import java.util.Hashtable;
import org.checkerframework.checker.nullness.qual.Nullable;

class HashtableTest {

    public static void main(String[] args) {

        // :: error: (type.argument.type.incompatible)
        Hashtable<@Nullable Integer, String> ht1 = new Hashtable<>();

        // Suffers null pointer exception
        ht1.put(null, "hello");

        // :: error: (type.argument.type.incompatible)
        Hashtable<Integer, @Nullable String> ht2 = new Hashtable<>();

        // Suffers null pointer exception
        ht2.put(42, null);
    }
}