Converting ArrayList to HashMap in Java 8 using a Lambda Expression
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 Expressionimport java.util.*;import java.util.ArrayList;import java.util.Scanner;Â
// here we are making a class , and we will makeclass ListItems {Â
    // key will act as an id of the value    private Integer key;Â
    // value will be the value of the above key    private String value;Â
    // create constructor for reference    public ListItems(Integer id, String name)    {Â
        // assigning the value of key and value        this.key = id;        this.value = name;    }Â
    // return private variable key    public Integer getkey() { return key; }Â
    // return private variable name    public String getvalue() { return value; }}class Main {    public static void main(String[] args)    {        // Write your code hereÂ
        // Creating a List of type ListItems using ArrayList        List<ListItems> list = new ArrayList<ListItems>();Â
        // 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"));Â
        // Map which will store the list items        Map<Integer, String> map = new HashMap<>();Â
        // 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()); });Â
        // 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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