@Test
  public void testKeySet() {
    CustomEntryConcurrentHashMap map = map5();
    Set s = map.keySet();
    assertEquals(5, s.size());
    assertTrue(s.contains(one));
    assertTrue(s.contains(two));
    assertTrue(s.contains(three));
    assertTrue(s.contains(four));
    assertTrue(s.contains(five));
  }
keySet returns a Set containing all the keys
geode-core/src/integrationTest/java/org/apache/geode/internal/util/concurrent/cm/ConcurrentHashMapJUnitTest.java