[java] New rule: SuspiciousEqualsCall

[UncleOwen]

Proposed Rule Name: SuspiciousEqualsCall? EqualsOnIncompatibleTypes?

Proposed Category: Error Prone

Description: This rule would trigger on calls of a.equals(b), where the static types of a and b don't have a common supertype (except Object).

Code like this usually comes up when you

• were thinking of slightly different objects (e.g. you meant to call a.getId().equals(b) or something like that
• messed up a refactoring.

Code Sample:

Integer i;
String s;
if (i.equals(s)) {    // violation
  // do stuff
}

Possible Properties:

• Maybe a stricter mode, where one of the types must be a supertype of the other?