How to define an `equals` method in a Scala class (object equality)

Table of Contents1 - Solution2 - A Scala `equals` method example3 - Discussion4 - Example 2: A Scala `equals` method with inheritance5 - Implementing hashCode6 - See Also

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

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If you’re new to Scala, a first thing to know is that object instances are compared with ==:

"foo" == "foo"   // true
"foo" == "bar"   // false
"foo" == null    // false
null == "foo"    // false
1 == 1           // true
1 == 2           // false
1d == 1.0d       // true

case class Person(name: String)
Person("Jess") == Person("Jessie")   // false

This is different than Java, which uses == for primitive values and equals for object comparisons.

A Scala method to create an MD5 hash of a string

If you happen to need Scala method to perform an MD5 hash on a string, here you go:

def md5HashString(s: String): String = {
    import java.security.MessageDigest
    import java.math.BigInteger
    val md = MessageDigest.getInstance("MD5")
    val digest = md.digest(s.getBytes)
    val bigInt = new BigInteger(1,digest)
    val hashedString = bigInt.toString(16)

A Java tuple class (Tuple2 or Pair, if you prefer)

After working with Scala for a long time, I had to come back to Java for a while to work on an Android app. Right away I missed a lot of things from the Scala world, including all of the built-in Scala collection methods, and other things as simple as the Scala Tuple classes.

If you haven’t used them before, a Scala Tuple class lets you write code like this:

Tuple<String, Integer> t = new Tuple<>("age", 41);

If you’re comfortable with generics, the Java implementation of a Tuple class like this is simple:

What is the difference between Nil and List() in Scala? alvin October 23, 2014 - 10:08am

Scala FAQ: What is the difference between Nil and List() in Scala?

Short answer: There isn’t any difference, as shown in the Scala REPL:

scala> Nil == List()
res0: Boolean = true

It’s more “idiomatic Scala” Scala to use Nil rather than List(). For instance, I wrote code like this last night using Nil in a Scala match/case expression:

Scala - How to find the unique items in a List, Array, Vector (sequence)

Scala FAQ: How do I find the unique items in a List, Array, Vector, or other Scala sequence?

Solution: Use the distinct method.

Here's a simple example using a List of integers:

scala> val x = List(1,1,1,2,2,3,3)
x: List[Int] = List(1, 1, 1, 2, 2, 3, 3)

scala> x.distinct
res0: List[Int] = List(1, 2, 3)

As you can see, res0 now contains only the unique elements in the list.

Eclipse can generate Java hashCode and equals methods

A nice feature of Eclipse is that you can easily generate hashCode and equals methods for your Java class using the editor. You'll really appreciate this capability when you need to create these methods whenever you're doing anything related to sorting, comparisons, comparators, etc.