Java Tutorial/File/Externalizable

implements Externalizable

import java.io.Externalizable;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInput;
import java.io.ObjectInputStream;
import java.io.ObjectOutput;
import java.io.ObjectOutputStream;
public class MainClass {
  public static void main(String[] args) throws IOException, ClassNotFoundException {
    System.out.println("Constructing objects:");
    MyBean myBean = new MyBean("A String ", 47);
    System.out.println(myBean);
    ObjectOutputStream o = new ObjectOutputStream(new FileOutputStream("MyBean.out"));
    System.out.println("Saving object:");
    o.writeObject(myBean);
    o.close();

    ObjectInputStream in = new ObjectInputStream(new FileInputStream("MyBean.out"));
    System.out.println("Recovering:");
    myBean = (MyBean) in.readObject();
    System.out.println(myBean);
  }
}
class MyBean implements Externalizable {
  private int i;
  private String s; // No initialization
  public MyBean() {
    System.out.println("MyBean Constructor");
    // s, i not initialized
  }
  public MyBean(String x, int a) {
    System.out.println("MyBean(String x, int a)");
    s = x;
    i = a;
    // s & i initialized only in nondefault constructor.
  }
  public String toString() {
    return s + i;
  }
  public void writeExternal(ObjectOutput out) throws IOException {
    System.out.println("MyBean.writeExternal");
    out.writeObject(s);
    out.writeInt(i);
  }
  public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
    System.out.println("MyBean.readExternal");
    s = (String) in.readObject();
    i = in.readInt();
  }
}
/**/

Constructing objects:
MyBean(String x, int a)
A String 47
Saving object:
MyBean.writeExternal
Recovering:
MyBean Constructor
MyBean.readExternal
A String 47

Reconstructing an externalizable object

import java.io.Externalizable;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInput;
import java.io.ObjectInputStream;
import java.io.ObjectOutput;
import java.io.ObjectOutputStream;
public class MainClass {
  public static void main(String[] args) throws IOException, ClassNotFoundException {
    System.out.println("Constructing objects:");
    MyBean myBean = new MyBean("A String ", 47);
    System.out.println(myBean);
    ObjectOutputStream o = new ObjectOutputStream(new FileOutputStream("MyBean.out"));
    System.out.println("Saving object:");
    o.writeObject(myBean);
    o.close();

    ObjectInputStream in = new ObjectInputStream(new FileInputStream("MyBean.out"));
    System.out.println("Recovering:");
    myBean = (MyBean) in.readObject();
    System.out.println(myBean);
  }
}
class MyBean implements Externalizable {
  private int i;
  private String s; // No initialization
  public MyBean() {
    System.out.println("MyBean Constructor");
    // s, i not initialized
  }
  public MyBean(String x, int a) {
    System.out.println("MyBean(String x, int a)");
    s = x;
    i = a;
    // s & i initialized only in nondefault constructor.
  }
  public String toString() {
    return s + i;
  }
  public void writeExternal(ObjectOutput out) throws IOException {
    System.out.println("MyBean.writeExternal");
    out.writeObject(s);
    out.writeInt(i);
  }
  public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
    System.out.println("MyBean.readExternal");
    s = (String) in.readObject();
    i = in.readInt();
  }
}
/**/

Constructing objects:
MyBean(String x, int a)
A String 47
Saving object:
MyBean.writeExternal
Recovering:
MyBean Constructor
MyBean.readExternal
A String 47

Simple use of Externalizable

import java.io.Externalizable;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInput;
import java.io.ObjectInputStream;
import java.io.ObjectOutput;
import java.io.ObjectOutputStream;
class A implements Externalizable {
  public A() {
    System.out.println("A Constructor");
  }
  public void writeExternal(ObjectOutput out) throws IOException {
    System.out.println("A.writeExternal");
  }
  public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
    System.out.println("A.readExternal");
  }
}
class B implements Externalizable {
  B() {
    System.out.println("B Constructor");
  }
  public void writeExternal(ObjectOutput out) throws IOException {
    System.out.println("B.writeExternal");
  }
  public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
    System.out.println("B.readExternal");
  }
}
public class MainClass {
  public static void main(String[] args) throws IOException, ClassNotFoundException {
    System.out.println("Constructing objects:");
    A b1 = new A();
    B b2 = new B();
    ObjectOutputStream o = new ObjectOutputStream(new FileOutputStream("File.out"));
    System.out.println("Saving objects:");
    o.writeObject(b1);
    o.writeObject(b2);
    o.close();
    ObjectInputStream in = new ObjectInputStream(new FileInputStream("File.out"));
    System.out.println("Recovering b1:");
    b1 = (A) in.readObject();
  }
}
/*
*/

Constructing objects:
A Constructor
B Constructor
Saving objects:
A.writeExternal
B.writeExternal
Recovering b1:
A Constructor
A.readExternal