@Test
public void testRemove() {

  this.map.put(LOWER_KEY, VALUE_1);
  this.map.remove(UPPER_KEY);
  assertThat(this.map.containsKey(LOWER_KEY)).isFalse();
  this.map.put(UPPER_KEY, VALUE_2);
  this.map.remove(LOWER_KEY);
  assertThat(this.map.containsKey(UPPER_KEY)).isFalse();
  assertThat(this.map.remove(WRONG_KEY)).isNull();
}