Good morning

I do not understand why the example foo is the only one that can be executed ... if I change the names or parameters eclipse tells me an error!!! ..

package Essai;

public class Foo {
 public int fooField;

 public Foo () {
 fooField = 0;
 }

}

package Essai;

public class FooMixin implements FooMixinInt {
 public FooMixin(){}

 public String fooMethod2 (int i) {
 System.out.print("Message");
 return Integer.toString ( 4- i);
 }

}

package Essai;
public interface FooMixinInt {
 public String fooMethod2 (int i);
}

package Essai;

public class Main {


 public static void main(String[] args) {

 Foo foo = new Foo ();
 FooMixinInt fooint = (FooMixinInt) foo;
 String s = foo.fooMethod2(-2);

 }

}

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<aop>
 <introduction class="Essai.Foo">
 <mixin>
 <interfaces>Essai.FooMixinInt</interfaces>
 <class>Essai.FooMixin</class>
 <construction>new Essai.FooMixin()</construction>
 </mixin>
 </introduction>
</aop>