public interface Foo {
   myMethod();
}

public class Parent() {
}

public class Child1 extends Base implements Foo {
   public myMethod() {
   }
}

ArrayList<Parent> myList = new ArrayList<Parent>();
myList.add(new Child1());
myList.add(new Child2());
myList.add(new Child3());

myList.get(0).myMethod();   // Ayto edw den doyleyei, giati to MyList.get(0) epistrefei 
                           // typo Parent, enw thn methodA() thn ekane implement to ChildX