What do you know about LinkedHashSet.md

LinkedHashSet

Hash table and linked list implementation of the Set interface, with predictable iteration order. This implementation differs from HashSet in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is the order in which elements were inserted into the set (insertion-order). Note that insertion order is not affected if an element is re-inserted into the set. (An element e is reinserted into a set s if s.add(e) is invoked when s.contains(e) would return true immediately prior to the invocation.)

This implementation spares its clients from the unspecified, generally chaotic ordering provided by HashSet, without incurring the increased cost associated with TreeSet. It can be used to produce a copy of a set that has the same order as the original, regardless of the original set's implementation:

void foo(Set s) {
     Set copy = new LinkedHashSet(s);
         ...
}

This technique is particularly useful if a module takes a set on input, copies it, and later returns results whose order is determined by that of the copy. (Clients generally appreciate having things returned in the same order they were presented.)

This class provides all of the optional Set operations, and permits null elements. Like HashSet, it provides constant-time performance for the basic operations (add, contains and remove), assuming the hash function disperses elements properly among the buckets. Performance is likely to be just slightly below that of HashSet, due to the added expense of maintaining the linked list, with one exception: Iteration over a LinkedHashSet requires time proportional to the size of the set, regardless of its capacity. Iteration over a HashSet is likely to be more expensive, requiring time proportional to its capacity.

A linked hash set has two parameters that affect its performance: initial capacity and load factor. They are defined precisely as for HashSet. Note, however, that the penalty for choosing an excessively high value for initial capacity is less severe for this class than for HashSet, as iteration times for this class are unaffected by capacity.

Initial capacity

The initial capacity means the number of buckets (in backing HashMap) when LinkedHashSet is created. The number of buckets will be automatically increased if the current size gets full.

Default initial capacity is 16. We can override this default capacity by passing default capacity in it’s constructor LinkedHashSet(int initialCapacity).

Load factor

The load factor is a measure of how full the LinkedHashSet is allowed to get before its capacity is automatically increased. Default load factor is 0.75.

This is called threshold and is equal to (DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY). When LinkedHashSet elements count exceed this threshold, LinkedHashSet is resized and new capacity is double the previous capacity.

With default LinkedHashSet, the internal capacity is 16 and load factor is 0.75. The number of buckets will automatically get increased when the table has 12 elements in it.

Synchronization

LinkedHashSet is not synchronized. If multiple threads access a hash set concurrently, and at least one of the threads modifies the set, it must be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the set. If no such object exists, the set should be "wrapped" using the Collections.synchronizedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the set:

Set s = Collections.synchronizedSet(new LinkedHashSet(...));

The iterators returned by this class's iterator method are fail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator's own remove method, the Iterator throws a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

Example of usage

import java.util.LinkedHashSet;

public class LinkedHashSetExample {

  public static void main(String[]args) {
    LinkedHashSet<String> linkedHashSet = new LinkedHashSet<>();

    linkedHashSet.add("India");
    linkedHashSet.add("Australia");
    linkedHashSet.add("South Africa");
    linkedHashSet.add("India");// adding duplicate elements
    linkedHashSet.add("Ukraine");
    linkedHashSet.add("Usa");
    linkedHashSet.add("France");

    System.out.println("Initial LinkedHashSet:");
    System.out.println(linkedHashSet);

    linkedHashSet.remove("Australia");
    linkedHashSet.remove("France");

    System.out.println("LinkedHashSet after removing elements:");
    System.out.println(linkedHashSet);
  }
}

Output:

Initial LinkedHashSet: 
[India, Australia, South Africa, Ukraine, Usa, France]
LinkedHashSet after removing elements: 
[India, South Africa, Ukraine, Usa]

Conclusion

LinkedHashSet are:

• Extends HashSet class which extends AbstractSet class;
• Implements Set interface;
• Implements Searlizable and Cloneable interfaces;
• Duplicate values are not allowed;
• Allowed null elements;
• It is an ordered collection which is the order in which elements were inserted into the set (insertion-order);
• Not synchronized;
• The initial default capacity is 16, and the load factor is 0.75;
• Time complexity average - O(1)
• Time complexity worse - O(n)

Links

https://docs.oracle.com/javase/7/docs/api/java/util/LinkedHashSet.html
https://howtodoinjava.com/java/collections/java-linkedhashset/