Thread: Serializable compiler warning

Hi,
I hope posted this is in the right area. My program is not dealing with File I/O but I got a warning message from the compiler indicating a serailVersionUID definition problem. Here is the warning.

C:\J21work>javac -Xlint Exercise22_6.java
Exercise22_6.java:6: warning: [serial] serializable class Exercise22_6 has no de
finition of serialVersionUID
public class Exercise22_6 extends JFrame {
^
1 warning

I'm not sure why my class is "serializable". Is it because I'm using a data structure from the JCF? I know they implement the Serializable interface. I've only read a little about that interface, it was covered briefly on a chapter about Binary I/O, but nothing was mentioned about the serialVersionUID. This is the second time I've run into this variable. The program ran fine but I was wondering what I have to do to get rid of this warning. I've listed the code below. I'd appreciate any help.

Thanks,
Casey

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.*;
 
public class Exercise22_6 extends JFrame  {
  private LinkedList<Integer> numberList = new LinkedList<Integer>();
  private JTextField jtfNumber = new JTextField();
  private JLabel jlblEnter = new JLabel("Enter a number: ", SwingConstants.RIGHT);
  private JTextArea jtaNumbers = new JTextArea();
  private JButton jbtSort = new JButton("Sort");
  private JButton jbtShuffle = new JButton("Shuffle");
  private JButton jbtReverse = new JButton("Reverse");
 
  public Exercise22_6() {
    JPanel p1 = new JPanel(new GridLayout(1, 2));
 
	p1.add(jlblEnter);
	p1.add(jtfNumber);
 
	JPanel p2 = new JPanel(new BorderLayout());
 
	p2.add(jtaNumbers, BorderLayout.CENTER);
 
	//JPanel p3 = new JPanel(new GridLayout(1, 3, 5, 5));
	JPanel p3 = new JPanel();
	p3.add(jbtSort);
	p3.add(jbtShuffle);
	p3.add(jbtReverse);
 
	JPanel panel = new JPanel(new BorderLayout());
	panel.add(p1, BorderLayout.NORTH);
	panel.add(p2, BorderLayout.CENTER);
	panel.add(p3, BorderLayout.SOUTH);
 
	setLayout(new BorderLayout());
	add(panel, BorderLayout.CENTER);
 
	jtfNumber.addActionListener(new ActionListener() {
	  public void actionPerformed(ActionEvent e) {
	    int num = Integer.parseInt(jtfNumber.getText());
		if(!numberList.contains(num)){
		  numberList.add(num);
		  displayNumbers();
		}
		jtfNumber.setText("");
	  }
	});
 
	jbtSort.addActionListener(new ActionListener() {
	  public void actionPerformed(ActionEvent e) {
	    Collections.sort(numberList);
		displayNumbers();
	  }
	});
 
	jbtShuffle.addActionListener(new ActionListener() {
	  public void actionPerformed(ActionEvent e) {
	    Collections.shuffle(numberList);
		displayNumbers();
	  }
	});
 
    jbtReverse.addActionListener(new ActionListener() {
	  public void actionPerformed(ActionEvent e) {
	    Collections.reverse(numberList);
		displayNumbers();
	  }
	});
 
  }
 
  public void displayNumbers() {
    String numbers = "";
	for(Object element: numberList)
	  numbers = numbers + element.toString() + " ";
	jtaNumbers.setText(numbers);
 
  }
 
  public static void main(String[] args) {
    Exercise22_6 frame = new Exercise22_6();
	frame.setSize(400, 125);
	frame.setLocationRelativeTo(null);
	frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
	frame.setTitle("Exercise22.6");
	frame.setVisible(true);
 
  }
 
 
}

Originally Posted by copeg

Inheritance: JFrame implements Serializable, your class extends JFrame, thus your class implements Serializable. Simply a) declare a serialVersionUID:
static final long serialVersionUID = 1L;
b) or use SuppressWarning annotation if you wish the compiler warning to go away,

That worked. Thanks!! is this a common problem with Seriallizable objects?

is this a common problem

It's not a problem, it's a feature! Read the API doc for java.io.Seralizable around the part that says:

However, it is strongly recommended that all serializable classes explicitly declare serialVersionUID values

You don't actually *have to* specify it, but if you're relying on default serialization for object communication / persistence, you really should know what might happen if you edit your code,

Originally Posted by Sean4u

It's not a problem, it's a feature! Read the API doc for java.io.Seralizable around the part that says:

You don't actually *have to* specify it, but if you're relying on default serialization for object communication / persistence, you really should know what might happen if you edit your code,

Gotcha!! Thank you.