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Converting ArrayList to HashMap using Method Reference in Java 8

  • Last Updated : 07 Jan, 2021

Data Structures are a boon to everyone who codes. But is there any way to convert a Data Structure into another? Well, it seems that there is! In this article, we will be learning how to convert an ArrayList to HashMap using method reference in Java 8.


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Elements in ArrayList are : [Pen, Pencil, Book, Headphones]
Elements in HashMap are :{Pen=3, Pencil=6, Book=4, Headphones=10}

In Java 8, there are two methods of achieving our goal:

  1. Converting an ArrayList to HashMap using Lambda expression.
  2. Converting an ArrayList to HashMap using method reference.

Method: Using Method Reference

Using Method Reference comparatively makes the code look cleaner when compared with Lambda Expression.

Lambda is nothing but code, and if you already have a method that does the same thing, then you can pass the method reference instead of a lambda expression.

  • Function.identity() refers to an element making itself as the key of the HashMap.
  • String::length allows storing the length of the element as its respected value.


// Java program for Converting ArrayList to
// HashMap using method reference in Java 8
import java.util.*;
import java.util.function.Function;
class GFG {
    public static void main(String[] args)
        // creating arraylist to add elements
        ArrayList<String> fruits = new ArrayList<>();
        // printing contents of arraylist before conversion
        System.out.println("Elements in ArrayList are : "
                           + fruits);
        // creating new hashmap and using method reference
        // with necessary classes for the conversion
        HashMap<String, Integer> res =
                Function.identity(), String::length,
                (e1, e2) -> e1, HashMap::new));
        // printing the elements of the hashmap
        System.out.println("Elements in HashMap are : "
                           + res);
Elements in ArrayList are : [Banana, Guava, Pineapple, Apple]
Elements in HashMap are : {Guava=5, Apple=5, Pineapple=9, Banana=6}

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