Java ArrayList subList() method
Syntax and Examples

Introduction

Sometimes you need to work with a portion of an ArrayList instead of the entire list. The subList() method provides exactly that functionality. It allows you to create a view of a section of the original ArrayList without creating a new copy of the data itself, making it memory-efficient.

Syntax

    
    public List<E> subList(int fromIndex,
                              int toIndex)
    
  

Parameters

Parameter	Description
fromIndex	The index of the first element included in the sublist (inclusive).
toIndex	The index of the last element *excluded* from the sublist. It's one past the last element to be included.

Return Value

The subList() method returns a List<E> object, which is a view of the portion of the original ArrayList.

Important Considerations

• The returned sublist is linked to the original list. Modifications to the sublist will affect the original list and vice-versa.
• The indices used in subList() must be within the bounds of the original list (0 <= fromIndex <= toIndex <= size()). Otherwise, an IndexOutOfBoundsException is thrown.
• The sublist doesn’t create a new ArrayList; it's essentially a view into the original one.

Examples

Example 1: Basic Sublist Creation

This example demonstrates creating a basic sublist from an existing ArrayList.

    
    import java.util.ArrayList;
    import java.util.List;

    public class SubListExample1 {
        public static void main(String[] args) {
            // Create an ArrayList
            List<String> names = new ArrayList<>();
            names.add("Alice");
            names.add("Bob");
            names.add("Charlie");
            names.add("Ram");
            names.add("Eve");

            // Create a sublist from index 1 (inclusive) to index 3 (exclusive)
            List<String> subNames = names.subList(1, 3);

            // Print the original list and sublist
            System.out.println("Original List: " + names);
            System.out.println("Sublist: " + subNames);
        }
    }
  

    Original List: [Alice, Bob, Charlie, Ram, Eve]
    Sublist: [Bob, Charlie]

In this example, subList(1, 3) creates a view of the list containing elements at indices 1 and 2 (i.e., "Bob" and "Charlie"). Notice that index 3 is *not* included.

Example 2: Modifying the Sublist

This example shows how changes to the sublist affect the original list, and vice versa.

    
    import java.util.ArrayList;
    import java.util.List;

    public class SubListExample2 {
        public static void main(String[] args) {
            // Create an ArrayList
            List<Integer> numbers = new ArrayList<>();
            numbers.add(10);
            numbers.add(20);
            numbers.add(30);
            numbers.add(40);

            // Create a sublist from index 1 to index 3
            List<Integer> subNumbers = numbers.subList(1, 3);

            // Modify the sublist
            subNumbers.set(0, 25); // Changes the element at index 1 in the sublist
            subNumbers.add(35);  // Adds an element to the end of the sublist

            // Print both lists
            System.out.println("Original List: " + numbers);
            System.out.println("Sublist: " + subNumbers);
        }
    }
  

    Original List: [10, 25, 30, 40, 35]
    Sublist: [25, 30, 35]

When we modified the sublist (changed element at index 1 to 25 and added 35), these changes were also reflected in the original list. This demonstrates that a sublist is linked to its parent ArrayList.

Example 3: Handling IndexOutOfBoundsException

This example illustrates what happens when you provide invalid indices to subList().

    
    import java.util.ArrayList;
    import java.util.List;

    public class SubListExample3 {
        public static void main(String[] args) {
            // Create an ArrayList
            List<String> colors = new ArrayList<>();
            colors.add("Red");
            colors.add("Green");
            colors.add("Blue");

            try {
                // Attempt to create a sublist with invalid indices
                List<String> invalidSublist = colors.subList(0, 5); // IndexOutOfBoundsException will be thrown

                System.out.println("Sublist: " + invalidSublist);
            } catch (IndexOutOfBoundsException e) {
                System.out.println("Caught an exception: " + e.getMessage());
            }
        }
    }
  

    Caught an exception: toIndex = 5 greater than size = 3

Because we attempted to create a sublist with `toIndex` (5) exceeding the list's size (3), an IndexOutOfBoundsException was thrown. Always ensure your indices are valid.