What is the CopyOnWriteArrayList.set method() in Java?

What is CopyOnWriteArrayList?

  • The CopyOnWriteArrayList is a thread-safe implementation of ArrayList, which does not require synchronization.
  • The whole content of the CopyOnWriteArrayList is duplicated on to a new internal copy when we use any of the altering methods, such as add() or delete(). Hence, the list can be iterated in a safe way, without any fear of losing data, while modifying the data simultaneously.
  • The data structure’s properties make it particularly helpful in situations when we iterate the data more frequently than we alter it.
  • If adding items is a typical activity, the CopyOnWriteArrayList isn’t the appropriate data structure to use because the extra copies will almost always result in poor performance.

set() is an instance method of the CopyOnWriteArrayList, which is used to replace the element at the specified position in the CopyOnWriteArrayList with the given element.

The set method is defined in the CopyOnWriteArrayList class. The CopyOnWriteArrayList class is defined in the java.util.concurrent package. To import the CopyOnWriteArrayList class, we check the following import statement:

import java.util.concurrent.CopyOnWriteArrayList;

Syntax

public E set(int index, E element)

Parameters

  • int index: This is the index of the element that needs to be replaced.
  • E element: This is the new element that needs to be stored at the given index.

Return value

This method returns the previous element at the specified position.

Code example

import java.util.concurrent.CopyOnWriteArrayList;
public class Main {
    public static void main(String[] args){
        CopyOnWriteArrayList<String> copyOnWriteArrayListObject = new CopyOnWriteArrayList<>();
        copyOnWriteArrayListObject.add("edpresso");
        copyOnWriteArrayListObject.add("educative");
        copyOnWriteArrayListObject.add("coding-interview");
        copyOnWriteArrayListObject.add("courses");
        System.out.println("List before modification - " + copyOnWriteArrayListObject);
        int index = 2;
        String newElement = "hello-world";
        String previousElement = copyOnWriteArrayListObject.set(index, newElement);
        System.out.printf("The previous element at index %s is '%s'.\n", index, previousElement);
        System.out.println("List after modification - " + copyOnWriteArrayListObject);
    }
}

Code explanation

  • Line 1: We import the CopyOnWriteArrayList class.
  • Line 6: We create an instance of the CopyOnWriteArrayList class and call it copyOnWriteArrayListObject.
  • Lines 8–11: We insert elements to the copyOnWriteArrayListObject, using the add() method.
  • Line 13: We print the original list called copyOnWriteArrayListObject.
  • Line 15: We define the index at which the element has to be replaced.
  • Line 16: We define the new value called newElement that needs to be stored.
  • Line 17: We use the set() method to replace the element at the index with the newElement. Then, the old element at the index called previousElement is returned.
  • Line 19: We print the previousElement found at the index.
  • Line 20: We print the modified copyOnWriteArrayListObject list.
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