How to Initialize an ArrayList with Default Values in Java

How to initialize an ArrayList with all elements set to zero in Java?

In Java, an ArrayList does not automatically populate its elements when initialized with a specific capacity. Instead, it starts as empty, which is why accessing any element throws an IndexOutOfBoundsException. To create an ArrayList filled with default values such as zeros, you will need to add the values explicitly after initialization.

// Initialize an ArrayList of a specific size filled with zeros
int size = 60;
ArrayList<Integer> list = new ArrayList<>(size);
for (int i = 0; i < size; i++) {
    list.add(0);
}

// Now you can safely access the list
int value = list.get(5); // This will return 0 without throwing an exception.

Causes

• The ArrayList is initialized with a specified capacity but does not contain any elements, leading to an IndexOutOfBoundsException when trying to access an element that does not exist.
• In Java, unlike C++, no default values are assigned to the array elements during initialization.

Solutions

• You can initialize the ArrayList and manually add zeros in a loop until you reach the desired size.
• Alternatively, consider using a standard array if you need fixed-size arrays with default values.

Mistake: Assuming that the ArrayList will automatically fill elements with zeros when initialized with a capacity.

Solution: Always add elements explicitly, as shown in the code snippet.

Mistake: Trying to access an index before adding elements leads to IndexOutOfBoundsException.

Solution: Ensure that you add elements to the ArrayList before accessing them.