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Tip of the Day
Language: Java
Expertise: Beginner
Feb 27, 1998

Object I/O

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 is a method of turning Java objects into a stream of bytes and a stream of bytes back into Java objects. You should read the serialization specification 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 implement the Serializable interface. ObjectInputStream and ObjectOutputStream will automatically take care of serializing and deserializing a class that implements the Serializable interface. However, that class must have a no-argument constructor. The following simple program writes two 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();
    }
  }

}
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