Shiva Prasad Gurram

What is HashSet in Java and How it works internally?

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Shiva Prasad Gurram
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Knowing about internal working behavior of HashSet will play a vital role to crack interviews on Java HashSet and to ease your day-to-day work on it. In this article I'm trying to demonstrate, what is HashSet and How it works internally in Java?

What is HashSet?

HashSet is a class in Java which implements Set Interface and extends AbstractSet. It allows you to store only unique elements that means no duplicates are allowed.

HashSet is a member of Java Collections Framework.

  • HashSet uses HashMap internally to store data in the form of Key-Value pair.

  • Elements are stored using hashing technique, therefore it is not stored in an ordered fashion and the elements will be returned in random order.

  • Elements are stored in the form of key-value pair (internally by HashMap) where key will the actual element value and value will be the PRESENT Constant.

  • It contains only unique elements.

  • Hashset is not synchronized (not thread-safe).

  • This class permits at most one null element.

  • HashSet has default initial capacity of 16.

  • HashSet has default loadfactor of 0.75 or 75%.

  • HashSet does not have any method to retrieve the object directly.

  • The only way to retrieve objects from the HashSet is through iterating the entire HashSet. This can be achieved by using iterator, for, for-each ,etc.

  • The iterators returned by HashSet class 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.

  • The class also offers constant time performance for the basic operations like add, remove, contains and size, assuming the hash function distribute the elements properly among the buckets.

Example of HashSet

HashSet<String> languages = new HashSet<>();
languages.add("Java");
languages.add("Python");
languages.add("JavaScript");
languages.add("C");
languages.add("C++");
languages.add(null);

Iterator<String> itr = languages.iterator();
while(itr.hasNext())
{
    System.out.println(itr.next());
}

Output:

null
Java
C++
C
JavaScript
Python

How HashSet works Internally?

In order to understand HashSet internal behavior , one should understand the HashMap internals. This is because HashSet internally uses the HashMap behavior.

I highly recommend to go through the HashMap internals How HashMap works internally? before proceeding.

The moment you create a HashSet the default constructor will get called and internally a backing HashMap object gets created automatically.

Set<String> hs = new HashSet<String>();

Constructors of HashSet:

1) Default Constructor :

Constructs a new, empty set; the backing HashMap instance has default initial capacity (16) and load factor (0.75)

public HashSet() {
map = new HashMap<>();
}

In HashSet class, a map instance can be created at class level like below

private transient HashMap<E,Object> map;

2) Parameterized constructor with initialCapacity :

Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and default load factor (0.75).

public HashSet(int initialCapacity) {
  map = new HashMap<>(initialCapacity);
 }

3) Parameterized constructor with initialCapacity & loadFactor :

Constructs a new, empty set; the backing HashMap instance has the specified initial capacity and the specified load factor.

public HashSet(int initialCapacity, float loadFactor) {
   map = new HashMap<>(initialCapacity, loadFactor);
}

4) Parameterized constructor with Collection:

Constructs a new set containing the elements in the specified collection.

public HashSet(Collection<? extends E> c) {
  map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
  addAll(c);
}

Lets see how add(E e) works:

Adds the specified element to the set if it is not already present and then it returns true. If this set already contains the specified element, no operation would be performed and it returns false.

Implementation: 

public boolean add(E e) {
return map.put(e, PRESENT)==null;
}

Whenever we try to add an element using HashSet, it stores internally as a key-value pair.

Here Key is an element which we are trying to add using HashSet and Value is a CONSTANT called PRESENT (Same for all the HashSet elements). The PRESENT is defined in the HashSet class as below.

private static final Object PRESENT = new Object();

Ex: add("Java")

In background "Java" will behave like a "key" for the map and PRESENT will be it's value. So, add("Java") is similar to map.put("Java",PRESENT).

return map.put(e, PRESENT)==null => returns true if the map doesn't contains the specified element(e) , else false will be returned which means element already exists.

Lets see how remove(Object o) works:

Removes the specified element from this set if it is present. This method returns true if this set changed as a result of the call.

Implementation: 

public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}

Once this method returns true the element will not be present in Set.

Why HashSet doesn't have get(Object o) method?

Internal Implementation of HashSet class

Reference : https://docs.oracle.com/javase/8/docs/api/java/util/HashSet.html

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