Page History

...

Consequently, the hashcode of an object need not remain consistent across different executions of the application. Similarly, if an object is serialized, its hashcode may not stay consistent with the original value. This introduces several hurdles; for example, upon deserialization of a hashtable it is impossible to retrieve the object contained objects because its the corresponding key value could values may have changed.

Noncompliant Code Example

...

This example follows the above advice but serialization is still at the mercy of the implementation of the hashcode() method. It is not required to produce a key value (hashcode) that is consistent across different executions of the program or during (de)serialization. Consequently, using the default serialized form of the hashtable may be is inappropriate. In this noncompliant code example, it is not possible to retrieve the value of the object using the original key after deserialization.

class Key implements Serializable {
  // Overrides hashcode and equals methods
}

class HashSer {
  public static void main(String[] args) throws IOException, ClassNotFoundException {
    Hashtable<Key,String> ht = new Hashtable<Key, String>();
    Key key = new Key();
    ht.put(key, "Value");
    System.out.println("Entry: " + ht.get(key)); // Retrieve using the key, works
	 
    // Serialize the Hashtable object
    FileOutputStream fos = new FileOutputStream("hashdata.ser");
    ObjectOutputStream oos = new ObjectOutputStream(fos);
    oos.writeObject(ht);
    oos.close();
	 
    // Deserialize the Hashtable object
    FileInputStream fis = new FileInputStream("hashdata.ser");
    ObjectInputStream ois = new ObjectInputStream(fis);
    Hashtable<Key, String> ht_in = (Hashtable<Key, String>)(ois.readObject());
    ois.close();
	 
    if(ht_in.contains("Value")) // Check if the object actually exists in the Hashtable
      System.out.println("Value was found in deserialized object.");
	 
    if (ht_in.get(key) == null) // Gets printed
      System.out.println("Object was not found when retrieved using the key.");	 
  }
}

Compliant Solution

One solution is to change This compliant solution changes the type of the key value so that it remains consistent across different runs of the program and a multitude of JVMs. This can be achieved by using an Integer object, for example, to hold the key values.

class HashSer {
  public static void main(String[] args) throws IOException, ClassNotFoundException {
    Hashtable<Integer, String> ht = new Hashtable<Integer, String>();
    ht.put(new Integer(1), "Value");
    System.out.println("Entry: " + ht.get(1)); // Retrieve using the key
	 
    // Serialize the Hashtable object
    FileOutputStream fos = new FileOutputStream("hashdata.ser");
    ObjectOutputStream oos = new ObjectOutputStream(fos);
    oos.writeObject(ht);
    oos.close();
	 
    // Deserialize the Hashtable object
    FileInputStream fis = new FileInputStream("hashdata.ser");
    ObjectInputStream ois = new ObjectInputStream(fis);
    Hashtable<Integer, String> ht_in = (Hashtable<Integer, String>)(ois.readObject());
    ois.close();
	 
    if(ht_in.contains("Value")) // Check if the object actually exists in the Hashtable
      System.out.println("Value was found in deserialized object.");
	 
    if (ht_in.get(1) == null)  // Does not get printed
      System.out.println("Object was not found when retrieved using the key.");	 
  }
}

This problem can also be avoided by overriding the equals() and the hashcode() method in the Key class, though it is best to avoid employing serializing hash tables that are known to use implementation defined parameters.

...