.set()

The .set() method replaces an element at a specified position with another element in an ArrayList instance. After execution, the replaced element is returned.

arrayListInstance.set(index, newElement);

An element at a specified index is replaced in an arrayListInstance with the newElement.

Note: The newElement must be of the same data type as the rest of the elements in arrayListInstance. Otherwise, an error will occur.

In the example below, an empty ArrayList instance studentList is created and can hold String-type elements. Next, a few elements are added with the .add() method. Lastly, two students are replaced with the .set() method:

import java.util.ArrayList;

public class Students {
  public static void main(String[] args) {

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

    // Add new values to the studentList
    studentList.add("John");
    studentList.add("Lily");
    studentList.add("Samantha");
    studentList.add("Tony");

    // `.set()` method returns replaced element's value
    String replacedStudentOne = studentList.set(1, "David");
    String replacedStudentTwo = studentList.set(2, "George");

    // Output updated ArrayList and replaced elements
    System.out.println("Updated ArrayList: " + studentList);
    System.out.println("Replaced Elements: " + replacedStudentOne + " and " + replacedStudentTwo);
  }
}

The output should look like this:

Updated ArrayList: [John, David, George, Tony]
Replaced Elements: Lily and Samantha