Converting ArrayList to HashMap in Java 8 using a Lambda Expression

Lambda expressions provide a concise way to perform operations on collections without writing boilerplate loops. Using lambdas, an ArrayList can be easily converted into a HashMap by mapping each element to a key–value pair.

Syntax

(parameters) -> expression

Example: Converting an ArrayList containing objects into a HashMap where: One field acts as the key, and Another field acts as the value.

Input : List : [1="I", 2="love", 3="Geeks" , 4="for" , 5="Geeks"]
Output: Map : {1=I, 2=Love, 3=Geeks, 4=For, 5=Geeks}

Approach

1. Create a list containing objects with key–value data.
2. Create an empty HashMap.
3. Iterate over the list using a lambda expression.
4. Insert elements into the map using map.put(key, value).
5. Print the resulting map.

import java.util.*;
class ListItems {

    private Integer key;
    private String value;
    public ListItems(Integer key, String value) {
        this.key = key;
        this.value = value;
    }
    public Integer getKey() {
        return key;
    }
    public String getValue() {
        return value;
    }
}
class Main {
    public static void main(String[] args) {
        List<ListItems> list = new ArrayList<>();

        list.add(new ListItems(1, "I"));
        list.add(new ListItems(2, "Love"));
        list.add(new ListItems(3, "Geeks"));
        list.add(new ListItems(4, "For"));
        list.add(new ListItems(5, "Geeks"));

        Map<Integer, String> map = new HashMap<>();
        list.forEach(item ->
            map.put(item.getKey(), item.getValue())
        );
        System.out.println("Map : " + map);
    }
}

Map : {1=I, 2=Love, 3=Geeks, 4=For, 5=Geeks}

Explanation

• Each object in the ArrayList contains a key and a value.
• The forEach() method iterates over the list.
• The lambda expression inserts each element into the HashMap.
• This approach replaces a traditional for loop with cleaner and more readable code.

Note:

• If duplicate keys exist, the latest value will overwrite the previous one.
• This approach is best suited when keys are already available in the list objects.
• Java 8 Streams (Collectors.toMap) can also be used for the same task.