[java] New rule: SuspiciousEqualsCall

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) subtype.

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?

Use the a.equals(b) call pattern in the issue as the entry point and review existing Java rule implementations for type comparisons. No files or tests are named, so locate the rule and test conventions first. Done means the rule reliably identifies equals calls between incompatible static types, with the handling of Object and any stricter mode decided and covered by tests.