Java reflection: creating an implementation class

Question

Class someInterface = Class.fromName("some.package.SomeInterface"); 

How do I create a new class that implements someInterface ?

I need to create a new class and pass it to a function that needs someInterface as an argument.

Answer 1

Creating something that pretends to be an on-the-fly interface is actually not too difficult. You can use java.lang.reflect.Proxy after implementing the InvocationHandler to handle any method calls.

Of course, you could create a real class with a library like BCEL .

If this is for test purposes, you should look at mocking frameworks like jMock and EasyMock .

Answer 2

Easy, java.lang.reflect.Proxy to the rescue!

Full working example :

 interface IRobot { String Name(); String Name(String title); void Talk(); void Talk(String stuff); void Talk(int stuff); void Talk(String stuff, int more_stuff); void Talk(int stuff, int more_stuff); void Talk(int stuff, String more_stuff); } public class ProxyTest { public static void main(String args[]) { IRobot robot = (IRobot) java.lang.reflect.Proxy.newProxyInstance( IRobot.class.getClassLoader(), new java.lang.Class[] { IRobot.class }, new java.lang.reflect.InvocationHandler() { @Override public Object invoke(Object proxy, java.lang.reflect.Method method, Object[] args) throws java.lang.Throwable { String method_name = method.getName(); Class<?>[] classes = method.getParameterTypes(); if (method_name.equals("Name")) { if (args == null) { return "Mr IRobot"; } else { return args[0] + " IRobot"; } } else if (method_name.equals("Talk")) { switch (classes.length) { case 0: System.out.println("Hello"); break; case 1: if (classes[0] == int.class) { System.out.println("Hi. Int: " + args[0]); } else { System.out.println("Hi. String: " + args[0]); } break; case 2: if (classes[0] == String.class) { System.out.println("Hi. String: " + args[0] + ". Int: " + args[1]); } else { if (classes[1] == String.class) { System.out.println("Hi. int: " + args[0] + ". String: " + args[1]); } else { System.out.println("Hi. int: " + args[0] + ". Int: " + args[1]); } } break; } } return null; } }); System.out.println(robot.Name()); System.out.println(robot.Name("Dr")); robot.Talk(); robot.Talk("stuff"); robot.Talk(100); robot.Talk("stuff", 200); robot.Talk(300, 400); robot.Talk(500, "stuff"); } } 

Answer 3

If you want to go beyond interfaces, you can take a look at cglib and objenesis . Together, they allow you to do some pretty powerful things, expanding the abstract class and creating it. ( jMock uses them for this purpose, for example.)

If you want to stick with the interfaces, do what John Skeet said :).

Answer 4

In fact, you need to use the class name in the Class.fromName () method and apply it to your interface type. See if the example below helps.

 public class Main { public static void main(String[] args) throws Exception { Car ferrari = (Car) Class.forName("Mercedez").newInstance(); System.out.println(ferrari.getName()); } } interface Car { String getName(); } class Mercedez implements Car { @Override public String getName() { return "Mercedez"; } } class Ferrari implements Car { @Override public String getName() { return "Ferrari"; } }