LinkedHashSet - Notes By ShariqSP

LinkedHashSet in Java

LinkedHashSet is an implementation of the Set interface in Java that maintains the insertion order of elements. It is similar to HashSet but provides predictable iteration order, making it suitable for scenarios where iteration order is important.

Definition:

A LinkedHashSet is a collection that combines the properties of a HashSet and a linked list. It stores unique elements and maintains the order in which they were inserted into the set. Duplicate elements are not allowed, and null elements are permitted.

Methods:

  • add(E element): Adds the specified element to the set if it is not already present.
  • remove(Object obj): Removes the specified element from the set if it is present.
  • contains(Object obj): Returns true if the set contains the specified element.
  • isEmpty(): Returns true if the set contains no elements.
  • size(): Returns the number of elements in the set.
  • clear(): Removes all elements from the set.

Example:


            import java.util.LinkedHashSet;
            
            public class Main {
                public static void main(String[] args) {
                    // Create a LinkedHashSet of strings
                    LinkedHashSet set = new LinkedHashSet<>();
            
                    // Add elements to the set
                    set.add("Java");
                    set.add("Python");
                    set.add("C++");
            
                    // Display the elements
                    System.out.println("Elements in the set:");
                    System.out.println(set);
            
                    // Remove an element
                    set.remove("Python");
            
                    // Display the modified set
                    System.out.println("\nElements after removal:");
                    System.out.println(set);
                }
            }

In this example, a LinkedHashSet is created to store strings. Elements are added to the set using the add() method and removed using the remove() method.

Significance of Maintaining Order in LinkedHashSet

LinkedHashSet is a variant of HashSet that maintains the insertion order of elements. Unlike HashSet, which does not guarantee any specific order of elements, LinkedHashSet preserves the order in which elements were added to the set. This feature provides several advantages:

Order Preservation:

The primary significance of maintaining order in LinkedHashSet is that it allows predictable iteration order. When iterating over a LinkedHashSet, the elements are returned in the same order in which they were inserted. This predictability can be crucial in scenarios where the order of elements matters, such as processing elements in a specific sequence or maintaining the sequence of user inputs.

FIFO Behavior:

LinkedHashSet follows a first-in-first-out (FIFO) behavior for iteration. This means that the elements are returned in the same order as they were added, resembling the behavior of a queue. This FIFO behavior can be useful in scenarios where elements need to be processed in the order of their arrival or where maintaining a history of operations is important.

Linked List Structure:

Internally, LinkedHashSet maintains a doubly-linked list in addition to the hash table to preserve insertion order. This linked list structure ensures efficient iteration and maintains the order of elements without sacrificing the performance benefits of hashing for fast lookup and insertion.

Overall, the significance of maintaining order in LinkedHashSet lies in its ability to provide predictable iteration order and FIFO behavior, making it suitable for scenarios where order preservation is important.

Advantages and Disadvantages of LinkedHashSet

Advantages:

  • Order Preservation: LinkedHashSet maintains the insertion order of elements, ensuring predictable iteration order. This feature is useful when the order of elements matters, such as when processing elements in a specific sequence or maintaining the sequence of user inputs.
  • Efficient Iteration: LinkedHashSet provides efficient iteration over elements due to its linked list structure. Iteration performance remains consistent regardless of the size of the set, making LinkedHashSet suitable for scenarios where frequent traversal or iteration is required.
  • Implements Set Interface: LinkedHashSet implements the Set interface, providing methods for performing set operations such as union, intersection, and difference. It allows developers to work with sets in a familiar and standardized way, simplifying code maintenance and interoperability.
  • Combines Hashing and Linked List: Internally, LinkedHashSet maintains a hash table for fast lookup and a linked list for order preservation. This combination of data structures allows LinkedHashSet to provide both efficient insertion, deletion, and lookup operations and predictable iteration order.

Disadvantages:

  • Memory Overhead: LinkedHashSet requires additional memory to maintain the linked list structure for order preservation. This overhead can become significant when dealing with large datasets, especially if the load factor is set too low, leading to increased memory consumption.
  • Not Thread-safe: LinkedHashSet is not thread-safe by default. If multiple threads access a LinkedHashSet concurrently, it can lead to data corruption or undefined behavior. Synchronization mechanisms need to be applied externally to ensure thread safety if needed, increasing complexity and potentially impacting performance.
  • Slower Lookup than HashSet: While LinkedHashSet provides predictable iteration order, the lookup performance may be slightly slower compared to HashSet due to the additional overhead of maintaining the linked list structure. If fast lookup is a priority and order preservation is not required, HashSet may be a more suitable choice.

Interview Questions and MCQs on LinkedHashSet in Java

Interview Questions:

  1. What is a LinkedHashSet in Java?
  2. How does LinkedHashSet differ from HashSet?
  3. What is the significance of maintaining insertion order in LinkedHashSet?
  4. How is LinkedHashSet implemented internally?
  5. Can LinkedHashSet contain duplicate elements?
  6. How do you add elements to a LinkedHashSet?
  7. How do you remove elements from a LinkedHashSet?
  8. What are the advantages and disadvantages of using LinkedHashSet?

Multiple Choice Questions (MCQs):

  1. Which interface does LinkedHashSet implement?
    a) List
    b) Set
    c) Map
    d) Collection
    Answer: b) Set
  2. How are elements stored internally in a LinkedHashSet?
    a) Based on their hash codes
    b) Based on their indexes
    c) In insertion order
    d) In natural order
    Answer: c) In insertion order
  3. What happens when you try to add a duplicate element to a LinkedHashSet?
    a) The element is added without any changes
    b) An exception is thrown
    c) The existing element is replaced with the new one
    d) The new element is ignored
    Answer: a) The element is added without any changes
  4. Which method is used to remove all elements from a LinkedHashSet?
    a) removeAll()
    b) clear()
    c) deleteAll()
    d) erase()
    Answer: b) clear()
  5. How do you check if a LinkedHashSet contains a specific element?
    a) containsElement()
    b) contains()
    c) containsKey()
    d) containsValue()
    Answer: b) contains()
☰