Question

i make simple game , and it consist of 2 files , first file is “Alibaba.java” which is extended from JFrame , i used it to display general contents of the game !,

and the second file is “intro.java” which is extended from JPanel , i used it to show intro of the game which include (title & background & person) ,

my problem occured when i tried to add a simple button in the intro ! , i did a function to create the button , but the problem is when i run the game , the button which i added it don’t appear !! , but when i tried add it from the first file which extended from JFrame , its appeared ! ,
so what is the problem in my code ? is JPanel don’t accept JButtons ! or i must create the buttons from the JFrame file ?!

so i need to know how to add Jbutton inside Jpanel instead of add Jbutton in JFrame Direct !!,

this is my samples of my codes which contain the problem :

1st file (Alibaba.java)

package alibaba;

import java.awt.Color;
import java.awt.GraphicsDevice;
import java.awt.GraphicsEnvironment;
import java.awt.Toolkit;
import javax.swing.JFrame;


public class Alibaba extends JFrame {

    public Alibaba(){
        super("Alibaba");    


        Intro intro =   new Intro();
        this.add(intro);


        GraphicsEnvironment environment =   GraphicsEnvironment.getLocalGraphicsEnvironment();
        GraphicsDevice  device  =   environment.getDefaultScreenDevice();

        device.setFullScreenWindow(this);    
    }

    public static void main(String[] args) { 

        Alibaba alibaba =   new Alibaba();
    }
}

2nd file(Intro.java) :

package alibaba;


import javax.swing.JButton;




public class Intro extends javax.swing.JPanel implements Runnable{

    Thread _intro_run;

    public Intro() {
        _intro_run  =   new Thread(this);
        _intro_run.start();
    }


    @Override
    public void run() {

    // Here i tried to add a button to the Intro !!! 

        this.add(this.createbutton("Exit"));
    }


    public JButton createbutton(String text){

        JButton _button  = new JButton(text);

        return _button;
    }

}

So please tell me what is the problem and how to solve it , sorry but iam new to java , new to programming games world ! ,, thank you 🙂

Answer 1

You have to add the JButton inside the main thread, cross thread Component manipulation is bad.

For example:

public Intro() {
    JButton exitButton = new JButton("Exit");
    this.add(exitButton);
} 

Alternatively, use SwingUtilities.invokeLater(Runnable). For example, in your run method:

@Override
public void run() {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            add(createbutton("Exit"));
        }
    }
} 

Based on discussion it seems that you are overriding the paint or paintComponent methods. You need to call super in them, like:

void paint(Graphics g) {
    super.paint(g);
    // do other stuff to g
}