Support Standard Invariants

[jlink]

Testing Problem

Some invariants show up again and again. A few examples:

• Idempotency
• Commutativity
• Associativity
• Equals/Hashcode dependency

Coding properties for them by hand requires very similar code again and again.

Suggested Solution

Provide a mechanism to apply standard invariants onto functions, types etc.

Different APIs are thinkable:

• Abstract template classes for an Invariant, e.g.

  class IdempotencyProperties<T> {
    abstract T createFunction();
  }

• Annotation based enhancement of Property classes
• ???

Discussion

Here's how this can be done in quickcheck (Haskell): https://github.com/knupfer/test-invariant/blob/master/src/Test/Invariant.hs