Table of Contents
In this post, we will see how to convert Java Collection to List.
There are many ways to do it, we will see two ways here.
Using ArrayList constructor
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ArrayList(Collection<? extends E> c) |
Above ArrayList‘s constructor creates a list having the elements of the specified collection, in the order they are returned by the collection’s iterator.
Using Java 8 Stream
You can use Java 8‘s stream as well to convert any collection to the List.
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List<Integer> intValuesJava8 = values.stream().collect(Collectors.toList()); |
Complete program for converting Java Collection to List.
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package org.arpit.java2blog; import java.util.ArrayList; import java.util.Collection; import java.util.HashMap; import java.util.HashSet; import java.util.List; import java.util.Map; import java.util.stream.Collectors; public class ConvertCollectionToListMain { public static void main(String[] args) { // HashSet Collection<String> s1 = new HashSet<>(); s1.add("John"); s1.add("Martin"); s1.add("Mary"); System.out.println("Set Elements:"); System.out.println(s1); // Converting Collection to list List<String> list=new ArrayList<String>(s1); System.out.println("List elements: "); System.out.println(list); // HashMap Map<String,Integer> m1 = new HashMap<>(); m1.put("John", 18); m1.put("Martin", 23); m1.put("Mary",34); m1.put("Tom", 32); System.out.println("==================="); Collection<Integer> values = m1.values(); // Converting Collection to list List<Integer> intValues = new ArrayList<>(values); System.out.println("Values of Map are: "); System.out.println(intValues); System.out.println("==================="); System.out.println("Using Java 8"); // Converting Collection to list using java 8 List<String> listJava8= s1.stream().collect(Collectors.toList()); System.out.println("List elements: "); System.out.println(listJava8); // Converting Collection to list using java 8 List<Integer> intValuesJava8 = values.stream().collect(Collectors.toList()); System.out.println("Values of Map are: "); System.out.println(intValuesJava8); } } |
When you run above program, you will get below output
Set Elements:
[John, Martin, Mary] List elements:
[John, Martin, Mary] ===================
Values of Map are:
[32, 18, 23, 34] ===================
Using Java 8
List elements:
[John, Martin, Mary] Values of Map are:
[32, 18, 23, 34]
[John, Martin, Mary] List elements:
[John, Martin, Mary] ===================
Values of Map are:
[32, 18, 23, 34] ===================
Using Java 8
List elements:
[John, Martin, Mary] Values of Map are:
[32, 18, 23, 34]
That’s all about Java collection to List.
