What is SplayTreeMap.putIfAbsent method in Dart?

Overview

The putIfAbsent method adds the key-value pair to the SplayTreeMap if an entry for the provided 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())

Parameters

This method takes two parameters.

  1. key: This is the key for which value is to be added if there is no entry already available.

  2. ifAbsent: This function will be invoked when there is no entry for the key present in the map. This method should return a value. That value is used when we add 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.

Example

The code below demonstrates how to use the putIfAbsent method:

import 'dart:collection';
void main() {
  //create a new SplayTreeMap which can have int key and string values
  SplayTreeMap map = new SplayTreeMap<int, String>((keya,keyb) =>  keyb.compareTo(keya));
  // add five entries to the map
  map[5] = "Five";
  map[4] = "Four";
  print('The map is $map');
  print('\nCalling put if absent for key 5');
  // using putIfAbsent to add entry for key 5
  var result = map.putIfAbsent(5, ()=> "Six");
  print('map.putIfAbsent(5, ()=> "Six") : $result');
  print('The map is $map');
  print('\nCalling put if absent for key 1');
  // using putIfAbsent to add entry for key 1
  result = map.putIfAbsent(1, ()=> "One");
  print('map.putIfAbsent(1, ()=> "One") : $result');
  print('The map is $map');
}

Explanation

Line 1: We import the collection library.

Line 4: We create a new SplayTreeMap object with the name map. We pass a compare function as an argument. This function is used for maintaining the order of the map entries. In our case, the compare function orders the elements in descending order.

Lines 7–8: We add two new entries to the map. Now, the map is {5=Five, 4=Four}.

Line 14: We use the puIfAbsent method with the following paramters:

  • 5 as the key.
  • The ifAbsent function returns Six as a return value.

In our case, there is an entry already available for the key 5. The old value Five associated with the key 5 is returned.

Line 20: We use the puIfAbsent method with the following parameters:

  • 1 as the key.
  • The ifAbsent function returns One as a return value.

In our case, there is no entry available for the key 1. So a new entry with key 1 and value One is added to map and the new value One is returned. Now, map is {5: Five, 4: Four, 1: One}.

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