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)
    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");
    }
  }
}