How to restrict types which are allowed as "T" in a generic class?

Question

I've got a class Foo<T>. How can I say that I want T to be some class implementing BarInterface? Writing simply class Foo<T implements BarInterface> doesn't compile.

Konrad Rudolph · Accepted Answer · 2010-07-20 08:07:47Z

11

Use extends instead of implements.

answered Jul 20, 2010 at 8:07

Konrad Rudolph

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4 Comments

Andreas Dolk Over a year ago

... if I had a compiler at hand I could test it right away: shouldn't it be class Foo<? extends BarInterface> - extends is a WildcardBound ...

helpermethod Over a year ago

+1 One note to add: Extends in the context of Generics has both the meaning of 'extends' AND 'implements', which I found quite confusing in the beginning.

Konrad Rudolph Over a year ago

@Andreas: no, it doesn’t work only on wildcards, it also works on template arguments.

Andreas Dolk Over a year ago

thanks for clarification. I was really unsure, my first intention was that it <T extends Something> is correct but I was quite irritated after reading this one chapter of the language spec.

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