### Question:
ObjectMultistack { public void push(String keyName, ValueWrapper valueWrapper) { Objects.requireNonNull(keyName, "Key name cannot be null."); Objects.requireNonNull(valueWrapper, "Value wrapper cannot be null."); MultistackEntry next = map.get(keyName); map.put(keyName, new MultistackEntry(valueWrapper, next)); } ObjectMultistack(); void push(String keyName, ValueWrapper valueWrapper); ValueWrapper pop(String keyName); ValueWrapper peek(String keyName); boolean isEmpty(String keyName);  }### Answer:
@Test void testAddingNullKey() { assertThrows(NullPointerException.class, () -> new ObjectMultistack().push(null, new ValueWrapper("a"))); }
@Test void testAddingNullValue() { assertThrows(NullPointerException.class, () -> new ObjectMultistack().push("string", null)); }