Convert HashMap to ArrayList 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 HashMap’s Node objects in ArrayList.

Table of Contents

Using Java 8’s Stream API
Using ArrayList’s constructor
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 8’s Stream API

Java 8: Convert HashMap’s keys to ArrayList

We can use HashMap’s keyset() method to get a set of keys and use Java 8‘s Stream API to convert it to ArrayList.


List<Integer> customerIdList = customerIdNameMap.keySet()
                                                    .stream()
                                                   .collect(Collectors.toList());

List<Integer> customerIdList = customerIdNameMap.keySet()

.stream()

.collect(Collectors.toList());

Convert HashMap’s values to ArrayList

We can use HashMap’s values() method to get a collection of values and use Java 8’s Stream API to convert it to ArrayList.


List customerNames = customerIdNameMap.values()
                                          .stream()
                                          .collect(Collectors.toList());

List customerNames = customerIdNameMap.values()

.stream()

.collect(Collectors.toList());

Convert HashMap’s Entry objects to ArrayList

We can use HashMap’s entrySet() method to get set of Entry object and use Java 8’s Stream API to convert it to ArrayList.


List<Entry<Integer, String>> entryCustomerList = customerIdNameMap.entrySet()
                                                                      .stream()
                                                                      .collect(Collectors.toList());

List<Entry<Integer, String>> entryCustomerList = customerIdNameMap.entrySet()

.stream()

.collect(Collectors.toList());

Let’s see the complete example.


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<Integer, String> customerIdNameMap = new HashMap<Integer, String>();
        // 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<Integer> customerIdList = customerIdNameMap.keySet()
                                                        .stream()
                                                        .collect(Collectors.toList());
        System.out.println("customerIds: "+customerIdList);

                // Convert values to ArrayList
        List<String> customerNames = customerIdNameMap.values()
                                                      .stream()
                                                      .collect(Collectors.toList());
        System.out.println("Customer Names: "+ customerNames);

                // Convert entry objects to ArrayList
        List<Entry<Integer, String>> entryCustomerList = customerIdNameMap.entrySet()
                                                                           .stream()
                                                                           .collect(Collectors.toList());
        System.out.println("Customer ID and Names: "+entryCustomerList);
    }
}

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<Integer, String> customerIdNameMap = new HashMap<Integer, String>();

// 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<Integer> customerIdList = customerIdNameMap.keySet()

.stream()

.collect(Collectors.toList());

System.out.println("customerIds: "+customerIdList);

// Convert values to ArrayList

List<String> customerNames = customerIdNameMap.values()

.stream()

.collect(Collectors.toList());

System.out.println("Customer Names: "+ customerNames);

// Convert entry objects to ArrayList

List<Entry<Integer, String>> 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 ArrayList’s constructor

Convert HashMap’s 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 keys to ArrayList

Set keySet = customerIdNameMap.keySet();

List customerIdList = new ArrayList(keySet);

Convert HashMap’s 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 values to ArrayList

Collection values = customerIdNameMap.values();

List customerNames = new ArrayList(values);

Convert HashMap’s Entry objects to ArrayList

Get entrySet() from HashMap and then convert it to ArrayList using its constructor.


// Convert entry objects to ArrayList
Set<Entry<Integer, String>> entrySet = customerIdNameMap.entrySet();
        List<Map.Entry<Integer,String>> entryCustomerList = new ArrayList<Map.Entry<Integer,String>>(entrySet);

// Convert entry objects to ArrayList

Set<Entry<Integer, String>> entrySet = customerIdNameMap.entrySet();

List<Map.Entry<Integer,String>> entryCustomerList = new ArrayList<Map.Entry<Integer,String>>(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<Integer, String> customerIdNameMap = new HashMap<Integer, String>();
        // 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<Integer> keySet = customerIdNameMap.keySet();
        List<Integer> customerIdList = new ArrayList<Integer>(keySet);
        System.out.println("customerIds: "+customerIdList);

        // Convert values to ArrayList
        Collection<String> values = customerIdNameMap.values();
        List<String> customerNames = new ArrayList<String>(values);
        System.out.println("Customer Names: "+ customerNames);

        // Convert entry objects to ArrayList
        Set<Entry<Integer, String>> entrySet = customerIdNameMap.entrySet();
        List<Map.Entry<Integer,String>> entryCustomerList = new ArrayList<Map.Entry<Integer,String>>(entrySet);
        System.out.println("Customer ID and Names: "+entryCustomerList);

    }
}

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<Integer, String> customerIdNameMap = new HashMap<Integer, String>();

// 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<Integer> keySet = customerIdNameMap.keySet();

List<Integer> customerIdList = new ArrayList<Integer>(keySet);

System.out.println("customerIds: "+customerIdList);

// Convert values to ArrayList

Collection<String> values = customerIdNameMap.values();

List<String> customerNames = new ArrayList<String>(values);

System.out.println("Customer Names: "+ customerNames);

// Convert entry objects to ArrayList

Set<Entry<Integer, String>> entrySet = customerIdNameMap.entrySet();

List<Map.Entry<Integer,String>> entryCustomerList = new ArrayList<Map.Entry<Integer,String>>(entrySet);

System.out.println("Customer ID and Names: "+entryCustomerList);

}

Java HashMap tutorial

That’s all about How to convert HashMap to ArrayList.

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Convert HashMap to ArrayList in java

Using Java 8’s Stream API

Java 8: Convert HashMap’s keys to ArrayList

Convert HashMap’s values to ArrayList

Convert HashMap’s Entry objects to ArrayList

Using ArrayList’s constructor

Convert HashMap’s keys to ArrayList

Convert HashMap’s values to ArrayList

Convert HashMap’s Entry objects to ArrayList

Java HashMap tutorial

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Using Java 8’s Stream API

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Convert HashMap’s values to ArrayList

Convert HashMap’s Entry objects to ArrayList

Using ArrayList’s constructor

Convert HashMap’s keys to ArrayList

Convert HashMap’s values to ArrayList

Convert HashMap’s Entry objects to ArrayList

Java HashMap tutorial

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