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 t

Answer 1

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  int getArrayListCapacity(ArrayList arrayList) {
        try {
            final E[] elementData = (E[]) field.get(arrayList);
            return elementData.length;
        } catch (Exception e) {
            throw new RuntimeException(e);
        }

    }
}