Java .isEmpty()

The .isEmpty() method is an inbuilt method of the Queue interface in Java that returns true if the queue contains no elements, and false otherwise. It is inherited from the Collection interface and provides a convenient way to check if a queue is empty before performing operations that require elements. The method has O(1) time complexity in most implementations, making it an efficient way to validate queue state.

Syntax

queueName.isEmpty()

Parameters:

• The .isEmpty() method does not accept any parameters.

Return value:

The .isEmpty() method returns a boolean value:

• true if the queue contains no elements
• false if the queue contains one or more elements

Example 1: Basic Usage of .isEmpty()

In this example, .isEmpty() is used to check if a queue is empty before and after adding elements:

import java.util.LinkedList;
import java.util.Queue;

public class Main {
  public static void main(String[] args) {
    Queue<String> queue = new LinkedList<String>();

    // Check if queue is empty initially
    System.out.println("Is queue empty? " + queue.isEmpty());
    System.out.println("Queue contents: " + queue);

    // Add elements to the queue
    queue.offer("First");
    queue.offer("Second");
    queue.offer("Third");

    // Check if queue is empty after adding elements
    System.out.println("Is queue empty after adding elements? " + queue.isEmpty());
    System.out.println("Queue contents: " + queue);
    System.out.println("Queue size: " + queue.size());

    // Remove all elements
    queue.poll();
    queue.poll();
    queue.poll();

    // Check if queue is empty after removing all elements
    System.out.println("Is queue empty after removing all elements? " + queue.isEmpty());
    System.out.println("Queue contents: " + queue);
  }
}

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The output of this code is:

Is queue empty? true
Queue contents: []
Is queue empty after adding elements? false
Queue contents: [First, Second, Third]
Queue size: 3
Is queue empty after removing all elements? true
Queue contents: []

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Example 2: Using .isEmpty() in a Loop

This example demonstrates how .isEmpty() can be used to safely process all elements in a queue:

import java.util.LinkedList;
import java.util.Queue;

public class Main {
  public static void main(String[] args) {
    Queue<Integer> numbers = new LinkedList<Integer>();

    // Add some numbers to the queue
    numbers.offer(10);
    numbers.offer(20);
    numbers.offer(30);
    numbers.offer(40);
    numbers.offer(50);

    System.out.println("Initial queue: " + numbers);
    System.out.println("Processing all elements in the queue:");

    int elementCount = 0;
    // Process all elements until queue is empty
    while (!numbers.isEmpty()) {
      Integer number = numbers.poll();
      elementCount++;
      System.out.println("Processing element #" + elementCount + ": " + number);
      System.out.println("Remaining elements: " + numbers.size());
    }

    System.out.println("Queue is now empty: " + numbers.isEmpty());
    System.out.println("Total elements processed: " + elementCount);
  }
}

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The output of this code is:

Initial queue: [10, 20, 30, 40, 50]
Processing all elements in the queue:
Processing element #1: 10
Remaining elements: 4
Processing element #2: 20
Remaining elements: 3
Processing element #3: 30
Remaining elements: 2
Processing element #4: 40
Remaining elements: 1
Processing element #5: 50
Remaining elements: 0
Queue is now empty: true
Total elements processed: 5

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