Is it possible to override a method at runtime?

Question

Is there anyway to override a method at run time? Even if it requires dynamically creating a subclass from that instance?

Answer 1

As others said, no, you can't override a method at runtime. However, starting with Java 8 you can take the functional approach. Function is a functional interface that allows you to treat functions as reference types. This means that you can create several ones and switch between them (dynamically) a-la strategy pattern.

Let's look at an example:

public class Example {

    Function<Integer, Integer> calculateFuntion;

    public Example() {

        calculateFuntion = input -> input + 1;
        System.out.println(calculate(10));
        // all sorts of things happen
        calculateFuntion = input -> input - 1;
        System.out.println(calculate(10));
    }

    public int calculate(int input) {
        return calculateFuntion.apply(input);
    }

    public static void main(String[] args) {
        new Example();
    }
}

Output:

11
9

I don't know under what circumstances and design you intend to override, but the point is that you replace the behavior of the method, which is what overriding does.

Answer 2

With plain Java, no.

With ByteBuddy(preferred), asm, cglib or aspectj, yes.

In plain Java, the thing to do in a situation like that is to create an interface-based proxy that handles the method invocation and delegates to the original object (or not).