final keyword in method parameters

Java is only pass-by-value. (or better - pass-reference-by-value)

So the passed argument and the argument within the method are two different handlers pointing to the same object (value).

Therefore if you change the state of the object, it is reflected to every other variable that's referencing it. But if you re-assign a new object (value) to the argument, then other variables pointing to this object (value) do not get re-assigned.

The final keyword on a method parameter means absolutely nothing to the caller. It also means absolutely nothing to the running program, since its presence or absence doesn't change the bytecode. It only ensures that the compiler will complain if the parameter variable is reassigned within the method. That's all. But that's enough.

Some programmers (like me) think that's a very good thing and use final on almost every parameter. It makes it easier to understand a long or complex method (though one could argue that long and complex methods should be refactored.) It also shines a spotlight on method parameters that aren't marked with final.

Java always makes a copy of parameters before sending them to methods. This means the final doesn't mean any difference for the calling code. This only means that inside the method the variables can not be reassigned.

Note that if you have a final object, you can still change the attributes of the object. This is because objects in Java really are pointers to objects. And only the pointer is copied (and will be final in your method), not the actual object.

There is a circumstance where you're required to declare it final --otherwise it will result in compile error--, namely passing them through into anonymous classes. Basic example:

public FileFilter createFileExtensionFilter(final String extension) {
    FileFilter fileFilter = new FileFilter() {
        public boolean accept(File pathname) {
            return pathname.getName().endsWith(extension);
        }
    };

    // What would happen when it's allowed to change extension here?
    // extension = "foo";

    return fileFilter;
}

Removing the final modifier would result in compile error, because it isn't guaranteed anymore that the value is a runtime constant. Changing the value from outside the anonymous class would namely cause the anonymous class instance to behave different after the moment of creation.