I am trying to understand interfaces in Java and have this task to do which I am a stack with. It must be something easy, but I don't seem to see the solution. Interface contains a few methods, one of them should return true if all elements of this set are also in the set. I.e. public interface ISet{ //some methods boolean isSubsetOf(ISet x); } Then the class: public class myClass implements ISet{ ArrayList<Integer> mySet; public myClass{ mySet = new ArrayList<Integer>(); } //some methods public boolean isSubsetOf(ISet x){ //method body } } What do I need to write in method body? How do I check that the instance of myClass is a subset of ISet collection? I was trying to cast, but it gives an error: ArrayList<Integer> param = (ArrayList<Integer>)x; return param.containsAll(mySet);