Java Collection to List

Table of Contents

Using ArrayList constructor
Using Java 8 Stream
Complete program for converting Java Collection to List.

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

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.

List<Integer> intValuesJava8 = values.stream().collect(Collectors.toList());

Complete program for converting Java Collection to List.

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]

That’s all about Java collection to List.

[Fixed] java.lang.ClassCastException: java.util.Arrays$ArrayList cannot be cast to java.util.ArrayList

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