What Is an ArrayList Data Structure?


Scott Campbell

An ArrayList is a data structure in Java that provides a dynamic array-like implementation. It is part of the Java Collections Framework and is commonly used when we need to store and manipulate a collection of elements.

An ArrayList can store objects of any type, including primitive types such as integers and characters. It allows us to add, remove, modify, and access elements based on their index. Unlike regular arrays, an ArrayList automatically resizes itself when elements are added or removed, making it more flexible and convenient to use.

Creating an ArrayList

To create an ArrayList in Java, we need to import the java.util.ArrayList package. Then, we can declare and initialize the ArrayList using the following syntax:

import java.ArrayList;

ArrayList<Type> arrayList = new ArrayList<>();

Here, “Type” can be replaced with the desired data type for the elements stored in the ArrayList. For example, if we want to store integers, we would replace “Type” with “Integer”. If we want to store strings, we would use “String”.

Adding Elements

Adding elements to an ArrayList is straightforward. We can use the add() method to append elements at the end of the list:


The element can be any valid object or value of the specified type. For example:

ArrayList<String> names = new ArrayList<>();

Accessing Elements

We can access individual elements in an ArrayList using their index. The index starts from 0 for the first element and goes up to size() – 1 for the last element. We can use the get() method to retrieve an element at a specific index:

Type element = arrayList.get(index);

For example, to retrieve the second element from the “names” ArrayList created earlier:

String secondName = names.get(1);

The variable secondName will now store the value “Bob”.

Removing Elements

To remove elements from an ArrayList, we can use the remove() method. It allows us to remove elements based on their index or directly by specifying the object itself.


If we want to remove an element at a specific index:


This will remove the first name, “Alice”, from the list.

ArrayList Size and Capacity

The size of an ArrayList refers to the number of elements currently stored in it. We can obtain it using the size() method:

int size = arrayList.size();

The capacity of an ArrayList refers to the number of elements it can hold without resizing. When we create an ArrayList, it has an initial capacity of 10 by default. However, as we add more elements beyond this capacity, it automatically increases its size.


  • An ArrayList is not synchronized by default, which means it is not suitable for concurrent access by multiple threads.
  • If we need synchronized access, we can use the Collections.synchronizedList() method to obtain a synchronized version of the ArrayList.

In conclusion, an ArrayList is a versatile data structure in Java that provides dynamic array-like behavior. It allows us to easily add, access, and remove elements.

With its resizing capability, it provides flexibility for handling collections of various sizes. Remember to import the java.ArrayList package and enjoy using this powerful data structure in your Java projects!

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