HashSet implements the empty Serializable interface : HashSet « Collections « Java Tutorial

If, and only if, all of the elements of a HashSet are Serializable, HashSet can be saved to an ObjectOutputStream and later read it in to an ObjectInputStream.

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

public class MainClass {

  public static void main(String[] a) throws Exception{
    String elements[] = { "A", "B", "C", "D", "E" };
    Set set = new HashSet(Arrays.asList(elements));
    FileOutputStream fos = new FileOutputStream("set.ser");
    ObjectOutputStream oos = new ObjectOutputStream(fos);
    FileInputStream fis = new FileInputStream("set.ser");
    ObjectInputStream ois = new ObjectInputStream(fis);
    Set anotherSet = (Set) ois.readObject();
[D, A, C, B, E]

9.19.1.Set and HashSet
9.19.2.Creating a HashSet
9.19.3.An easy way to initialize a set without manually adding each element
9.19.4.Adding Single Elements: the add() method
9.19.5.Adding Another Collection with the addAll() method: public boolean addAll(Collection c)
9.19.6.Removing All Elements: public void clear()
9.19.7.Removing Single Elements: public boolean remove(Object element)
9.19.8.Removing Another Collection: public boolean removeAll(Collection c)
9.19.9.Retaining Another Collection: public boolean retainAll(Collection c)
9.19.10.Fetching Elements: to work with all of the elements of the set
9.19.11.Checking for Existence: the contains() method reports if a specific element is within the set
9.19.12.the containsAll() method checks if a set contains another whole collection
9.19.13.To find out how many elements are in a set, use the size() method
9.19.14.Checking for no elements in the set: use the isEmpty() method instead
9.19.15.Copying and Cloning Sets: public Object clone()
9.19.16.HashSet implements the empty Serializable interface
9.19.17.Converting elements in a set to Array: public Object[] toArray()
9.19.18.public Object[] toArray(Object[] a)
9.19.19.The HashSet class defines equality through its equals() method: public boolean equals(Object o)
9.19.20.Find maximum element of HashSet
9.19.21.Find Minimum element of HashSet
9.19.22.Get Enumeration over HashSet
9.19.23.Get Synchronized Set from HashSet
9.19.24.Check if a particular element exists in HashSet
9.19.25.Copy all elements of HashSet to an Object Array
9.19.26.Get Size of HashSet
9.19.27.Iterate through elements of HashSet
9.19.28.Remove all elements from HashSet
9.19.29.Remove specified element from HashSet
9.19.30.Integer value set
9.19.31.Listing the Elements of a Collection(iterate over the elements of set or list)
9.19.32.Making a Collection Read-Only
9.19.33.Convert array to Set
9.19.34.Create unique lists of items?