Java ArrayList set() Method

The set() method is a fundamental operation in the Java ArrayList class. It allows you to replace an element at a specific position in the list with a new value, making it essential for updating or modifying the contents of an ArrayList without altering its structure.

What Is the set() Method in ArrayList?

The set() method replaces the element at a given index with a new specified element.
It is useful when you want to update the value at a particular position while preserving the order and size of the list.
The method returns the element that was replaced.

Syntax

public E set(int index, E element)

Where:

index is the position of the element to replace (zero-based).
element is the new value to store at that position.
E is the type of elements contained in the list.

Parameters

Parameter	Description
`index`	The position of the element to replace. Must be between `0` and `size() - 1`.
`element`	The new value to be assigned at the given index.

Return Value

Returns the element that was previously at the specified position.

Exceptions

IndexOutOfBoundsException: Thrown if the specified index is less than 0 or greater than or equal to the list size.
No checked exceptions for standard usage.

How Does set() Work Internally?

Validates that the index is within bounds.
Sets the value at the specified index to the new element.
Returns the old element that was replaced.

Examples of the set() Method

1. Replace an Element by Index

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> cars = new ArrayList<>();
        cars.add("Volvo");
        cars.add("BMW");
        cars.add("Ford");
        cars.add("Mazda");

        // Replace the first element (index 0)
        String oldCar = cars.set(0, "Opel");

        // Output: [Opel, BMW, Ford, Mazda]
        System.out.println("Updated list: " + cars);

        // Output: Volvo
        System.out.println("Replaced element: " + oldCar);
    }
}

Output:

Updated list: [Opel, BMW, Ford, Mazda]
Replaced element: Volvo

2. Update Multiple Elements

import java.util.ArrayList;

public class Example {
    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<>();
        numbers.add(1); 
        numbers.add(2);
        numbers.add(3);
        numbers.add(4); 
        numbers.add(5);

        // [1, 2, 3, 4, 5]
        System.out.println("Before: " + numbers);

        // Update elements at positions 0, 2, 4
        numbers.set(0, 10);
        numbers.set(2, 30);
        numbers.set(4, 50);

        // [10, 2, 30, 4, 50]
        System.out.println("After: " + numbers); 
    }
}

3. set() vs add() (Key Difference)

import java.util.ArrayList;

public class Comparison {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("a"); list.add("b"); list.add("c");

        list.set(1, "z");    // Replaces element at index 1 ("b") with "z"
        System.out.println("After set: " + list); // [a, z, c]

        list.add(1, "y");    // Inserts "y" at index 1, shifts elements right
        System.out.println("After add: " + list); // [a, y, z, c]
    }
}