Old but gold since JDK 1.3^^

DynamicProxies

package App;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class Main {
    public static void main(String[] args) {
        MyInvocationHandler handler = new MyInvocationHandler();
        MyProxy proxy = (MyProxy) Proxy.newProxyInstance(MyProxy.class.getClassLoader(), new Class[]{MyProxy.class}, handler);
        System.out.println(proxy.getAnswer());
    }

    private static class MyInvocationHandler implements InvocationHandler {
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            return 42;
        }
    }
}