How to Retrieve the Capacity of an ArrayList in Java?

Question

How can I determine the capacity of an ArrayList in Java?

// The following code helps demonstrate how to check the capacity of an ArrayList in Java:
import java.lang.reflect.Field;
import java.util.ArrayList;

public class ArrayListCapacity {
    public static void main(String[] args) throws Exception {
        ArrayList<Integer> list = new ArrayList<>(10);
        // Add elements to the list
        list.add(1);
        list.add(2);

        // Get capacity using reflection
        int capacity = getArrayListCapacity(list);
        System.out.println("Current capacity of ArrayList: " + capacity);
    }

    private static int getArrayListCapacity(ArrayList<?> list) throws Exception {
        Field field = ArrayList.class.getDeclaredField("elementData");
        field.setAccessible(true);
        Object[] elementData = (Object[]) field.get(list);
        return elementData.length;
    }
}

Answer

In Java, the `ArrayList` does not provide a direct method to retrieve its capacity. However, by using reflection, it is possible to access the underlying array and determine its capacity. The capacity refers to the size of the internal array used to store the elements of the ArrayList, while the size is the total number of elements added to the ArrayList.

// The following code helps demonstrate how to check the capacity of an ArrayList in Java:
import java.lang.reflect.Field;
import java.util.ArrayList;

public class ArrayListCapacity {
    public static void main(String[] args) throws Exception {
        ArrayList<Integer> list = new ArrayList<>(10);
        // Add elements to the list
        list.add(1);
        list.add(2);

        // Get capacity using reflection
        int capacity = getArrayListCapacity(list);
        System.out.println("Current capacity of ArrayList: " + capacity);
    }

    private static int getArrayListCapacity(ArrayList<?> list) throws Exception {
        Field field = ArrayList.class.getDeclaredField("elementData");
        field.setAccessible(true);
        Object[] elementData = (Object[]) field.get(list);
        return elementData.length;
    }
}

Causes

Understanding the difference between size and capacity of the ArrayList.
Need for performance optimization during extensive operations with large data.

Solutions

Use reflection to access the private field of the ArrayList that stores the array elements.
Create a utility method to encapsulate the reflection code for better reusability.

Common Mistakes

Mistake: Attempting to directly call a method to get the capacity of ArrayList.

Solution: Understand that the ArrayList API does not provide a capacity method; instead, use reflection.

Mistake: Forgetting to handle exceptions when using reflection.

Solution: Wrap reflection calls in try-catch blocks to appropriately handle exceptions.

Helpers

Java ArrayList capacity
Retrieve ArrayList capacity in Java
ArrayList capacity vs size
Java programming
working with ArrayLists in Java