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Java Abstract Data Structures (ADT) 20-11-2023

9 min read

The Java Queue Interface

  1. Definition and Use: Queue is used to represent a sequence of elements in which operations are performed in a First-In-First-Out (FIFO) manner. It’s primarily used for holding elements prior to processing.

  2. Primary Operations:

    • enqueue (offer): Adds an element to the end of the queue. Similar to push in python.
    • dequeue (poll): Removes and returns the head of the queue. Returns null if the queue is empty. Similar to pop in Python
    • peek: Retrieves, but does not remove, the head of the queue, returning null if the queue is empty.

  3. Key Characteristics:

    • Queues typically, but not necessarily, order elements in a FIFO (first-in-first-out) manner. There are exceptions like priority queues.
    • They do not allow null elements.

  4. Implementations: Common implementations in the Java Collections Framework include LinkedList, PriorityQueue, and ArrayDeque. Each has its own specific characteristics:

    • LinkedList: Doubly-linked list implementation, can be used as a Queue.
    • PriorityQueue: Elements are ordered according to their natural ordering or by a Comparator provided at queue construction time.
    • ArrayDeque: Resizable-array implementation, can be used as both a stack and a queue.

  5. Use Cases: Queues are often used in scenarios like breadth-first search in algorithms, handling asynchronous data (like in IO Buffers), or in implementing resource sharing (like CPU scheduling).

  6. Thread Safety: The basic implementations like LinkedList and PriorityQueue are not thread-safe. For a thread-safe Queue, ConcurrentLinkedQueue or BlockingQueue implementations like ArrayBlockingQueue and LinkedBlockingQueue are used.

This summary provides an overview of the Queue interface in Java, covering its basic usage, operations, characteristics, and common implementations. Remember, specific behaviors can vary slightly based on the concrete implementation used.

Double-ended Queues AKA Deques

Commonly known as Deques, are a type of queue that allows insertion and removal of elements at both ends. Here’s a detailed overview:

  1. Definition: A Deque (pronounced “deck”) is a linear collection that supports element insertion and removal at both ends. It can be used both as a queue (FIFO - First-In-First-Out) and as a stack (LIFO - Last-In-First-Out).

  2. Java Interface: In Java, Deque is an interface that extends the Queue interface. It is part of the Java Collections Framework.

  3. Primary Operations:

    • Addition:
      • At the front: addFirst() and offerFirst()
      • At the rear: addLast() and offerLast()
    • Removal:
      • From the front: removeFirst() and pollFirst()
      • From the rear: removeLast() and pollLast()
    • Retrieval (without removal):
      • First element: getFirst() and peekFirst()
      • Last element: getLast() and peekLast()

  4. Characteristics:

    • Deques can be either bounded or unbounded. Bounded deques have a fixed size limit.
    • They do not support concurrent access by multiple threads without external synchronization.
    • Null elements are generally prohibited in deque implementations.

  5. Common Implementations:

    • ArrayDeque: Resizable-array implementation of the Deque interface. It has no capacity restrictions and is faster than Stack and LinkedList.
    • LinkedList: Implements both List and Deque interfaces and provides a linked-list-based implementation.

  6. Use Cases:

    • Deques are particularly useful in scenarios where you need to frequently add or remove elements from both ends of the collection, such as in certain algorithm implementations (e.g., sliding window algorithms).
    • They are also used in applications like undo mechanisms, where you need to add or remove operations from both ends.

  7. Performance Considerations:

    • Operations like insert, remove, and retrieve operations run in constant time (O(1)) for ArrayDeque.
    • In LinkedList, operations at the ends are constant time, but others are linear time due to traversal.

Deques provide a versatile and efficient means of handling collections where both ends are utilized for insertion and removal, offering more flexibility than standard queues or stacks.

  1. Methods that Throw Exceptions: These methods (addFirst, removeFirst, getFirst, etc.) are used when failure or an empty deque is an exceptional condition. They throw exceptions such as NoSuchElementException or IllegalStateException when the operation cannot be completed, like adding to a full deque or removing from an empty one. These are suitable when you need to handle such conditions explicitly.
  2. Methods that Return Special Values:
    These methods (offerFirst, pollFirst, peekFirst, etc.) return special values (usually null or false) to indicate failure or an empty deque. They don’t throw exceptions, making them useful in situations where failure is a normal, expected occurrence, allowing the program to continue running without the need for additional exception handling.

The choice between these two types of methods depends on how your program should respond to certain conditions like a full or empty deque. Exception-throwing methods are best when such conditions are not expected and should be handled as errors, while methods returning special values are more appropriate when these conditions are part of normal program flow.

How the Comparator interface contributes to Queues

The Comparator interface in Java plays a crucial role in determining the order of elements within certain types of queues, particularly in priority queues. Here’s how it works:

  1. Purpose of Comparator: A Comparator is used to define a custom order for objects, which might not have a natural ordering or when you want an order different from their natural order. It is especially useful for objects that do not implement the Comparable interface.

  2. Defining a Comparator: A Comparator is implemented by overriding its compare method. This method takes two arguments (objects of the type being compared) and returns:

    • A negative integer if the first argument is less than the second.
    • Zero if the first argument is equal to the second.
    • A positive integer if the first argument is greater than the second.

  3. Usage in Queues: In queues like PriorityQueue, a Comparator can be provided at the time of queue instantiation. This Comparator determines how the elements of the queue are ordered. For example:

PriorityQueue<MyObject> queue = new PriorityQueue<>(new MyObjectComparator());

Here, MyObjectComparator is a user-defined class that implements Comparator and defines the order for MyObject instances within the queue.

  1. Effect on Queue Behavior:

    • In a priority queue, elements are ordered based on their priority, and the priority is determined by the Comparator.
    • The queue’s poll or remove operations retrieve the element with the highest priority, as defined by the Comparator.

  2. Fallback to Natural Ordering: If no Comparator is provided, the queue falls back to the natural ordering of its elements. This requires that the elements implement the Comparable interface.

  3. Custom Ordering: By using a Comparator, you can implement complex ordering rules that might consider multiple fields or specific calculations, giving you fine-grained control over how elements are prioritized in the queue.

In summary, the Comparator interface allows you to define a specific ordering for elements in a queue, especially in priority queues where the order of elements is significant. By implementing the compare method, you can control how elements are prioritized and retrieved from the queue.

Abstract lists and sets in Java

AbstractList:

  • AbstractList is an abstract superclass that implements most of the List interface.
  • It is designed to make it easier to create concrete implementations of lists.
  • AbstractList provides a skeletal implementation of the List interface, so to create a list implementation, you only need to implement a few methods (get(int index), set(int index, E element), add(int index, E element), and remove(int index)).
  • It includes implementations for methods like add(E e), equals(Object o), and hashCode() that use the list’s iterator.

AbstractSet:

  • AbstractSet is an abstract superclass that implements most of the Set interface.
  • It simplifies the process of implementing the Set interface by providing a skeletal version.
  • AbstractSet ensures that the set cannot contain duplicate elements and provides implementations for methods like equals(Object o) and hashCode() based on the iterator provided by the set.

Difference between AbstractList and ArrayList:

  • AbstractList is an abstract class that cannot be instantiated on its own and is intended to be subclassed when creating a custom list implementation.
  • ArrayList is a concrete implementation of the List interface that uses a dynamic array to store its elements and is ready to use.
  • ArrayList extends AbstractList and provides complete implementations for all list operations, allowing for fast random access, adding, and searching.
  • ArrayList is part of the Java Collections Framework and is typically used when a resizable array is needed.

So, while AbstractList provides a template for creating list implementations, ArrayList is an out-of-the-box implementation of the List interface, optimized for performance and general use.

Doubly LinkedList Insertion & Deletion keynotes

  1. Inserting into a Doubly Linked List:

    • To insert a new element, create a new node.
    • Adjust the next pointer of the new node to point to the current node’s next.
    • Adjust the previous pointer of the current node’s next to point to the new node.
    • Set the next pointer of the current node to the new node.
    • Set the previous pointer of the new node to the current node.
    • Be careful with the head and tail cases.

  2. Deleting from a Doubly Linked List:

    • To delete an element, locate the node you want to remove.
    • Adjust the next pointer of the node’s previous to point to the node’s next.
    • Adjust the previous pointer of the node’s next to point to the node’s previous.
    • Ensure proper handling of head and tail cases where previous or next might be null.

  3. Modifying Pointers in a Doubly Linked List:

    • Always be cautious when changing the pointers to maintain the integrity of the list.
    • Update pointers in the correct order to avoid losing reference to parts of the list.
    • For insertion, update the pointers of the surrounding nodes after setting the pointers of the new node.
    • For deletion, ensure other nodes’ pointers are correctly reassigned before discarding the node to be deleted.

Considerations for pointers:

  • Head Pointers: Should always point to the first node of the list or be null if the list is empty.
  • Tail Pointers: Should always point to the last node of the list or be null if the list is empty.
  • Previous Node’s Next: In insertion, this should be updated to point to the new node.
  • Next Node’s Previous: In both insertion and deletion, this should point to the correct node before or after the operation.
  • New Node’s Next and Previous: Should point respectively to the nodes that will be after and before it in the list after insertion.

Understanding these key points will help you navigate the operations in a doubly linked list and answer typical quiz questions effectively.

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