Home / FAQ / Core Java Technology / Serialization

Tim Rohaly

The following code defines a simple Externalizable class that provides a main() method which instantiates an object, writes it to a file, restores it, then verifies that the restored object contains the same values as the original.

import java.io.*;
import java.util.*;

/**
 * This program demonstrates how to write an Externalizable class
 */
public class TestExternal implements Externalizable {

    private String comment = "A serializable data member";
    private Vector collection;

    /**
     * A no-argument constructor is required for an Externalizable class
     */
    public TestExternal() {
        this(null);
    }

    public TestExternal(Vector collection) {
        this.collection = collection;
    }

    /**
     * This method is responsible for saving the
     * entire state of the object
     */
    public void writeExternal(ObjectOutput out) throws IOException {
        out.writeObject(comment);
        //
        // You could also just do out.writeObject(collection) below,
        // but I prefer to loop over the contents and write them one
        // at a time.
        //
        out.writeInt(collection.size());
        Enumeration e = collection.elements();
        while (e.hasMoreElements()) {
            out.writeObject(e.nextElement());
        }
    }

    /**
     * This method is responsible for restoring the
     * entire state of the object
     */
    public void readExternal(ObjectInput in)
                 throws ClassNotFoundException, IOException {
        //
        // What was written in writeExternal() must be read back here.
        //
        comment    = (String) in.readObject();
        int size   = in.readInt();
        collection = new Vector(size);
        for (int i=0; i<size; i++) {
            collection.addElement(in.readObject());
        }
    }

    /**
     * Delegates equality check to the objects stored within
     * this instance
     */
    public boolean equals(Object o) {
        if (o instanceof TestExternal) {
            TestExternal other = (TestExternal) o;
            if (comment.equals(other.comment) &&
                collection.size() == other.collection.size() ) {
                for (int i=0; i<collection.size(); i++) {
                    if (!collection.elementAt(i).equals(other.collection.elementAt(i)))
                        return false;
                }
                return true;
            }
        }
        return false;
    }

    public static void main(String[] args)
                       throws IOException, ClassNotFoundException {
        //
        // Create the data to store in this object,
        // then create and fill the object
        //
        Vector bunchOfIntegers = new Vector();
        for (int i=0; i<10; i++) {
            Integer obj = new Integer(i);
            bunchOfIntegers.addElement(obj);
        }
        TestExternal original = new TestExternal(bunchOfIntegers);

        //
        // Write the object to a file
        //
        ObjectOutputStream out = new ObjectOutputStream(
                                     new FileOutputStream("testexternal.ser"));
        out.writeObject(original);
        out.flush();
        out.close();

        //
        // Now read the object back in from the file
        //
        ObjectInputStream in = new ObjectInputStream(
                                   new FileInputStream("testexternal.ser"));
        TestExternal restored = (TestExternal) in.readObject();

        //
        // Compare original object with restored object to
        // check that they are the same
        //
        if (restored.equals(original)) {
            System.out.println("The original is the same as the restored");
        }
        else {
            System.out.println("There was a problem with Externalization:");
            System.out.println("	The original and the restored aren't equal");
        }
    }
}