Ghosts

Ghosts are experimental, and off unless you ask for them:

mutastic run --experimental=ghosts

A mutant changes code that’s there. It can’t show code that isn’t: a setter that forgot to tell its listeners has no call to remove. A ghost is the other way round. Where most writes like this one fire a property change and this one doesn’t, mutastic inserts the call the others make, and runs the tests that reach the write against it.

• A ghost passes unseen when every one of those tests still passes. Nothing says whether listeners should hear of that write, so mutastic asks.
• A ghost appears when a test fails with the call in: that test expects the write to stay quiet, and mutastic says which one.

A ghost is a question about what the code should do, not a gap in the tests, so it’s never in the score, the counts or the gate. Each says why mutastic thinks the call belongs there, what running the tests with it found, and both answers: fire the change there, or write a test that listens and expects no event.

Where mutastic looks

Today, ghosts are about JavaBeans’ bound properties: classes that call PropertyChangeSupport’s firePropertyChange, directly or through a method of their own or a superclass’s. Two kinds of evidence make a ghost:

• The same field: at least two writes to a field fire a property change, and this one doesn’t.
• Its siblings: at least three of the class’s other fields fire on every write, and this setter doesn’t.

Writes in a constructor, readObject or clone aren’t counted, as no listener can be there yet, and neither is a method that fires a change with no name (everything changed at once), or one marked @DoNotMutate. The call inserted is the one most of the peers make, with the name they give it, the old value read before the write and the value written.

What a ghost says

Ghosts (experimental, not in the score)

  Account.java:67  Account.applyInterest() writes `balance` without telling listeners
    3 of 5 writes to `balance` fire a property change: setBalance (:42) fires "balance", deposit (:51) fires "balance", withdraw (:60) fires "balance"
    With firePropertyChange("balance", old, new) added after it, the 1 test that runs it still passes.
    If listeners should hear of it, fire the change there. If it's quiet on purpose, a test that listens and expects no event says so.

In --format=jsonl, the run-finished event lists them under ghosts, each with "experimental": true and a status of passedUnseen, appeared or notCovered. A run that finds none says so: its summary’s Ghosts row reads none found, and ghosts is an empty list.