Converting ArrayList to HashMap in Java 8 using a Lambda Expression
Last Updated :
17 Aug, 2021
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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.
// 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 constructor 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);
}
}
// Converting ArrayList to HashMap// in Java 8 using a Lambda Expressionimport java.util.*;import java.util.ArrayList;import java.util.Scanner;// here we are making a class , and we will makeclass 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 constructor 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.
