Category >> Collections  

How to Convert Array to ArrayList in Java?

This is a question that is worth to take a look for myself, because it is one of the top viewed and voted questions in stackoverflow. The one who accidentally asks such a question could gain a lot of reputation which would enable him to do a lot of stuff on stackoverflow. This does not make sense so much for me, but let’s take a look at the question first.

The question asks how to convert the following array to an ArrayList.

Element[] array = {new Element(1),new Element(2),new Element(3)};

1. Most popular and accepted answer

The most popular and the accepted answer is the following:

ArrayList<Element> arrayList = new ArrayList<Element>(Arrays.asList(array));

First, let’s take a look at the Java Doc for the constructor method of ArrayList.

- ArrayList(Collection c)
Constructs a list containing the elements of the specified collection, in the order they are returned by the collection’s iterator.

So what the constructor does is the following:
1. Convert the collection c to an array
2. Copy the array to ArrayList’s own back array called “elementData”

If the add() method is invoked NOW, the size of the elementData array is not large enough to home one more element. So it will be copied to a new larger array. As the code below indicates, the size grows 1.5 times of old array.

public void ensureCapacity(int minCapacity) {
      int oldCapacity = elementData.length;
      if (minCapacity > oldCapacity) {
          Object oldData[] = elementData;
          int newCapacity = (oldCapacity * 3)/2 + 1;
          if (newCapacity < minCapacity)
              newCapacity = minCapacity;
             // minCapacity is usually close to size, so this is a win:
             elementData = Arrays.copyOf(elementData, newCapacity);

2. Next popular answer

The next popular answer is:

List<Element> list = Arrays.asList(array);

It is not the best, because the size of the list returned from asList() is fixed. We know ArrayList is essentially implemented as an array, and the list returned from asList() is a fixed-size list backed by the original array. In this way, if add or remove elements from the returned list, an UnsupportedOperationException will be thrown.

list.add(new Element(4));
Exception in thread "main" java.lang.ClassCastException: java.util.Arrays$ArrayList cannot be cast to java.util.ArrayList
	at collection.ConvertArray.main(

3. Indications of the question

The problem is not hard, and kind of interesting. Every Java programmer knows ArrayList, it is simple but easy to make such a mistake. I guess that is why this question is so popular. If a similar question asked about a Java library in a specific domain, it would be less likely to become so popular.

There are several answers that basically indicate the same solution. This is true for a lot of questions, I guess people just don’t care, they like answering!

Reference: The stackoverflow question link

Category >> Collections  
  • Kumarbaki

    How to convert list array to byte array ? can you give any example for this.?

  • ryanlr

    Thanks for comment. I don’t expect any other way. This post only shows how it works now.

  • Matt Passell

    I looked over your post again and saw that I misunderstood you. Yes, the new ArrayList is sized to exactly fit the original array, so if someone adds an additional element, the ArrayList is resized. Seems reasonable. How else would you expect it to work?

  • ryanlr
  • Matt Passell

    Came here from the DZone posting. What version of Java are you basing this on? I took a look at the source for Array(Collection) and Arrays.asList(T…) in Java 6 and 7 and it looks like they use very direct, efficient approaches:

    public ArrayList(Collection c) {
    elementData = c.toArray();
    size = elementData.length;
    // c.toArray might (incorrectly) not return Object[] (see 6260652)
    if (elementData.getClass() != Object[].class)
    elementData = Arrays.copyOf(elementData, size, Object[].class);

    In Arrays:
    public static List asList(T… a) {
    return new ArrayList(a);

    ArrayList(E[] array) {
    if (array==null)
    throw new NullPointerException();
    a = array;