### Question:
SimpleKeyCryptoBuilder extends KeyCryptoBuilder { @Override public KeyCryptoBuilder setUsername(final String username) { if (username == null) { throw new NullPointerException("Username cannot be null."); } this.username = username; markAsSet(Attribute.USERNAME); return this; }  @Override KeyCryptoProduct build(); @Override KeyCryptoBuilder setKeySalt(final String keySalt); @Override KeyCryptoBuilder setPrivateKey(final URI privateKey); @Override KeyCryptoBuilder setServerAddress(final URI serverAddress); @Override KeyCryptoBuilder setUsername(final String username);  }### Answer:
@Test(expected = NullPointerException.class) public void testNullUsername() { builder.setUsername(null); }