Java编译器如何检查接口中的所有方法是否都已实现？

Question

If I declare that my object implements an interface, but fail to implement those methods, when I compile my code, I get a compiler error. How does the java compiler know that I haven't implemented all of the methods of an interface?

Answer 1

How does the java compiler know that I haven't implemented all of the methods of an interface?

It knows all methods that your class has implemented because it has found and analyzed them during the compilation.

It knows all of the methods that were defined in all of the superclasses and interfaces of your class because:

1. it has either just compiled their source code, or loaded their ".class" files, and

2. it has analysed the interfaces / classes and figured out which methods need to be implemented by your class.

Then it compares the two sets of methods. (Note that the methods don't need to be exactly the same. For example, the actual method could return a subtype of the return type of the method in the interface. The set comparison needs to take account of that.)

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(Actually, that is just one approach to doing this check. An actual Java compiler might do the checking a bit differently.)