Java Program to Implement HashSet API

This Java program is to Implement HashSet Collection API. This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.
This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets. Iterating over this set requires time proportional to the sum of the HashSet instance’s size (the number of elements) plus the “capacity” of the backing HashMap instance (the number of buckets). Thus, it’s very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

Here is the source code of the Java program to Implement HashSet Collection API. The Java program is successfully compiled and run on a Linux system. The program output is also shown below.

```
import java.util.Collection;
```
```
import java.util.HashSet;
```
```
import java.util.Iterator;
```
```
import java.util.Set;
```
```
 
```
```
public class HashSetImpl<E>
```
```
{
```
```
    private Set<E> hashSet;
```
```
 
```
```
    /*
```

     * Constructs a new, empty set; the backing HashMap instance has default

     * initial capacity (16) and load factor (0.75).

```
     */
```
```
    public HashSetImpl()
```
```
    {
```
```
        hashSet = new HashSet<E>();
```
```
    }
```
```
 
```

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

    public HashSetImpl(Collection<? extends E> c)

```
    {
```
```
        hashSet = new HashSet<E>(c);
```
```
    }
```
```
 
```
```
    /*
```

     * constructs a new, empty set; the backing HashMap instance has the

     * specified initial capacity and the specified load factor.

```
     */
```

    public HashSetImpl(int initialCapacity, float loadFactor)

```
    {
```

        hashSet = new HashSet<E>(initialCapacity, loadFactor);

```
    }
```
```
 
```
```
    /*
```

     * Constructs a new, empty set; the backing HashMap instance has the

     * specified initial capacity and default load factor (0.75).

```
     */
```

    public HashSetImpl(int initialCapacity)

```
    {
```

        hashSet = new HashSet<E>(initialCapacity);

```
    }
```
```
 
```

    /* adds the specified element if not already present */

```
    public boolean add(E eobj)
```
```
    {
```
```
        return hashSet.add(eobj);
```
```
    }
```
```
 
```

    /* return true if this set contains the specified element */

```
    public boolean contains(Object obj)
```
```
    {
```
```
        return hashSet.contains(obj);
```
```
    }
```
```
 
```

    /* returns true if the set is empty */

```
    public boolean isEmpty()
```
```
    {
```
```
        return hashSet.isEmpty();
```
```
    }
```
```
 
```

    /* returns an iterator over the elements in the set */

```
    public Iterator<E> iterator()
```
```
    {
```
```
        return hashSet.iterator();
```
```
    }
```
```
 
```

    /* removes the specified element from this set if present */

```
    public boolean remove(Object obj)
```
```
    {
```
```
        return hashSet.remove(obj);
```
```
    }
```
```
 
```

    /* returns the number of elements in set */

```
    public int size()
```
```
    {
```
```
        return hashSet.size();
```
```
    }
```
```
 
```

    /* removes all elements from this set */

```
    public void clear()
```
```
    {
```
```
        hashSet.clear();
```
```
    }
```
```
 
```

    /* Returns an array containing all of the elements in this set. */

```
    public Object[] toArray()
```
```
    {
```
```
        return hashSet.toArray();
```
```
    }
```
```
 
```
```
    /*
```

     * Adds all of the elements in the specified collection to this set if

```
     * they're not already present
```
```
     */
```

    public boolean addAll(Collection<? extends E> c)

      throws UnsupportedOperationException, ClassCastException,NullPointerException,IllegalArgumentException

```
    {
```
```
        return hashSet.addAll(c);
```
```
    }
```
```
 
```
```
    /*
```

     * Retains only the elements in this set that are contained in the specified

```
     * collection
```
```
     */
```

    public boolean retainAll(Collection<?> c)

      throws UnsupportedOperationException, ClassCastException,NullPointerException

```
    {
```
```
        return hashSet.retainAll(c);
```
```
    }
```
```
 
```
```
    /*
```

     * Removes from this set all of its elements that are contained in the

```
     * specified collection
```
```
     */
```

    public boolean removeAll(Collection<?> c)

      throws UnsupportedOperationException, NullPointerException,ClassCastException

```
    {
```
```
        return hashSet.retainAll(c);
```
```
    }
```
```
 
```
```
    /*
```

     * Returns an array containing all of the elements in this set; the runtime

     * type of the returned array is that of the specified array

```
     */
```
```
    public <T> T[] toArray(T[] a)
```

      throws ArrayStoreException,NullPointerException

```
    {
```
```
        return hashSet.toArray(a);
```
```
    }
```
```
 
```

    public static void main(String... arg)

```
    {
```

        HashSet<Integer> hashSet = new HashSet<Integer>();

```
        if (hashSet.add(10))
```

            System.out.println("element 10 added");

```
        if (hashSet.add(20))
```

            System.out.println("element 20 added");

```
        if (hashSet.add(30))
```

            System.out.println("element 30 added");

```
 
```

        System.out.println("the size of set is " + hashSet.size());

```
 
```
```
        if (hashSet.contains(40))
```

            System.out.println("set contains 40");

```
        else
```

            System.out.println("set does not contain 40");

```
 
```
```
        if (hashSet.remove(20))
```

            System.out.println("element 20 removed");

```
        else
```

            System.out.println("element 20 not removed");

```
 
```

        System.out.println("the element of set are");

        Iterator<Integer> iterator = hashSet.iterator();

```
        while (iterator.hasNext())
```
```
        {
```

            System.out.print(iterator.next() + "\t");

```
        }
```
```
        System.out.println();
```
```
 
```

        Set<Integer> removedSet = new HashSet<Integer>();

```
        removedSet.add(10);
```
```
        removedSet.add(20);
```
```
 
```

        System.out.println("the elements after removing");

```
        hashSet.removeAll(removedSet);
```

        Iterator<Integer> riterator = hashSet.iterator();

```
        while(riterator.hasNext())
```
```
        {
```

            System.out.print(riterator.next() + "\t");

```
        }
```
```
        System.out.println();
```
```
 
```
```
        hashSet.clear();
```

        System.out.println("hashSet cleared");

```
        if (hashSet.isEmpty())
```

            System.out.println("hashSet is empty");

```
        else
```

            System.out.println("hashSet is not empty");

```
    }
```
```
}
```

$javac HashSetImpl.java
$java HashSetImpl
 
element 10 added
element 20 added
element 30 added
the size of set is 3
set does not contain 40
element 20 removed
the element of set are
10	30	
the elements after removing
30	
hashSet cleared
hashSet is empty

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