import java.io.*;
import java.util.SortedMap;
import org.checkerframework.checker.nullness.qual.*;
import org.checkerframework.checker.nullness.qual.EnsuresNonNullIf;

public class NonEmptyCollection {

    public static @NonNull String returnRemove(@NonNull PriorityQueue1<@NonNull String> pq) {
        return pq.remove();
    }

    public static @NonNull String returnPoll1(PriorityQueue1<@NonNull String> pq) {
        // :: error: (return.type.incompatible)
        return pq.poll();
    }

    public static @NonNull String returnPoll2(PriorityQueue1<@NonNull String> pq) {
        if (pq.isEmpty()) {
            return "hello";
        } else {
            return pq.poll();
        }
    }

    public static @NonNull String returnFirstKey(SortedMap<String, String> sm) {
        return sm.firstKey();
    }

    ///////////////////////////////////////////////////////////////////////////
    /// Helper classes copied from JDK
    ///

    public class PriorityQueue1<E> {
        @SuppressWarnings("purity") // object creation is forbidden in pure methods
        @org.checkerframework.dataflow.qual.Pure
        public @Nullable E poll() {
            throw new RuntimeException("skeleton method");
        }

        public E remove() {
            throw new RuntimeException("skeleton method");
        }

        @EnsuresNonNullIf(result = false, expression = "poll()")
        public boolean isEmpty() {
            throw new RuntimeException("skeleton method");
        }
    }
}