Java ObjectInputStream .readFields ()

public ObjectInputStream.GetField readFields()     throws IOException ,      ClassNotFoundException

/*from   ww w .j  a v a 2  s.  co  m*/
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.ObjectStreamField;
import java.io.Serializable;

public class Main implements Serializable {

  public static void main(String[] args) throws Exception {

    FileOutputStream out = new FileOutputStream("test.txt");
    ObjectOutputStream oout = new ObjectOutputStream(out);

    oout.writeObject(new Example());
    oout.flush();
    oout.close();

    ObjectInputStream ois = new ObjectInputStream(new FileInputStream(
        "test.txt"));

    Example a = (Example) ois.readObject();

    System.out.println(a.isDefault);

    System.out.println(a.string);

    ois.close();
  }

}

class Example implements Serializable {

   String string = "Hello World!";
    boolean isDefault;

  private static final ObjectStreamField[] serialPersistentFields = { new ObjectStreamField(
      "string", String.class) };

  private void readObject(ObjectInputStream in) throws IOException,
      ClassNotFoundException {

    ObjectInputStream.GetField fields = in.readFields();

    isDefault = fields.defaulted("string");
    string = (String) fields.get("string", null);
  }

  private void writeObject(ObjectOutputStream out) throws IOException {

    ObjectOutputStream.PutField fields = out.putFields();
    fields.put("string", string);
    out.writeFields();

  }
}