Mixin solutions...

gistfile1.txt

package novoda.mixin;
public class TestMixin extends Mixable implements WithTest {

    /**
     * @param args
     */
    public static void main(String[] args) {
        TestMixin mixin = new TestMixin();
        mixin.sayHello("test");
    }

    @Override
    public void sayHello(String what) {
        super.call(what);
    }
}
---
package novoda.mixin;

import java.lang.reflect.Method;

public abstract class Mixable {

    private Method method;

    @SuppressWarnings({
            "rawtypes", "unchecked"
    })
    public void call(Object... parameters) {
        StackTraceElement stackTraceElement = Thread.currentThread().getStackTrace()[2];
        for (Class klass : this.getClass().getInterfaces()) {
            try {
                klass.getMethod(stackTraceElement.getMethodName(), String.class);
                Class c = getClass().getClassLoader().loadClass(klass.getCanonicalName() + "Impl");
                Method m = c.getMethod(stackTraceElement.getMethodName(), String.class);
                Object obj = c.newInstance();
                m.invoke(obj, parameters);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}
---

package novoda.mixin;

public interface WithTest {
    public void sayHello(String what);
}
----

package novoda.mixin;

public class WithTestImpl implements WithTest {
    
    public WithTestImpl() {
        System.out.println("hello world with test");
    }
    @Override
    public void sayHello(String what) {
       System.out.println("From with mixin");
       System.out.println(what);
    }
}