@Test
  public void whenCreateClassToInstanceMap_thenCreated() {
    final ClassToInstanceMap<Number> numbers = MutableClassToInstanceMap.create();
    numbers.putInstance(Integer.class, 1);
    numbers.putInstance(Double.class, 1.5);

    assertEquals(1, numbers.get(Integer.class));
    assertEquals(1.5, numbers.get(Double.class));
  }
  public void testNull() {
    try {
      map.put(null, new Integer(1));
      fail();
    } catch (NullPointerException expected) {
    }
    map.putInstance(Integer.class, null);
    assertNull(map.get(Integer.class));
    assertNull(map.getInstance(Integer.class));

    map.put(Long.class, null);
    assertNull(map.get(Long.class));
    assertNull(map.getInstance(Long.class));
  }