gistfile1.java

import java.util.Collections;
import java.util.Set;

class IncompatibleTypes {

        interface A {}
        interface B<T> {}

        static abstract class C<T> {
            abstract Set<A> getAsForB(B<T> b);
        }

        static class B1 implements B<String> {}
        static class B2 implements B<Integer> {}

        static class C1 extends C<String> {
            Set<A> getAsForB(B<String> bstr) {
                return Collections.emptySet();
            }
        }

        static class C2 extends C<Integer> {
            Set<A> getAsForB(B<Integer> bint) {
                return Collections.emptySet();
            }
        }

        public static void main(String[] args) {

            C c = new C1();
            B b = new B1();
            

            // $ javac IncompatibleTypes.java 
            // IncompatibleTypes.java:43: incompatible types
            // found   : java.lang.Object
            // required: IncompatibleTypes.A
            //       for ( A a : c.getAsForB(b) ) {
            //                              ^
            // Note: IncompatibleTypes.java uses unchecked or unsafe operations.
            // Note: Recompile with -Xlint:unchecked for details.
            // 1 error
            for ( A a : c.getAsForB(b) ) {
                System.out.println(a);
            }
        }



}

No compiler error: Since "c" is delcared as C without type params, the return type of getAsForB is just a plain Set...