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

1. Java Tutorial
2. Collections
3. HashSet

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);
    oos.writeObject(set);
    oos.close();
    
    FileInputStream fis = new FileInputStream("set.ser");
    ObjectInputStream ois = new ObjectInputStream(fis);
    Set anotherSet = (Set) ois.readObject();
    ois.close();
    System.out.println(anotherSet);
  }
}

[D, A, C, B, E]