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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
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