public class Parent {}
public class Child extends Parent {}
interface Store {
    public Set<Parent> getParents(); //PROBLEM!! This needs to change
}

Set<Parent> parents = store.getParents();
parents.add(new Parent());
parents.add(new Child());
store.getParents().add(new Child()); //Note the lack of generics

for(Parent curEntry : store.getParents()) {

}

interface Store {
    public Set<Parent> getParents();
}

class ConcreteStore implements Store {
    Set<Child> childs;
    public Set<Parent> getParents() {
        return (Set<? extends Parent>)childs; //ERROR: inconvertible types
    }
}

interface Store {
    public Set<? extends Parent> getParents();
}

class ConcreteStore implements Store {
    Set<Child> childs;
    public Set<Child> getParents() {
        return childs;
    }
}

Store store = new ConcreteStore();
Set<? extends Parent> parents = store.getParents();
parents.add(new Child()); //ERROR: cannot find symbol
parents.add(new Parent()); //ERROR: cannot find symbol. What?!

for (Parent curEntry : store.getParents()) {
}

interface Store<T extends Parent> {
    public Set<T> getParents();
}

public static class ConcreteStore implements Store<Child> {
    Set<Child> childs;

    @Override
    public Set<Child> getParents() {
        return childs;
    }
}

Store store = new ConcreteStore();
Set<Parent> parents = store.getParents();
parents.add(new Parent());
parents.add(new Child());

for (Parent curEntry : store.getParents()) { //ERROR: incompatible types. Found object, requited Parent
}

Set<Parent> parents = store.getParents();
parents.add(new Parent());
parents.add(new Child());
store.getParents().add(new Child()); //Note the lack of generics

for(Parent curEntry : store.getParents()) {

}

public class Parent {}
public class Child extends Parent {}
interface Store {
    public Set<Parent> getParents();
}

interface Store<X extends Parent> {
   public Set<X> getParents();
}

class ConcreteStore implements Store<Child> {
    Set<Child> childs;
    public Set<Child> getParents() {
        return childs;
    }
}

class ConcreteStore implements Store {
    Set<Parent> parents;
    public Set<Parent> getParents() {
        return parents;
    }
}