Input<TextInput>

What I'm trying to do is to return this as E (TextInput in this case)-

public E setTextColor(Color color) { this.colorText = color; return (E)this; }

So, If I call for example:

new TextInput().setColor(Color.black)

It should return TextInput. It is working but, it shows the following warning -

warning: [unchecked] unchecked cast
return (E) this; required: E
found: Input
where E is a type-variable:
E extends Object declared in class Input

In relation to the following line of code -

 return (E) this;

Does anybody knows how to solve it?
Thanks

You don't need <E extends Input> for the method, E is already parametrised. — Oliver Charlesworth
– Oliver Charlesworth, Commented Jun 8, 2014 at 8:09
@OliCharlesworth I've removed what you've said and now when I'm compiling lint gives me a warning of unchecked cast. And ButtonInput is just another class the represents a graphic button — Yehonatan
– Yehonatan, Commented Jun 8, 2014 at 8:12
But ButtonInput isn't in the code you've shown us. Please construct a complete test-case. — Oliver Charlesworth
– Oliver Charlesworth, Commented Jun 8, 2014 at 8:13
shouldn't you return "Input" instead of "E" ? I suggest you to watch this video: sites.google.com/site/io/effective-java-reloaded , presentation here: 14b1424d-a-62cb3a1a-s-sites.googlegroups.com/site/io/… — android developer
– android developer, Commented Jun 8, 2014 at 8:20

Robby Cornelissen · Accepted Answer · 2014-06-08 09:02:53Z

This is a pattern I've used in the past to solve this type of issue in abstract builder patterns. It relies on an abstract me() method that is overridden to return a reference to the concrete object.

abstract class Foo<T> { protected abstract T me(); public T baz() { return me(); } } class Bar extends Foo<Bar> { protected Bar me() { return this; } }

Solved my problem. It's a bit tricky but working just fine and removes the warnings . Thanks.

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