| The contract of `Object.equals()` states that for any non-null reference value |
| `x`, `x.equals(null)` should return `false`. Thus code such as |
| |
| ``` |
| if (x.equals(null)) { |
| ... |
| } |
| ``` |
| |
| either returns `false`, or throws a `NullPointerException` if `x` is `null`. The |
| nested block may never execute. |
| |
| This check replaces `x.equals(null)` with `x == null`, and `!x.equals(null)` |
| with `x != null`. If the author intended for `x.equals(null)` to return `true`, |
| consider this as fragile code as it breaks the contract of `Object.equals()`. |
| |
| See [Effective Java 3rd Edition ยง10: Objey the general contract when overriding |
| equals][ej3e-10] for more details. |
| |
| [ej3e-10]: https://books.google.com/books?id=BIpDDwAAQBAJ |
