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 §10: Objey the general contract when overriding equals for more details.