CODEWITHSUNDEEP
scalahashmap
Bruno
Bruno

Reputation: 864

Specifying the size of a HashMap in Scala

I'm using Scala's mutable HashMap to incrementally add up to millions of key-value pairs. The resizing of these HashMaps is now the slowest part of my program. How can I tell Scala to create a very large HashMap from the start so that it (almost) never needs to be resized?

I would appreciate also any ideas that propose another Scala/Java collection that would suit my needs. Adding a new key-value pair and retrieving a value given a key should both be doable in approximately constant time.

Upvotes: 12

Views: 3843

Answers (3)

Abhijit Sarkar
Abhijit Sarkar

Reputation: 24637

java.util.HashMap has a constructor that lets you specify an initial size. Unlike Yuriy's answer, there's no reason to write code that you don't need to.

jifeng.yin's answer is also acceptable, although since OpenHashMap uses an unspecified form of Open Addressing, you should benchmark for your use case and see how the change from chaining to open addressing matters, if it all.

Upvotes: 0

jifeng.yin
jifeng.yin

Reputation: 2151

scala.collection.mutable.OpenHashMap supports initialSize

Upvotes: 3

Yuriy
Yuriy

Reputation: 2769

One of possible way:

import scala.collection.mutable.{HashTable, DefaultEntry}

trait BigHashTable[A, B] extends HashTable[A, DefaultEntry[A, B]] {
  override def initialSize: Int = 1024 // 16 - by default
}

val x = new HashMap[Int, String] with BigHashTable[Int, String]

another one:

class MyHashMap[A, B](initSize : Int) extends HashMap[A, B] {
  override def initialSize: Int = initSize // 16 - by default
}

val x = new MyHashMap[Int, String](1024)

Upvotes: 11

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