# Find capacity of an ArrayList in Java. Size Vs Capacity in Java List example

This tutorial covers that ** what is size and capacity** of an **ArrayList** and **Vector **in java and also covers that how to **find the** **size and capacity** of an arraylist and vector using java code. You can also find the formula here that is used to calculate the capacity of an arraylist and vector.

** What is size?** The number of elements stored in the List .

**What is capacity ?** The current capacity of a List that means maximum number of elements it can hold currently. Initial capacity is decided by the integer number passed to Constructor of the ArrayList & Vector.

Vector uses the below formula to increase the capacity when it crosses the current capacity.

** int newCapacity = (capacityIncrement > 0) ? (oldCapacity + capacityIncrement) : (oldCapacity * 2) ** ———>**(A)**

ArrayList uses the below formula to increase the capacity.

**int newCapacity = (oldCapacity * 3)/2 + 1 ** ——–>**(B)**

Vector have methods **size() & capacity()** to return the current size & capacity where as ArrayList has **size()** method but it does not have **capacity()** method. Now let us find capacity of an ArrayList. ArrayList uses an internal array buffer (Object **elementData[]**) to store the elements. The capacity of the ArrayList is the length of this array buffer which can be returned by length property of an array. As **elementData[]** is a private array field in the ArrayList, how to access the private field and find the length?. We can use the **reflection** feature of Java to find the length which is the capacity of the arraylist.

package net.javaonline; import java.lang.reflect.Field; import java.util.*; public class CapacityArrayList { public static void main(String[] args) throws Exception { ArrayList<Integer> al = new ArrayList<Integer>(100); Vector<Integer> vector = new Vector<Integer> (100); for (int i = 0; i < 125; i++) { al.add(i); vector.add(i); } System.out.println("Array List : Size = "+ al.size() + " Capacity = "+ findCapacity(al)); System.out.println("Vector : Size = "+ vector.size() + " Capacity = "+ vector.capacity()); } //returns capacity of an ArrayList static int findCapacity(ArrayList<Integer> al) throws Exception { Field field = ArrayList.class.getDeclaredField("elementData"); field.setAccessible(true); return ((Object[]) field.get(al)).length; } }

**Output of the above program:**

D:\Test>java CapacityArrayList

Array List : Size = 125 Capacity = 151

Vector : Size = 125 Capacity = 200

**How the capacity is calculated ?**

Initial Capacity of both AL and Vector is 100;

125 elements are added which crosses the initial capacity.

1. now the capacity of ArrayList is calculated as follows

oldcapacity = 100;

newCapacity = (100*3)/2 +1 = 151. refer the formula B

2. capacity of Vector is calculated as follows

capacityIncrement=0;

so newCapacity = (100*2) = 200. refer the formula A

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