I agree -- I was just trying to guess what OP might have meant by "monkey-patching" when talking about Go (which is TTBOMK statically typed, which precludes what I'd personally call "monkey-patching" at the language level).
Java does have "agents", which enable arbitrary control of bytecode at class loading time -- bytecode in existing .class files can be transformed, or completely new bytecode generated on-the-fly. But generating bytecode just to replace a class with a test double would be insane.
From another comment I learned there's also a separate "Tool Interface" that can do this bytecode-replacement job, but again, it's a huge amount of tricksiness and bother for something more easily and clearly accomplished in source code with a boring old anonymous subclass. (Though it won't work if the class is sealed, or final, or your code uses reflection to check the class name, or...)
Java does have "agents", which enable arbitrary control of bytecode at class loading time -- bytecode in existing .class files can be transformed, or completely new bytecode generated on-the-fly. But generating bytecode just to replace a class with a test double would be insane.
From another comment I learned there's also a separate "Tool Interface" that can do this bytecode-replacement job, but again, it's a huge amount of tricksiness and bother for something more easily and clearly accomplished in source code with a boring old anonymous subclass. (Though it won't work if the class is sealed, or final, or your code uses reflection to check the class name, or...)