What is HashMap.putIfAbsent in Dart?

A Hash-table-based implementation of MapMap contains a list of key-value pairs as an element.. Read more about HashMap here.

The putIfAbsent method adds a key-value pair to a map if the key is not present in the map.

If the key is already present, then it skips the operation.

Syntax

V putIfAbsent(K key, V ifAbsent())

Arguments

This method takes two arguments.

  1. key: The key, together with its value, that is added if there is no entry already available.

  2. ifAbsent: A function that will be invoked when no entry for the key is present in the map. This method will return a value, which will be used when adding a new entry.

Return value

If the key is already present in the map, then the old value associated with the key is returned and insertion will be skipped.

If the key is not present in the map, then a new key-value pair is inserted and the new value is returned.

Code

The code below demonstrates how to use the putIfAbsent method:

import 'dart:collection';
void main() {
  //create a new hashmap which can have string type as key, and int type as value
  HashMap map = new HashMap<String, int>();
  // add two entries to the map
  map["one"] =  1;
  map["two"] =  2;
  print('The map is $map');
  print('\nCalling put if absent for key "one"');
  // using putIfAbsent to add entry for key "one"
  var result = map.putIfAbsent("one", ()=> 10);
  print('map.putIfAbsent("one", ()=> 10) : $result');
  print('The map is $map');
  print('\nCalling put if absent for key "three"');
  // using putIfAbsent to add entry for key "three"
  result = map.putIfAbsent("three", ()=> 3);
  print('map.putIfAbsent("one", ()=> 10) : $result');
  print('The map is $map');
}

Explanation

In the code above,

  • In line 1, we import the collection library.

  • In line 4, we create a new HashMap object with the name map.

  • In lines 7-8, we add two new entries to the map. Now the map is {one: 1, two: 2}

  • In line 14, we use the puIfAbsent method with the following arguments:

    • one as key
    • ifAbsent function which returns 10 as return value.

In our case, there is an entry already present for the key one. Therefore, the old value associated with the key one, i.e. 1, is returned.

  • In line 20, we use the puIfAbsent method with the following arguments:
    • three as key
    • ifAbsent function returns three as a return value.

In our case, there is no entry available for the key three. So a new entry with key three and value 3 is added to the map, and the new value 3 is returned. Now the map will be {three: 3, one: 1, two: 2}.

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