ArrayList ensureCapacity() Method in Java with Examples

The ensureCapacity() method of the ArrayList class is used to increase the internal capacity of an ArrayList so that it can accommodate a specified minimum number of elements. Preallocating capacity helps improve performance by reducing the number of internal array reallocations and copying operations when adding a large number of elements.

• Does not change the current size of the ArrayList.
• Does not remove or modify existing elements.

import java.util.ArrayList;

public class GFG {
    public static void main(String[] args) {
      
        // create an ArrayList of Integers
        ArrayList<Integer> l = new ArrayList<>();

        // preallocate capacity for 5 elements
        l.ensureCapacity(5);

        // Add elements to the ArrayList
        for (int i = 1; i <= 5; i++) {
            l.add(i);
        }

        System.out.println(l);
    }
}

[1, 2, 3, 4, 5]

Explanation: In the above example, we preallocate capacity for an ArrayList of integers before adding 5 elements to optimize memory usage.

Syntax

public void ensureCapacity(int minCapacity)

• Parameter: minCapacity: The minimum desired capacity for the ArrayList.
• Return Value: The ensureCapacity() method does not return any value (void).

Example: Using the ensureCapacity() method to preallocate space for a String ArrayList and to add elements.

import java.util.ArrayList;

public class GFG {
    public static void main(String[] args) {
      
        // Create an ArrayList of Strings
        ArrayList<String> n = new ArrayList<>();

        // Preallocate capacity for 7 elements
        n.ensureCapacity(7);

        // Add elements to the ArrayList
        n.add("Ram");
        n.add("Shyam");
        n.add("Hari");

        System.out.println(n);
    }
}

[Ram, Shyam, Hari]

Explanation: In the above example, we preallocate capacity for an ArrayList of strings and then add a few names to the list.

Example: Program to demonstrate the use of ensureCapacity() with a large dataset

import java.util.ArrayList;

public class GFG {
    public static void main(String[] args) {
      
        // Create an ArrayList of Integers
        ArrayList<Integer> l = new ArrayList<>();

        // Preallocate capacity for 200 elements
        l.ensureCapacity(200);

        // Add 200 elements to the ArrayList
        for (int i = 1; i <= 200; i++) {
            l.add(i);
        }

        System.out.println("Size of an ArrayList: " + l.size());
    }
}

Size of an ArrayList: 200

Explanation: In the above example, we preallocate the capacity for an ArrayList to handle a large dataset of 200 elements.

Difference Between ensureCapacity() and trimToSize()

Advantages of ensureCapacity() Method

• Improves performance by reducing resizing operations.
• Minimizes internal array copying.
• Efficient for handling large datasets.
• Helps optimize memory allocation when the required size is known.
• Reduces the overhead of repeatedly expanding the ArrayList.