The HashSet class extends AbstractSet class and implements the Set interface. The AbstractSet class itself extends the AbstractCollection class. It also implements Cloneable and Serializable marker interfaces. The HashSet hierarchy diagram can be shown in the below picture.

HashSet class declaration

Let’s see the declaration for java.util.HashSet class.
public class HashSetextends AbstractSetimplements Set, Cloneable, Serializable. 

Constructors of Java HashSet class

The following constructors are defined for Java HashSet class:
1. HashSet(): It constructs an empty hash set. i.e. default hash set. The default capacity is 16.

2. HashSet(int initialCapacity): It initializes the capacity of the HashSet. When the set capacity reaches full and a new element is added, the capacity of the hash set is expended automatically. The internal structure will double in size before adding a new element.

3. HashSet(int initialCapacity, float fillRatio): It initializes the capacity and the fill ratio(also called load factor or load capacity) of the hash set. When the number of elements is greater than the capacity of the HashSet, the size of the hash set is grown automatically by multiplying the capacity with the load factor. The default value of the load factor is 0.75. The value of the fill ratio ranges from 0.0 to1.0.

4. HashSet(Collection c): It initializes the hash set by using the elements of c. This constructor acts as a copy constructor. It copies the elements from one collection into the newly created collection. We cannot provide a custom initial capacity or fill ratio. If the original collection had duplicate elements, only one duplicate element will be allowed in the final created set. 
Note: The constructors that do not take a load factor, 0.75 is used. 

Methods of Java HashSet class

The HashSet class does not define any additional methods. It inherits the methods of its parent classes and the methods of the implemented interface. The various methods provided by its superclasses and interfaces are as follows:
1. Adding a single element
If you want to add a single element, call the add() method.
Syntax:
   public boolean add(Object o)
The add() method takes a single argument of the element to add. This method returns true if the set does not already contain a specified element.

2. Adding another collection of elements
If you want to add a group of elements from another collection to the set, call addAll() method.
Syntax:
   public boolean addAll(Collection c)
Each element in the collection will be added to the current set via calling add() method on each element. It returns true if the current set changes. If no elements are added, false is returned. A UnsupportedOperationException will be thrown when a current set does not support adding elements.

3. Removing Single Element
If you wish to remove a single element at a time, use the remove() method.
Syntax:
    public boolean remove(Object o) 
To remove an element from the set, Set internally use equals() method for each element. If the element is found, the element is removed from the set and returns true. If not found, false is returned. If the removal is not supported, you will get UnsupportedOperationException. 

4. Removing all elements
To remove all the elements from a set, use clear() method.
Syntax:
    public void clear() // It returns nothing.

5. Checking for Existing element
To check a specific element is present or not in the set, use contains() method.
Syntax:
    public boolean contains(Object element)
If the specified element is found in the set, it returns true otherwise false. The equality checking is done through the element’s equal method.

6. Checking size
If you wish to know how many elements are in a set, call size() method.
Syntax:
    public int size()
7. Checking for Empty
If you wish to check that whether the HashSet contains elements or not.
Syntax:
    public boolean isEmpty()

8. Coping and Cloning sets
Since the HashSet implements the Cloneable interface. So we can create a shallow copy of that hash set by calling the clone() method of the hash set. 

Java HashSet Example Program