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How to Convert TreeMap to an ArrayList in Java?
  • Last Updated : 07 Jan, 2021

TreeMap  is a part of Java Collection framework. Java TreeMap contains values based on the key. It implements the NavigableMap interface and extends AbstractMap class. It provides an efficient means of storing key-value pairs in sorted order. Java TreeMap contains only unique elements. It cannot have a null key but can have multiple null values. TreeMap is not synchronized, we have to synchronize it explicitly in order to use it in a multi threading environment. TreeMap maintains the ascending order of the elements.

The way to convert TreeMap to ArrayList:

  • TreeMap having keys and values can be converted into two ArrayLists of keys and values.
  • Also two ArrayLists, one with keys and the other with values, can be converted into a TreeMap.

Example:

TreeMap : {1=Welcome, 2=To, 3= Geeks, 4=For, 5=Geeks}
keyList : [1, 2, 3, 4, 5]
valueList : [Welcome, To, Geeks, For, Geeks]

Approach:

  • Create a TreeMap object and insert some keys and values.
  • Extract the keys from the TreeMap using TreeMap.keySet() method and put them into an ArrayList object which has been created for storing keys.
  • Extract the values from the TreeMap using TreeMap.values() method and put them into another ArrayList object which has been created for storing values.

Java




// Java program to demonstrate conversion of
// TreeMap to ArrayList
  
import java.util.*;
class GFG {
    // a class level treeMap object
    static TreeMap<Integer, String> treeMap
        = new TreeMap<Integer, String>();
  
    // Method to convert TreeMap to ArrayList
    static void convertMapToList()
    {
        // Extract the keys from the TreeMap
        // using TreeMap.keySet() and
        // assign them to keyList of type ArrayList
        ArrayList<Integer> keyList
            = new ArrayList<Integer>(treeMap.keySet());
  
        // Extract the values from the TreeMap
        // using TreeMap.values() and
        // assign them to valueList of type ArrayList
        ArrayList<String> valueList
            = new ArrayList<String>(treeMap.values());
  
        // printing the keyList
        System.out.println(
            "List of keys of the given Map : " + keyList);
  
        // printing the valueList
        System.out.println(
            "List of values of the given Map : "
            + valueList);
    }
  
    // Driver Method
    public static void main(String args[])
    {
        // inserting data into the TreeMap
        // using TreeMap.put() method
        treeMap.put(1, "Welcome");
        treeMap.put(2, "To");
        treeMap.put(3, "Geeks");
        treeMap.put(4, "For");
        treeMap.put(5, "Geeks");
  
        // printing the TreeMap
        System.out.println("The TreeMap is : " + treeMap);
  
        // calling convertMapToList() method
        convertMapToList();
    }
}
Output
The TreeMap is : {1=Welcome, 2=To, 3=Geeks, 4=For, 5=Geeks}
List of keys of the given Map : [1, 2, 3, 4, 5]
List of values of the given Map : [Welcome, To, Geeks, For, Geeks]

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