Scala Book

The ArrayBuffer Class

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If you’re an OOP developer coming to Scala from Java, the ArrayBuffer class will probably be most comfortable for you, so we’ll demonstrate it first. It’s a mutable sequence, so you can use its methods to modify its contents, and those methods are similar to methods on Java sequences.

To use an ArrayBuffer you must first import it:

import scala.collection.mutable.ArrayBuffer

After it’s imported into the local scope, you create an empty ArrayBuffer like this:

val ints = ArrayBuffer[Int]()
val names = ArrayBuffer[String]()

Once you have an ArrayBuffer you add elements to it in a variety of ways:

val ints = ArrayBuffer[Int]()
ints += 1
ints += 2

The REPL shows how += works:

scala> ints += 1
res0: ints.type = ArrayBuffer(1)

scala> ints += 2
res1: ints.type = ArrayBuffer(1, 2)

That’s just one way create an ArrayBuffer and add elements to it. You can also create an ArrayBuffer with initial elements like this:

val nums = ArrayBuffer(1, 2, 3)

Here are a few ways you can add more elements to this ArrayBuffer:

// add one element
nums += 4

// add multiple elements
nums += 5 += 6

// add multiple elements from another collection
nums ++= List(7, 8, 9)

You remove elements from an ArrayBuffer with the -= and --= methods:

// remove one element
nums -= 9

// remove multiple elements
nums -= 7 -= 8

// remove multiple elements using another collection
nums --= Array(5, 6)

Here’s what all of those examples look like in the REPL:

scala> import scala.collection.mutable.ArrayBuffer

scala> val nums = ArrayBuffer(1, 2, 3)
val nums: ArrayBuffer[Int] = ArrayBuffer(1, 2, 3)

scala> nums += 4
val res0: ArrayBuffer[Int] = ArrayBuffer(1, 2, 3, 4)

scala> nums += 5 += 6
val res1: ArrayBuffer[Int] = ArrayBuffer(1, 2, 3, 4, 5, 6)

scala> nums ++= List(7, 8, 9)
val res2: ArrayBuffer[Int] = ArrayBuffer(1, 2, 3, 4, 5, 6, 7, 8, 9)

scala> nums -= 9
val res3: ArrayBuffer[Int] = ArrayBuffer(1, 2, 3, 4, 5, 6, 7, 8)

scala> nums -= 7 -= 8
val res4: ArrayBuffer[Int] = ArrayBuffer(1, 2, 3, 4, 5, 6)

scala> nums --= Array(5, 6)
val res5: ArrayBuffer[Int] = ArrayBuffer(1, 2, 3, 4)

More ways to work with ArrayBuffer

As a brief overview, here are several methods you can use with an ArrayBuffer:

val a = ArrayBuffer(1, 2, 3)         // ArrayBuffer(1, 2, 3)
a.append(4)                          // ArrayBuffer(1, 2, 3, 4)
a.append(5, 6)                       // ArrayBuffer(1, 2, 3, 4, 5, 6)
a.appendAll(Seq(7,8))                // ArrayBuffer(1, 2, 3, 4, 5, 6, 7, 8)
a.clear                              // ArrayBuffer()

val a = ArrayBuffer(9, 10)           // ArrayBuffer(9, 10)
a.insert(0, 8)                       // ArrayBuffer(8, 9, 10)
a.insert(0, 6, 7)                    // ArrayBuffer(6, 7, 8, 9, 10)
a.insertAll(0, Vector(4, 5))         // ArrayBuffer(4, 5, 6, 7, 8, 9, 10)
a.prepend(3)                         // ArrayBuffer(3, 4, 5, 6, 7, 8, 9, 10)
a.prepend(1, 2)                      // ArrayBuffer(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
a.prependAll(Array(0))               // ArrayBuffer(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

val a = ArrayBuffer.range('a', 'h')  // ArrayBuffer(a, b, c, d, e, f, g)
a.remove(0)                          // ArrayBuffer(b, c, d, e, f, g)
a.remove(2, 3)                       // ArrayBuffer(b, c, g)

val a = ArrayBuffer.range('a', 'h')  // ArrayBuffer(a, b, c, d, e, f, g)
a.trimStart(2)                       // ArrayBuffer(c, d, e, f, g)
a.trimEnd(2)                         // ArrayBuffer(c, d, e)

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