Java generics: Using the type of class an interface is applied to?

Question

I have an interface, whose method could conveniently return the instance on which the method is called. For instance, a setter could return "this".

interface OriginWise<T> {
      T setOrigin( Origin origin );
}

Is there some way to get an instance of T automatically from the class it's applied to, without passing it as shown below?

class MyBean implements OriginWise<MyBean> {
    MyBean setOrigin( Origin o ){ ...; return this; }
}

Also, what's the progress with default implementations in interfaces (methods which only use other interface's methods; not the same as abstract class)?

What do you mean "passing it in as shown below" ? The setter value is being passed into a method. The returnn object is "this". I don't grok. — NimChimpsky, May 31 '13 at 14:49
I don't really understand the second part of your question ("Also, what's the progress..."), please, reformulate it. — thSoft, May 31 '13 at 14:59
possible duplicate of [Is there a way to refer to the current type with a type variable?](http://stackoverflow.com/questions/7354740/is-there-a-way-to-refer-to-the-current-type-with-a-type-variable) — Paul Bellora, May 31 '13 at 15:13

score 1 · Answer 1 · answered May 31 '13 at 14:47

1

default implementations in interfaces

in jdk 8, which you can download and try out.

I don't understand the rest of the question, as the code looks good to me.

answered May 31 '13 at 14:47

NimChimpsky

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score 0 · Answer 2 · edited May 23 '17 at 12:11

0

That's the best you can do. If you want to return this (which implements OriginWise), don't forget to let T extend OriginWise. See also this answer and http://passion.forco.de/content/emulating-self-types-using-java-generics-simplify-fluent-api-implementation.

edited May 23 '17 at 12:11

Community

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answered May 31 '13 at 14:58

thSoft

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its not a good point; the interface is implented by Mybean. Nothing is supposed to extend originwise. – NimChimpsky May 31 '13 at 15:07
@NimChimpsky They're implying that `T` should be a "self type", e.g. `T extends OriginWise`. – Paul Bellora May 31 '13 at 15:09
@PaulBellora I know. But that would completely change the functionality. – NimChimpsky May 31 '13 at 15:10
@NimChimpsky Would it? That seems to be what the OP's trying to do, for better or worse. – Paul Bellora May 31 '13 at 15:11
@PaulBellora yeah maybe actually +1 – NimChimpsky May 31 '13 at 15:11
`T extends OriginWise` adds no type safety to what's already there – newacct May 31 '13 at 20:19

Java generics: Using the type of class an interface is applied to?

2 Answers2