How to Add an Object to an ArrayList at a Specified Index in Java?

How do I add an object to a Java ArrayList at a specified index?

ArrayList<Object> list = new ArrayList<Object>();

In Java, the ArrayList class allows you to manage dynamically resizable arrays easily. However, when adding elements at specified indices, you must ensure that the indices are within the current bounds of the ArrayList. This guide explains how to add elements to an ArrayList without causing an IndexOutOfBoundsException.

ArrayList<Object> list = new ArrayList<Object>();

// Fill the ArrayList with placeholders (optional)
for (int i = 0; i < 5; i++) {
    list.add(null);
}

// Now add objects to the specific indices as needed
list.set(1, new Object()); // Set an object at index 1
list.set(3, new Object()); // Set an object at index 3
list.set(2, new Object()); // Set an object at index 2

Causes

• Attempting to add an element at an index that is greater than the current size of the ArrayList.
• Using an empty ArrayList and trying to add at any index other than 0.
• Not initializing the ArrayList with sufficient elements if specific indices are required.

Solutions

• Use the "add(int index, E element)" method for inserting elements directly at a specified index, ensuring that the index is between 0 and the current size of the list. To avoid an IndexOutOfBoundsException, the index must be less than or equal to the size of the list and not negative.
• You can fill the ArrayList with placeholder objects (e.g., null values or default instances) until it reaches your desired size if you want to insert at specific positions while maintaining order. However, initializing values ahead of time is usually more efficient.

Mistake: Trying to add an element at an index that exceeds the current size of the ArrayList.

Solution: Always check the current size of the ArrayList before trying to insert at a specific index.

Mistake: Using the 'add' method for setting an element at a specific index without initializing the list with enough elements.

Solution: Use 'set' instead of 'add' if you're replacing elements or use placeholders to fill the ArrayList.