Scala Cookbook

4.15. Defining an equals Method (Object Equality)

Problem

You want to define an equals method for your class so you can compare object instances to each other.

Solution

Like Java, you define an equals method (and hashCode method) in your class to compare two instances, but unlike Java, you then use the == method to compare the equality of two instances.

There are many ways to write equals methods. The following example shows one possible way to define an equals method and its corresponding hashCode method:

class Person (name: String, age: Int) {

  def canEqual(a: Any) = a.isInstanceOf[Person]

  override def equals(that: Any): Boolean =
    that match {
      case that: Person => that.canEqual(this) && this.hashCode == that.hashCode
      case _ => false
   }

  override def hashCode:Int = {
    val prime = 31
    var result = 1
    result = prime * result + age;
    result = prime * result + (if (name == null) 0 else name.hashCode)
    return result
  }

}

This example shows a modified version of a hashCode method that Eclipse generated for a similar Java class. It also uses a canEqual method, which will be explained shortly.

With the equals method defined, you can compare instances of a Person with ==, as demonstrated in the following tests:

import org.scalatest.FunSuite

class PersonTests extends FunSuite {

  // these first two instances should be equal
  val nimoy = new Person("Leonard Nimoy", 82)
  val nimoy2 = new Person("Leonard Nimoy", 82)
  val shatner = new Person("William Shatner", 82)
  val ed = new Person("Ed Chigliak", 20)

  // all tests ...