28 Class identity

Class variables can refer to the same entity or to distinct entities with equal-looking fields.

class Counter {
  Integer value
}

main() {
  Counter first = Counter(value: 2)
  Counter alias = first
  Counter equalFields = Counter(value: 3)
  alias.value = 3
  printLine(first.value)
  printLine(first == alias)
  printLine(first == equalFields)
}

Expected output:

3
true
false

alias shares the exact object with first, so writing through it changes first.value to three. A separately constructed object also holds three but has a distinct identity.

Try it: Assign equalFields = first and predict the last comparison.

Precise rules: Reference.