In Java 9 and later, we can declare private methods in interfaces. These private methods have a body and are mainly used to implement other methods within the interface or to hide implementation details. This helps to reduce code duplication and encapsulate the implementation of the interface. However, I noticed that private methods in interfaces cannot be declared as final. Why is this the case? What is the reason behind this restriction?
I asked GPT for an answer to this question, but it didn’t give me a clear answer.
>Solution :
It’s not about interfaces. All private methods are inherently final!
This isn’t just an observation – the JVM acts accordingly and e.g. hardcodes (skipping dynamic dispatch) when JITting calls to private methods the same way it does for final ones.
A private method cannot be extended because it is invisible to everything outside of the class.
There is one academic case where you can extend a private method: private doesn’t quite mean ‘invisible to everything outside this class’. It more closely means ‘invisible to everything outside this source file’, and in theory you can squeeze more than one class in a single source file. However, at the JVM level it doesn’t work that way (if you squeeze more than 1 class into a source file, compiling that one .java file results in more than one .class file), and because of that, javac actually makes your method non-private, or makes non-private bridger synthetic methods instead. Allowing private methods to be final therefore makes no sense unless you know about that weirdness, which is probably not a good language feature.
javac fails to compile private final void foo() {} in an interface because it’s nonsense. javac generates a warning when you write: int x = (int) 5; for the same reason: That code is nonsense. It’s either stating the obvious to a ridiculous degree, or, more likely, it indicates the coder is confused about a concept. Same here: Marking a private method as final is either ridiculous (all private methods inherently always are, you’re stating the obvious), or indicative of misunderstanding. Or a typo.
Assuming programmers aren’t being ridiculous, it’s correct to error (or possibly it is more correct to warn but that is a separate discussion).