Loop isn't getting updated boolean value from actionPerformed in java

Question

I'm trying to create a simple chat program in java, using a frame. The user can either host a server (by clicking the server button) or connect as a client (using the connect button). the problem I'm having is the server button. My goal is that when the Start server button is clicked, all of the other buttons and fields are disabled, and the startServer method should run.

The entire program is inside of a while (!kill) loop, and right now, it's just checking the isServer boolean.

    while (!kill)
    {
        if (isServer)
        {
            startServer();
        }
    }

isServer is set to true inside of actionPerformed when the startServerButton is pressed.

my problem is that startServer() is never run because the while (!kill) loop isn't getting the updated isServer value when startServerButton is clicked.

Here's my run method:

public void run()
{   
    conversationBox.appendText("Session Start.\n");
    inputBox.requestFocus();

    while (!kill)
    {
        if (isServer)
        {
            startServer();
        }
    }       

}

and here's my actionPerformed:

public void actionPerformed(ActionEvent e) throws NumberFormatException
{
    Object o = e.getSource();

    if (o == sendButton || o == inputBox)
    {
        if(inputBox.getText() != "")
        {
            clientSendMsg = inputBox.getText();
            inputBox.setText("");
        }
    }
    if (o == changeHost || o == hostField)
    {
        if (hostField.getText() != "" && hostField.getText() != host)
        {
            host = hostField.getText();
            conversationBox.appendText("Host changed to " + host + "\n");
        }
    }
    if (o == changePort || o == portField)
    {
        if (portField.getText() != "" && Integer.valueOf(portField.getText()) != port)
        {
            try
            {
                port = Integer.valueOf(portField.getText());
                conversationBox.appendText("Port changed to " + port + "\n");
            }
            catch(NumberFormatException up)
            {
                throw up; //blargh enter a real value
            }
        }
    }
    if (o == startServerButton)
    {
        isServer = true;
        startServerButton.enable(false);
        connectButton.enable(false);
        changeHost.enable(false);
        changePort.enable(false);
        sendButton.enable(false);
        hostField.enable(false);
        portField.enable(false);
        inputBox.enable(false);
    }
    inputBox.requestFocus();
}

Obviously, the program is nowhere near complete, but this is a pretty big hurdle to ignore, so I figured it would be best to get it fixed before chugging along. Also, it should be noted that I have a new Thread(this).start(); inside the Chat() object that creates the frame layout. I'm not 100% sure how effective this is.

Can you try declaring `isServer` and `kill` fields with `volatile` modififer? — Tomasz Nurkiewicz, Dec 02 '12 at 20:23
So, using volatile worked, but I have no idea what happened, haha. Which ever of you posts it in an answer with an explanation of what volatile does gets my points. — Alex Kibler, Dec 02 '12 at 20:28

score 2 · Answer 1 · answered Dec 02 '12 at 20:29

2

In this line: if (isServer)

do you mean to say if (isServer == true) if when the button is clicked isServer is set to true?

In my experience "if" statements should always have a condition, like if (variable1.equals(variable2)) or if (boolean == true) for example.

Or the problem could be in your while (!kill) loop. In that case just take out the if statement and see if the method runs then.

answered Dec 02 '12 at 20:29

CodeAddict

194
2
5
19

I did mean `(isServer == true)`, but I tried it both with and without the `==true` and it worked either way. my problem was that I needed to use the `volatile` keyword as described above. either way, thanks for the answer! – Alex Kibler Dec 02 '12 at 20:32

score 1 · Accepted Answer · edited May 23 '17 at 12:18

1

Volatile keyword in Java is used as an indicator to Java compiler and Thread that do not cache value of this variable and always read it from main memory. So if you want to share any variable in which read and write operation is atomic by implementation e.g. read and write in int or boolean variable you can declare them as volatile variable.

Here is a link:

Explanation about volatile

edited May 23 '17 at 12:18

Community

1
1

answered Dec 02 '12 at 20:30

Narendra Pathai

41,187
18
82
120

but also take care that unnecessary use of `volatile` also is bad as requires a lot of operations (CPU cycles) – Narendra Pathai Dec 02 '12 at 20:34

Loop isn't getting updated boolean value from actionPerformed in java

2 Answers2