Java memory allocation in Arraylist

Question

How does java handle getting new memory space when adding a new element in Arraylist? For example, there is no free space after the list.

Tx

• rocketboy: why not look at the sourcecode!?
• Sotirios Delimanolis: These are JVM native calls. It's not implemented in Java.
• Steve Kuo: Since when is ArrayList "not implemented in Java" ?

Answer 1

So when you add element in ArrayList internally it calls following method :

/**
 * Increases the capacity of this <tt>ArrayList</tt> instance, if
 * necessary, to ensure that it can hold at least the number of elements
 * specified by the minimum capacity argument.
 *
 * @param minCapacity the desired minimum capacity
 */
public void ensureCapacity(int minCapacity) {
 modCount++;
 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);
 }
 }

And in above method , Arrays.copyOf method further reaches to following native method ,

 public static native void arraycopy(Object src, int srcPos,
 Object dest, int destPos,
 int length);

So for java you have to see openjdk native method code.

Answer 2

Basically Java's ArrayList usually ensures enough space in an array for the elements to fit. If the array is not long enough then it provide more space for them: create new array with the double size of the original array and copies the elements into it. (DEFAULT_CAPACITY = 10)

public void ensureCapacity(int minCapacity){
 int current = data.length;
 if (minCapacity > current)
 {
 E[] newData = (E[]) new Object[Math.max(current * 2, minCapacity)];
 System.arraycopy(data, 0, newData, 0, size);
 data = newData;
 }
}

You can see it from the implementation ensureCapacity method of Arraylist:

http://developer.classpath.org/doc/java/util/ArrayList-source.html

If it can not provide enough space then it will throw an "java.lang.OutOfMemoryError: Java heap space"

You can check this here: http://javarevisited.blogspot.com/2011/09/javalangoutofmemoryerror-permgen-space.html