Converting ArrayList to HashMap in Java 8 using a Lambda Expression

Last Updated : 04 Jan, 2021

A lambda expression is one or more line of code which works like function or method. It takes a parameter and returns the value. Lambda expression can be used to convert ArrayList to HashMap.

Syntax:

(parms1, parms2) -> expression

Examples:

Input : List : [1="1", 2="2", 3="3"]
Output: Map  : {1=1, 2=2, 3=3, 4=4, 5=5}

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:

Get the List to be converted into Map
Create an empty Map
Put the list values to the map using Lambda Expression
Return the formed Map

Below is the implementation of the above approach.

Java

// Converting ArrayList to HashMap 
// in Java 8 using a Lambda Expression 
import java.util.*; 
import java.util.ArrayList; 
import java.util.Scanner; 
  
// here we are making a class , and we will make 
class ListItems { 
  
    // key will act as an id of the value 
    private Integer key; 
  
    // value will be the value of the above key 
    private String value; 
  
    // create constuctor for reference 
    public ListItems(Integer id, String name) 
    { 
  
        // assigning the value of key and value 
        this.key = id; 
        this.value = name; 
    } 
  
    // return private variable key 
    public Integer getkey() { return key; } 
  
    // return private variable name 
    public String getvalue() { return value; } 
} 
class Main { 
    public static void main(String[] args) 
    { 
        // Write your code here 
  
        // Creating a List of type ListItems using ArrayList 
        List<ListItems> list = new ArrayList<ListItems>(); 
  
        // add the member of list 
        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 which will store the list items 
        Map<Integer, String> map = new HashMap<>(); 
  
        // for (ListItems n : list) { map.put(n.getkey(), 
        // n.getvalue()); } 
        // the below lambda  performs the same task as the 
        // above given for loop will do 
        // put the list items in the Map 
        list.forEach( 
            (n) -> { map.put(n.getkey(), n.getvalue()); }); 
  
        // Printing the given map 
        System.out.println("Map : " + map); 
    } 
}

Output

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

Time Complexity: O(N), where N is the length of Arraylist.

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