Convert HashMap to ArrayList in Java

In this tutorial, we will see how to convert HashMap to ArrayList with or without using java 8 Stream API. HashMap and ArrayList both are part of Java’s Collection Framework under java.util package. The HashMap is the collection of key-value pairs. It implements the Map interface. On the other hand, ArrayList is a list of items of similar datatype and implements List interface.

Read Also:  Collection Interview Questions

To convert HashMap to ArrayList we need to convert HashMap keys and HashMap values to ArrayList. Alternatively, we can convert key-value pairs into ArrayList. In this article, we will learn several methods to perform the above conversions with or without using Java 8 Stream API. Let's dive deep into the topic.

Convert HashMap to ArrayList using Java 8

In Java 8, we can use below two methods to convert HashMap into ArrayList:

1. Conversion of HashMap Keys to ArrayList

import java.util.*;
import java.util.Map.Entry;
import java.util.stream.Collectors;

public class JavaHungry
{
    public static void main(String args[])
    {
        //Creating a HashMap Object
        HashMap<Integer,Integer> map = new HashMap<>();
        //Adding Values to HashMap
        map.put(1,10);
        map.put(2,20);
        map.put(3,30);
        map.put(4,40);
        map.put(5,50);
        //Creating an ArrayList Object and adding HashMap keys to it using Java 8
        ArrayList<Integer> list = (ArrayList<Integer>)map.keySet().stream().collect(Collectors.toList());
        System.out.println("ArrayList of Keys: ");
        //Displaying the contents of ArrayList of keys in Java 8
        list.forEach(System.out::println);
    }
}

Output:
ArrayList of Keys:
1
2
3
4
5

2. Conversion of HashMap Values to ArrayList

import java.util.*;
import java.util.Map.Entry;
import java.util.stream.Collectors;

public class JavaHungry
{
    public static void main(String args[])
    {
        //Creating a HashMap Object
        HashMap<Integer,Integer> map = new HashMap<>();
        //Adding Values to HashMap
        map.put(1,10);
        map.put(2,20);
        map.put(3,30);
        map.put(4,40);
        map.put(5,50);
        //Creating an ArrayList Object and adding HashMap items to it using Java 8
        ArrayList<Integer> list = (ArrayList<Integer>)map.values().stream().collect(Collectors.toList());
        System.out.println("ArrayList of Values: ");
        //Displaying the contents of ArrayList of Values in Java 8
        list.forEach(System.out::println);
    }
}

Output:
ArrayList of Values:
10
20
30
40
50

Convert HashMap to ArrayList Without Using Stream API

1. Conversion of HashMap Keys into ArrayList

In this method, we will create one ArrayList and add all the keys into it using the keySet() method of HashMap.

import java.util.*;

public class JavaHungry
{
    public static void main(String args[])
    {
        //Creating a HashMap Object
        HashMap<Integer,Integer> map = new HashMap<>();
        //Adding Values to HashMap
        map.put(1,10);
        map.put(2,20);
        map.put(3,30);
        map.put(4,40);
        map.put(5,50);
        //Getting Set of Keys from HashMap
        Set<Integer> keyset = map.keySet();
        //Creating an ArrayList Object
        ArrayList<Integer> list = new ArrayList<>();
        //Adding each key to ArrayList
        for(int key: keyset)
        {
            list.add(key);
        }
        //Display the content of ArrayList
        System.out.println("ArrayList of Keys:");
        for(int data: list)
        {
            System.out.print(data+" ");
        }
    }
}

Output:
ArrayList of Keys:
1 2 3 4 5

2. Conversion of HashMap Values into ArrayList

In this method, we will create one ArrayList and add all the values into it using values() method of HashMap.

import java.util.*;

public class JavaHungry
{
    public static void main(String args[])
    {
        //Creating a HashMap Object
        HashMap<Integer,Integer> map = new HashMap<>();
        //Adding Values to HashMap
        map.put(1,10);
        map.put(2,20);
        map.put(3,30);
        map.put(4,40);
        map.put(5,50);
        //Getting Collection of Values from HashMap
        Collection<Integer> val = map.values();
        //Creating an ArrayList Object
        ArrayList<Integer> list = new ArrayList<>();
        //Adding each Value to ArrayList
        for(int value: val)
        {
            list.add(value);
        }
        //Display the content of ArrayList
        System.out.println("ArrayList of Values:");
        for(int data : list)
        {
            System.out.print(data+" ");
        }
    }
}

Output:
ArrayList of Values:
10 20 30 40 50 


3. Conversion of Key-Value Pairs into ArrayList

For this, we are going to use the entrySet() method of HashMap class. This method returns the set of entries containing key-value pairs i.e. Entry. 

import java.util.*;
import java.util.Map.Entry;

public class JavaHungry
{
    public static void main(String args[])
    {
        //Creating a HashMap Object
        HashMap<Integer,Integer> map = new HashMap<>();
        //Adding Values to HashMap
        map.put(1,10);
        map.put(2,20);
        map.put(3,30);
        map.put(4,40);
        map.put(5,50);
        //Getting Set of Entry<Key,Value> from HashMap
        Set<Entry<Integer,Integer>> entryset = map.entrySet();
        //Creating an ArrayList Object
        ArrayList<Entry<Integer,Integer>> list = new ArrayList<>();
        //Adding each Value to ArrayList
        for(Entry<Integer,Integer> entry: entryset)
        {
            list.add(entry);
        }
        //Display the content of ArrayList
        System.out.println("ArrayList of Key-Value Pairs:");
        for(Entry<Integer,Integer> data : list)
        {
            System.out.print(data+" ");
        }
    }
}

Output:
ArrayList of Key-Value Pairs:
1=10 2=20 3=30 4=40 5=50

That's all for today, using the above methods, we can convert HashMap to ArrayList by using any current version of Java. Please mention in the comments in case you know any other way.

References:
HashMap Java doc
ArrayList Java doc