Scala Case Classes and Pattern Matching

Before you start

Scala functions.

Define a data class and take it apart with match.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A case class declares an immutable data type with generated helpers: you can create one without new, compare by value, and print it readably. Its fields are public vals.

match tests a value against patterns in order. case Point(0, 0) matches a literal, case Point(x, 0) matches any point on the x-axis and binds its x, and case _ is the catch-all.

Code example

Scala

object Main {
  case class Point(x: Int, y: Int)

  def describe(p: Point): String = p match {
    case Point(0, 0) => "origin"
    case Point(x, 0) => s"on x-axis at $x"
    case _ => "elsewhere"
  }

  def main(args: Array[String]): Unit = {
    println(describe(Point(0, 0)))
    println(describe(Point(4, 0)))
    println(describe(Point(1, 2)))
  }
}

Output

origin
on x-axis at 4
elsewhere

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

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Guided walkthroughReady

1object Main {

2 case class Point(x: Int, y: Int)

4 def describe(p: Point): String = p match {

5 case Point(0, 0) => "origin"

6 case Point(x, 0) => s"on x-axis at $x"

7 case _ => "elsewhere"

8 }

10 def main(args: Array[String]): Unit = {

11 println(describe(Point(0, 0)))

12 println(describe(Point(4, 0)))

13 println(describe(Point(1, 2)))

14 }

15}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
An immutable data type with matching support.	`—`	—
The first pattern matches.	`x = 0, y = 0`	origin
The second pattern matches and binds x = 4.	`x = 4, y = 0`	origin on x-axis at 4
Nothing earlier matches, so the catch-all runs.	`x = 1, y = 2`	origin on x-axis at 4 elsewhere

Behind the scenes

How Scala processes this code

Scala compiles for the JVM, which invokes the main entry point.

Cost and common mistakes

O(1) time and space per call.

Patterns are tried in order, so a broad case placed first hides the later ones.
Without a catch-all, an unmatched value throws a MatchError at runtime.

Try it yourself

Add a case for points on the y-axis.

Reveal answer and reasoning

Add case Point(0, y) => s"on y-axis at $y" before the catch-all.