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Can we convert ArrayList to map in Java?

In this post, we will see how to convert HashMap to ArrayList in java. Many times, you need to store keys or values into ArrayList or Store HashMaps Node objects in ArrayList.

Table of Contents

Using Java 8s Stream API
- Java 8: Convert HashMaps keys to ArrayList
- Convert HashMaps values to ArrayList
- Convert HashMaps Entry objects to ArrayList
Using ArrayLists constructor
- Convert HashMaps keys to ArrayList
- Convert HashMaps values to ArrayList
- Convert HashMaps Entry objects to ArrayList
Java HashMap tutorial

HashMap and ArrayList both are the most used data structure in java and both have different Hierarchy.

Here are the ways to convert HashMap to ArrayList in java.

Using Java 8s Stream API

Java 8: Convert HashMaps keys to ArrayList

We can use HashMaps keyset[] method to get a set of keys and use Java 8s Stream API to convert it to ArrayList.

Java

List customerIdList = customerIdNameMap.keySet[]

.stream[]

.collect[Collectors.toList[]];

Convert HashMaps values to ArrayList

We can use HashMaps values[] method to get a collection of values and use Java 8s Stream API to convert it to ArrayList.

List customerNames = customerIdNameMap.values[]

.stream[]

.collect[Collectors.toList[]];

Convert HashMaps Entry objects to ArrayList

We can use HashMaps entrySet[] method to get set of Entry object and use Java 8s Stream API to convert it to ArrayList.

List entryCustomerList = customerIdNameMap.entrySet[]

.stream[]

.collect[Collectors.toList[]];

Lets see the complete example.

HashMapToArrayListMainJava8.java

package org.arpit.java2blog;

import java.util.ArrayList;

import java.util.Collection;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

import java.util.Map.Entry;

import java.util.Set;

import java.util.stream.Collectors;

public class HashMapToArrayListMainJava8 {

public static void main[String[] args] {

HashMap customerIdNameMap = new HashMap[];

// Putting key-values pairs in HashMap

customerIdNameMap.put[1001, "Arman"];

customerIdNameMap.put[1002, "Javin"];

customerIdNameMap.put[1003, "Mat"];

customerIdNameMap.put[1004, "Joe"];

// Java 8

// Convert keys to ArrayList

List customerIdList = customerIdNameMap.keySet[]

.stream[]

.collect[Collectors.toList[]];

System.out.println["customerIds: "+customerIdList];

// Convert values to ArrayList

List customerNames = customerIdNameMap.values[]

.stream[]

.collect[Collectors.toList[]];

System.out.println["Customer Names: "+ customerNames];

// Convert entry objects to ArrayList

List entryCustomerList = customerIdNameMap.entrySet[]

.stream[]

.collect[Collectors.toList[]];

System.out.println["Customer ID and Names: "+entryCustomerList];

}

Output:

customerIds: [1001, 1002, 1003, 1004] Customer Names: [Arman, Javin, Mat, Joe] Customer ID and Names: [1001=Arman, 1002=Javin, 1003=Mat, 1004=Joe]

Using ArrayLists constructor

Convert HashMaps keys to ArrayList

Get keySet[] from HashMap and then convert it to ArrayList using its constructor.

// Convert keys to ArrayList

Set keySet = customerIdNameMap.keySet[];

List customerIdList = new ArrayList[keySet];

Convert HashMaps values to ArrayList

Get values[]from HashMap and then convert it to ArrayList using its constructor.

// Convert values to ArrayList

Collection values = customerIdNameMap.values[];

List customerNames = new ArrayList[values];

Convert HashMaps Entry objects to ArrayList

Get entrySet[] from HashMap and then convert it to ArrayList using its constructor.

// Convert entry objects to ArrayList

Set entrySet = customerIdNameMap.entrySet[];

List entryCustomerList = new ArrayList[entrySet];

Here is the complete program to convert HashMap to ArrayList.

package org.arpit.java2blog;

import java.util.ArrayList;

import java.util.Collection;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

import java.util.Map.Entry;

import java.util.Set;

public class HashMapToArrayListMain {

public static void main[String[] args] {

HashMap customerIdNameMap = new HashMap[];

// Putting key-values pairs in HashMap

customerIdNameMap.put[1001, "Arman"];

customerIdNameMap.put[1002, "Javin"];

customerIdNameMap.put[1003, "Mat"];

customerIdNameMap.put[1004, "Joe"];

// Before Java 8

// Convert keys to ArrayList

Set keySet = customerIdNameMap.keySet[];

List customerIdList = new ArrayList[keySet];

System.out.println["customerIds: "+customerIdList];

// Convert values to ArrayList

Collection values = customerIdNameMap.values[];

List customerNames = new ArrayList[values];

System.out.println["Customer Names: "+ customerNames];

// Convert entry objects to ArrayList

Set entrySet = customerIdNameMap.entrySet[];

List entryCustomerList = new ArrayList[entrySet];

System.out.println["Customer ID and Names: "+entryCustomerList];

}

Java HashMap tutorial

HashMap in java
HashMap internal working
hash and indexfor method in HashMap
hashcode and equals in javasort
HashMap by keys and values
Difference between HashMap and HashSet
Difference between HashMap and Hashtable
How to iterate over HashMap

Thats all about How to convert HashMap to ArrayList.

import_contacts

Using Java 8s Stream API

Java 8: Convert HashMaps keys to ArrayList

Convert HashMaps values to ArrayList

Convert HashMaps Entry objects to ArrayList

Using ArrayLists constructor

Convert HashMaps keys to ArrayList

Convert HashMaps values to ArrayList

Convert HashMaps Entry objects to ArrayList

Java HashMap tutorial

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