How to get the capacity of the ArrayList in Java?

Question

Its known that Java ArrayList is implemented using arrays and initializes with capacity of 10 and increases its size by 50% . How to get the current ArrayList capacity not the Size of the ArrayList.

Thx

Answer 1

I don't think this is possible. What is your use case? I believe C# ArrayLists have a .capacity property, but the Java ArrayList class doesn't expose this information.

You have the constructor that takes an initial capacity argument, and you have the ensureCapacity() method which you could use to reduce the amount of incremental reallocation.

You also have the trimToSize() method you can use if you are really worried about memory usage.

Answer 2

You can get it by reflection:

public abstract class ArrayListHelper {      static final Field field;     static {         try {             field = ArrayList.class.getDeclaredField("elementData");             field.setAccessible(true);         } catch (Exception e) {             throw new ExceptionInInitializerError(e);         }     }      @SuppressWarnings("unchecked")     public static <E> int getArrayListCapacity(ArrayList<E> arrayList) {         try {             final E[] elementData = (E[]) field.get(arrayList);             return elementData.length;         } catch (Exception e) {             throw new RuntimeException(e);         }      } }