Is there a way to listen to another class' method execution?

Is it possible to listen to a method execution of an instance or all instances of a class, without modifying their code ? Something like:

someInstance.addMethodExecutionListener('methodName', handler);
SomeClass.addMethodExecutionListener('methodName', handler);

It would be for logging purposes...

How to call a method in another class Java?

In Java, a method can be invoked from another class based on its access modifier. For example, a method created with a public modifier can be called from inside as well as outside of a class/package. The protected method can be invoked from another class using inheritance.

What you're asking about is a small subset of what you can do with Aspect Oriented Programming. It's not supported in plain Java and its implementations, but it's the central reason for AspectJ.