Internal implementation of ArrayList

An ArrayList is a very common data structure used by Java developers. It is an implementation of the List interface and is used to store a list of values/ objects. Unlike normal arrays in Java, it is a dynamic data structure which means its size dynamically increases when its current capacity is exhausted.

The AbstractList class

The ArrayList extends the AbstractList class, which provides a skeletal implementation of the List interface to provides random access to the objects in the list. For sequential access AbstractSequentialList can be used, since ArrayList provides random access it extends the AbstractList class.

The AbstractList class also provides implementations of Iterator and ListIterator on top of random access methods like get(), set(), add() and remove(), so that the programmer does not have to provide his own implementation while implementing a List.

Internal Storage and Growth Policy

An ArrayList stores data in an internal array, which it initializes to a default size of 10, if not otherwise specified in the constructor.

transient Object[] elementData;

private static final int DEFAULT_CAPACITY = 10;

It dynamically increases size of the internal array when the available size is exhausted. Shown below is the code snippet from ArrayList class which is used to grow the internal array size.

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;
elementData = Arrays.copyOf(elementData, newCapacity); } }

Note that the growth policy may differ with different versions of JDK, the above code shows the growth policy of ArrayList in JDK 6.


ArrayList is roughly similar to Vector, except that it is unsynchronized. If an ArrayList is supposed to be used in a multi-threaded application it should be synchronized externally.

To synchronize an ArrayList we can use the Collections.synchronizedList() method as shown below:

List myList = Collections.synchronizedList(new ArrayList());

It maintains a ‘modCount’ variable, declared in AbstractList, to keep track of the structural modifications made to it. The modCount increases each time the ArrayList is modified structurally. A structural modification is any operation that adds or deletes one or more elements in the ArrayList.

The ‘modCount’ serves as a version that keeps track of modification to the ArrayList and is used to throw ConcurrentModificationException in a multi-threaded environment, when any thread tries to modify the ArrayList, while another thread is iterating on it.


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