Java Reflection: Create an implementing class

Creating something which pretends to implement an interface on the fly actually isn't too hard. You can use java.lang.reflect.Proxy after implementing InvocationHandler to handle any method calls.

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

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

If you want to go beyond interfaces, you might want to take a look at cglib and objenesis. Together, they will allow you to do some pretty powerful stuff, extending an abstract class and instantiating it. (jMock uses them for that purpose, for example.)

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

Actually, you have to use the class name in Class.fromName() method and cast to your interface type. See if the sample 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";
}


}

Easily, 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");
}
}