Starting in JDK 9 build 54, compiler support for private interface methods have been resurrected. This was one feature that was planned for Java 8 and did not make the final cut. It is now part of JEP 213. The primary motivation for this proposal is to enable code sharing between non abstract methods in an interface.

Interface evolution was always a tricky problem to solve in earlier versions of Java. You either need to maintain multiple versions of APIs which creates a technical debt over time or brute force your clients to accommodate new methods which breaks binary compatibility. The need to enhance an interface needed support in interface themselves instead of resorting to other options. This was fulfilled by default methods in interfaces in Java 8. Taking this further, private methods come in handy when we have a reusable block of code that default methods could benefit from. Latest Intellij 14.1 builds started to support JDK 9, so lets code an interface!.

An interface now can contain these:

– constants
– method signatures
– nested types
– default methods (since 1.8)
– static methods (since 1.8)
– private methods (since 1.9)
– private static methods (since 1.9)

Some of the restrictions of private methods include:
– private methods must contain method body
– combination of modifiers not allowed : abstract and private
– private methods in an interface does not disqualify it from being a @FunctionalInterface
– annotations types cannot declare private methods
– name clash not permitted

– cannot reduce visibility of private methods

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