code: public void toContainKey(final K key) {
          expectNotNull(this.getValue(), "Expected null to contain the key '%s'", key);
  		expectTrue(this.getValue().containsKey(key), "Expected '%s' to contain key '%s'", this.getValue(), key);
  	}
docs: Checks if the stored {@code Map} contains the given {@code key}.
  <p>This method throws an {@code AssertionError} if
  <ul>
  <li>the stored {@code Map} does not contain the given {@code key}</li>
  <li>the stored {@code Map} is {@code null}</li>
  </ul>
  @param key the expected key