Convert HashMap, ArrayList

The HashMap collection supports fast lookups. But with an ArrayList we can sort and iterate over elements more efficiently.

To convert a HashMap to an ArrayList we can create an ArrayList and then populate it. We use the entrySet method. We must introduce the Entry class.

An example

To begin we create a HashMap and specify Integer keys and String values. We then use put() to add 3 entries to the HashMap.

import java.util.HashMap;
import java.util.Map.Entry;
import java.util.ArrayList;

public class Program {
    public static void main(String[] args) {

        // Create a HashMap.
        HashMap<Integer, String> hash = new HashMap<>();
        hash.put(100, "one-hundred");
        hash.put(1000, "one-thousand");
        hash.put(50, "fifty");

        // Use Entry type.
        // ... Create an ArrayList and call addAll to add the entrySet.
        ArrayList<Entry<Integer, String>> array = new ArrayList<>();
        array.addAll(hash.entrySet());

        // Loop over ArrayList of Entry elements.
        for (Entry<Integer, String> entry : array) {
            // Use each ArrayList element.
            int key = entry.getKey();
            String value = entry.getValue();

            System.out.println("Key = " + key + "; Value = " + value);
        }
    }
}
Key = 50; Value = fifty
Key = 100; Value = one-hundred
Key = 1000; Value = one-thousand

Notes, for-loop

In the above example we use a for-loop on the ArrayList. It is easier to iterate over an ArrayList than a HashMap. We access the keys and values from each Entry.

A HashMap offers important advantages for lookups. But with an ArrayList, we have better looping and sorting. We can convert a HashMap to an ArrayList with addAll() and entrySet.