skaffman skaffman - 4 months ago 6
Java Question

Why no static methods in Interfaces, but static fields and inner classes OK? [pre-Java8]

There have been a few questions asked here about why you can't define static methods within interfaces, but none of them address a basic inconsistency: why can you define static fields and static inner types within an interface, but not static methods?

Static inner types perhaps aren't a fair comparison, since that's just syntactic sugar that generates a new class, but why fields but not methods?

An argument against static methods within interfaces is that it breaks the virtual table resolution strategy used by the JVM, but shouldn't that apply equally to static fields, i.e. the compiler can just inline it?

Consistency is what I desire, and Java should have either supported no statics of any form within an interface, or it should be consistent and allow them.


An official proposal has been made to allow static methods in interfaces in Java 7. This proposal is being made under Project Coin.

My personal opinion is that it's a great idea. There is no technical difficulty in implementation, and it's a very logical, reasonable thing to do. There are several proposals in Project Coin that I hope will never become part of the Java language, but this is one that could clean up a lot of APIs. For example, the Collections class has static methods for manipulating any List implementation; those could be included in the List interface.

Update: In the Java Posse Podcast #234, Joe D'arcy mentioned the proposal briefly, saying that it was "complex" and probably would not make it in under Project Coin.

Update: While they didn't make it into Project Coin for Java 7, Java 8 does support static functions in interfaces.