Verify that two equal objects produce the same hashcode.

← Operators and Expressions · Ref: Q1096

Hashcode prefix operator:

  h1 <- #?p1
  h2 <- #?p2
  assert h1 == h2

Equal objects must have equal hashes. 'default operator' generates #? and == from the same fields, ensuring consistency. See Q966 for hashcode details, Q1095 for using records as Dict keys.

Example

defines module qa.operators.hashcodeoperator

  defines class

    Point
      x <- Integer()
      y <- Integer()

      Point()
        ->
          x as Integer
          y as Integer
        this.x :=: x
        this.y :=: y

      default operator

  defines program

    HashcodeOperatorDemo()
      stdout <- Stdout()

      p1 <- Point(10, 20)
      p2 <- Point(10, 20)
      p3 <- Point(99, 20)

      //Equal objects must have equal hashcodes
      h1 <- #?p1
      h2 <- #?p2
      stdout.println(`p1 hash: ${h1}`)
      stdout.println(`p2 hash: ${h2}`)

      if h1 == h2
        stdout.println("Hash contract holds: equal objects, equal hashes")

      //Different objects likely have different hashes
      h3 <- #?p3
      stdout.println(`p3 hash: ${h3}`)

Common mistakes

E50060 — EK9 uses the '#?' prefix operator for hashcodes, not a '.hashCode()' method — 'p1.hashCode()' does not resolve. See ek9 -h E50060 for details.

Incorrect:

h1 <- p1.hashCode()

Correct:

h1 <- #?p1

Other ways to ask this

I need to confirm the hash contract holds for my custom class
In Java I'd test that hashCode() is consistent with equals(). Show the EK9 equivalent
Given two objects with identical field values, check their #? hashcodes match
Test hashcode consistency between equal objects using #?

Coming from another language?

Java: obj.hashCode(). Python: hash(obj). Rust: Hash trait. EK9: #?obj prefix operator — no method call needed.

Keywords: hashcode, consistency, #?, equal, hash, contract