Queue offer vs add in Java

A Queue is a data structure that allows users to add elements in the First in First out manner. In Java, a queue is an interface present in the java.util package. It is a linear structure that allows ordered elements in the data structure.

The collection is open from both the ends, or the rear, and the front end. The property enables objects to be added at the front and removed from the rear end.

The structure offers various methods, and its description is in the code below.

import java.util.Queue;
import java.util.concurrent.ArrayBlockingQueue;

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

        Queue queue = new ArrayBlockingQueue(2);
        boolean value = false;
        value = queue.offer(1);
        System.out.println("Offer Returned Value: " + value);
        value = queue.offer(2);
        System.out.println("Offer Returned Value: " + value);
        value = queue.offer(3);
        System.out.println("Offer Returned Value: " + value);
      
        Queue blockingQueue = new ArrayBlockingQueue(2);
        blockingQueue.add(5);
        blockingQueue.add(10);
        blockingQueue.add(15); 

    }
}

The code block above contains both implementations of the add and offer method and provides the difference between both. The code block above instantiates a queue instance using the new keyword.

The first statement creates an object of the ArrayBlockingQueue with the fixed capacity of size two and the default access policy. It takes a size value as a parameter. It throws an IllegalArgumentException if the capacity is smaller than one.

The offer method inserts the specified element into this queue after checking the capacity restrictions. The function takes the parameter as the element to be added. This function returns true if the value is added to the end of the queue; else, false.

It throws a ClassCastException if the class of the specified element prevents it from getting added to it. The NullPointerException is called if the specified value is null. The IllegalArgumentException is thrown if some element property prevents it from being added to the queue.

The print statement next to the offer function prints the Boolean value true or false. Again, a new ArrayBlockingQueue is instantiated with the capacity of size two. It appends until two successful insertions and then throws an Exception trace.

This method takes a parameter as an element to add to the queue. It returns true each time once the successful addition is made; else, it throws an IllegalStateException if the value cannot be added.

The ClassCastException is thrown if the class of the specified element prevents it from addition. The NullPointerException if the specified value is null, and IllegalArgumentException if a property of this element prevents it from being added to the queue.

The only difference between the methods is that the offer method throws true or false if the addition is made. As opposed to this, the add method throws an exception when no more additions are possible in the queue.

Below is the resultant output from the code block above.

Offer Returned Value: true
Offer Returned Value: true
Offer Returned Value: false
Exception in thread "main" java.lang.IllegalStateException: Queue full
    at java.util.AbstractQueue.add(AbstractQueue.java:98)
    at java.util.concurrent.ArrayBlockingQueue.add(ArrayBlockingQueue.java:312)
    at F11.QueueAddVsOffer.main(QueueAddVsOffer.java:25)