Implementing Serializable

[Yed Su]

Is it mandatory to implement Serializable interface if an object is to be serialized ? I worked with ObectInput/ObjectOutput and FileInput/FileOutput without implementing Serializable interface for my class and it worked perfect. Just curious to understand the Serializable.

[Ernest Friedman-Hill]

Hi,

Yes, to be serialized, a class must be marked Serializable. If you observed otherwise, I suspect either (1) you weren't actually using serialization, or (2) your classes inherited Serializable from a parent class.

[Yed Su]

Thanks Ernest,

But I dont have any super class and Im using serialization as well. Please check the following example:

public class SimpleSerialization {

    /**
     * Create a simple Hashtable and serialize it to a file called
     * HTExample.ser.
     */
	
	
	SimpleSerialization(){
		
		System.out.println("Constructor called");
	}
   private static void doSave() {
	
        System.out.println();
        System.out.println("+------------------------------+");
        System.out.println("| doSave Method                |");
        System.out.println("+------------------------------+");
        System.out.println();
        
        Hashtable h = new Hashtable();
        h.put("string", "Oracle / Java Programming");
        h.put("int", new Integer(36));
        h.put("double", new Double(Math.PI));

        try {

            System.out.println("Creating File/Object output stream...");
            
            FileOutputStream fileOut = new FileOutputStream("HTExample.ser");
            ObjectOutputStream out = new ObjectOutputStream(fileOut);

            System.out.println("Writing Hashtable Object...");
            out.writeObject(h);

            System.out.println("Closing all output streams...\n");
            out.close();
            fileOut.close();
            
        } catch(FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }


    /**
     * Loads the contents of a previously serialized object from a file called
     * HTExample.ser.
     */
   private static void doLoad() {
	
        System.out.println();
        System.out.println("+------------------------------+");
        System.out.println("| doLoad Method                |");
        System.out.println("+------------------------------+");
        System.out.println();
        
        Hashtable h = null;


        try {

            System.out.println("Creating File/Object input stream...");
            
            FileInputStream fileIn = new FileInputStream("HTExample.ser");
            ObjectInputStream in = new ObjectInputStream(fileIn);

            System.out.println("Loading Hashtable Object...");
            h = (Hashtable)in.readObject();

            System.out.println("Closing all input streams...\n");
            in.close();
            fileIn.close();
            
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch(FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        System.out.println("Printing out loaded elements...");
        for (Enumeration e = h.keys(); e.hasMoreElements(); ) {
            Object obj = e.nextElement();
            System.out.println("  - Element(" + obj + ") = " + h.get(obj));
        }
        System.out.println();

    }


    /**
     * Sole entry point to the class and application.
     * @param args Array of String arguments.
     */
   public static void main(String[] args) {
        doSave();
        doLoad();
    }

}

[Henry Wong]

Hashtable instances, along with String, Integer, and Double instances are all Serializable -- try putting something that is not serializable into that hashtable.

Henry

[Ernest Friedman-Hill]

Hashtable, String, Integer, and Double all implement Serializable, so all the objects you're writing are, indeed, serializable.

[Yed Su]

Ernest and Henry Thanks for making me realize Serialize.. !