ArrayList

The ArrayList class uses dynamic arrays that are resizable, unlike traditional fixed arrays. However, each element must still be of the same type. Elements can be added or removed at any time, making the ArrayList more flexible.

Some other important points about the ArrayList class include:

• It has the ability to contain duplicate elements.
• It maintains insertion order.
• It is non-synchronized and, therefore, not safe for multiple threading.
• It allows random access since arrays work on an index basis.
• The space/time complexity is a bit slower than a LinkedList due to the nature of adding/removing elements.

Syntax

This class is imported with the java.util package.

import java.util.ArrayList;

The syntax for defining an instance of ArrayList is similar to defining a traditional array. A key difference is that angle brackets <> are used instead of square brackets:

ArrayList<DataType> arrayListInstance = new ArrayList<DataType>();

Example

In the example below, a new ArrayList instance arr is defined. There are print statements throughout to show elements being added to and removed from arr before it is ultimately cleared:

// Import from the java.util package
import java.util.ArrayList;

public class Example {
  public static void main(String []args) {
    ArrayList<String> arr = new ArrayList<>();

    System.out.println(arr);  // Currently empty

    arr.add("hello");
    arr.add("India");

    System.out.println(arr);

    arr.remove("hello");

    System.out.println(arr.size());  // Prints size of ArrayList

    arr.clear();

    System.out.println(arr); // Empty again
  }
}

The output would be:

[]
[hello, India]
1
[]