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Asked: 2026-06-11T02:58:05+00:00

I am trying to change an interface in Proj2. However, Proj1 is already using

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I am trying to change an interface in Proj2.
However, Proj1 is already using this interface, and won’t be compiled with my new version.

Looks like problems occur only if I change/delete existing methods but adding new methods seems to be not causing any issues.

So as long as I only “add” methods to the interface, I can expect Proj1 to be fine with the latest Jar of Proj2.

Is that a rock solid safe assumption?

Update:
Both the interface and impl reside in proj2. Proj1 has no implementation of that that interface.

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1 Answer

  1. Editorial Team

    By “using,” you mean “references an object using the interface and invokes interface-defined methods using that reference,” not “implements the interface” (yes, you’ve clarified in comments, but I just want to repeat, because all the other answers are about implementing the interface).

    In that case, as long as you have not removed methods from the interface or change their signature, your existing code will work.

    The reason that this will work is that the compiler translates the method calls into an invokeinterface bytecode, which references the interface class and method by name. At runtime, the JVM simply validates that the actual reference implements the interface, and invokes the method.

    If you remove or change a referenced interface method, one of two things will happen: either the JVM will refuse to load the class in proj1, because the interface reference could not be resolved, or it will give a NoSuchMethodError (I believe it’s the second). Same thing if you remove or change a referenced method in a concrete class.

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