Question:
How can i add many java objects to a file then retrieve them as Java objects from the file?

Answer:
Java 1.1 introduced a concept called serialization. Serialization isa method of turning Java objects into a stream of bytes and a streamof bytes back into Java objects. You should read the serializationspecification that accompanies JDK 1.1.x for full details. But you will want to become familiar with java.io.Serializable, java.io.Externalizable,java.io.ObjectInputStream and java.io.ObjectOutputStream.

The simplest way to perform object I/O is to make a class implementthe Serializable interface. ObjectInputStream and ObjectOutputStreamwill automatically take care of serializing and deserializing a classthat implements the Serializable interface. However, that class musthave a no-argument constructor. The following simple program writestwo objects to a file and reads them back in, printing out their values.

import java.io.*;public final class Serialization implements Serializable {  int a, b, c;  public Serialization() {    a = b = c = 0;  }  public void setValues(int a, int b, int c) {    this.a = a; this.b = b; this.c = c;  }  public String toString() {    return a + " " + b + " " + c;  }  public static final void main(String[] args) {    Serialization tmp;    ObjectOutputStream output;    ObjectInputStream input;    try {      output = new ObjectOutputStream(new FileOutputStream("tmp.dat"));      tmp = new Serialization();      tmp.setValues(1, 2, 3);      output.writeObject(tmp);      tmp = new Serialization();      tmp.setValues(3, 2, 1);      output.writeObject(tmp);      output.close();      input = new ObjectInputStream(new FileInputStream("tmp.dat"));      tmp = (Serialization)input.readObject();      System.out.println(tmp.toString());      tmp = (Serialization)input.readObject();      System.out.println(tmp.toString());      input.close();    } catch(IOException e) {      e.printStackTrace();    } catch(ClassNotFoundException e) {      e.printStackTrace();    }  }}