Queue - Generic Queue Implementation in Java

A fully featured generic Queue implementation in Java using a singly linked list.

This queue follows the FIFO (First In, First Out) principle and supports common operations as well as useful utility methods for flexibility and convenience.

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Table of Contents

• Queue - Generic Queue Implementation in Java
  • Table of Contents
  • Features
  • Usage
  • Running Tests
  • Design Notes
  • Example
  • Author
  • License
  • Contact

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Features

• Generic: works with any type (Queue<T>)
• Core queue operations:
  • enqueue(T item) — add an element to the rear of the queue
  • dequeue() — remove and return the front element
  • peek() — return (without removing) the front element
  • peekLast() — return (without removing) the rear element
  • isEmpty() — check if the queue is empty
  • size() — number of elements in the queue
• Iterable: supports iteration from front to rear with enhanced for-loops
• Utility methods:
  • clear() — empties the queue
  • contains(T item) — checks if the queue contains a given element (supports null)
  • remove(T item) — removes the first occurrence of a specified element
  • offer(T item) — adds an element (alias for enqueue)
  • poll() — removes and returns the front element or returns null if empty
  • toArray() — returns an array of elements in FIFO order
  • clone() — creates a shallow copy of the queue
  • reverse() — reverses the order of elements in the queue
• Properly overrides equals(Object) and hashCode()
• Throws NoSuchElementException when dequeuing or peeking from an empty queue
• Well documented with Javadoc comments

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Usage

Queue<Integer> queue = new Queue<>();
queue.enqueue(10);
queue.enqueue(20);
queue.enqueue(30);

System.out.println(queue.peek());  // Output: 10

while (!queue.isEmpty()) {
    System.out.println(queue.dequeue());  // Dequeues and prints: 10, 20, 30
}

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Running Tests

The implementation comes with comprehensive unit tests using JUnit 5.

To run the tests:

1. Ensure you have JUnit 5 set up in your build system (Maven, Gradle, or your IDE).
2. Run QueueTest.java as a JUnit test suite.
3. Tests cover normal operations, edge cases (empty queue, null elements), and utility methods.

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Design Notes

• Internally implemented with a singly linked list (Node<T>) for dynamic size and efficient enqueue/dequeue.
• Iterator returns elements from front to rear.
• Supports null elements safely in methods like contains(), equals(), and hashCode().
• clone() creates a shallow copy preserving element order.
• reverse() modifies the queue in-place reversing the element order.

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Example

Queue<String> names = new Queue<>();
names.enqueue("Alice");
names.enqueue("Bob");
names.enqueue("Charlie");

names.reverse();

for (String name : names) {
    System.out.println(name);
}
// Prints:
// Charlie
// Bob
// Alice

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Author

trigologiaa

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License

This project is released under the MIT License. Feel free to use, modify, and distribute.

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Contact

For questions or contributions, please open an issue or contact the author.