Understanding the Internal Mechanics of Java's ArrayList

How Arraylist works internally?

An ArrayList in Java is a resizable array implementation of the List interface. It is part of the Java Collections Framework and provides a way to store a dynamically sized collection of elements. Here's how it works internally:

Internal Structure

1. Backing Array:

   • An ArrayList uses an array to store its elements. When you create an ArrayList, it initializes an internal array (often called the "backing array") to hold the elements.
   • The initial capacity of the array can be specified when creating the ArrayList, or it defaults to a certain size (usually 10).

2. Size and Capacity:

   • Size: This is the number of elements currently stored in the ArrayList.
   • Capacity: This is the total number of elements that the backing array can hold before needing to resize. The capacity is always greater than or equal to the size.

Dynamic Resizing

When you add elements to an ArrayList, if the size exceeds the current capacity, the ArrayList needs to resize the backing array. This is how it works:

1. Doubling the Capacity: When the current capacity is reached, the ArrayList creates a new array that is typically double the size of the old array.
2. Copying Elements: It then copies the elements from the old array to the new array.
3. Updating References: Finally, it updates the reference to point to the new array and sets the new capacity.

Operations

• Adding Elements:

  • When you add an element using add(), it checks if there is enough capacity. If there is, it simply places the element in the next available index. If not, it resizes the array as described above.

• Accessing Elements:

  • Accessing an element by index (using get(index)) is a constant-time operation, O(1), because it directly accesses the array at the specified index.

• Removing Elements:

  • When you remove an element, the ArrayList shifts all subsequent elements to fill the gap, which is an O(n) operation in the worst case.

• Iterating:

  • Iterating over an ArrayList can be done using a for-loop, enhanced for-loop, or an iterator. The underlying array structure allows for efficient iteration.

Performance Considerations

• Amortized Time Complexity: The average time complexity for adding an element is O(1) due to the amortized cost of resizing.
• Memory Overhead: Since the ArrayList may allocate more space than it currently needs (to accommodate future growth), it can lead to memory overhead.
• Thread Safety: ArrayList is not synchronized, meaning it is not thread-safe. If multiple threads access an ArrayList concurrently, and at least one of the threads modifies it, it must be synchronized externally.

Summary

In summary, an ArrayList is a flexible and efficient way to manage a list of elements in Java. It uses a dynamic array to store elements, automatically resizing when necessary, and provides fast access and iteration capabilities. However, it is important to be aware of its performance characteristics and limitations, especially in multi-threaded environments.