problems with JPanel and JFrame

I seem to be doing something wrong, because my code compiles but all I get is a blank box, I've been going around in circles trying to figure out what I'm doing wrong.

This is a program that accepts a value from the user and uses it as the radius to draw a circle, in addition to displaying varius stats about the circle that is being shown.

Any help greatly appreciated.

Code:

---

import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.JLabel;
import java.awt.Color;
import java.awt.Graphics;
import javax.swing.SwingConstants;

public class circle extends JPanel
{
    public double radius;
    public double diameter;
    public double circum;
    public double area;
    public int diam;
    public JTextField input;
    public JLabel display;
   
    public void input()
    {       
        //input dialog box sits at top of screen
        input = new JTextField( 20 );
        input.setText( "Please input radius" );
        input.selectAll();
        add( input );
       
        //JLabel displays dimensions of drawn circle
        display = new JLabel( "circumference = " + circum +
                              "radius = " + radius +
                              "diameter = " + diameter +
                              "area = " + area, SwingConstants.BOTTOM );
       
        //a listener for input
        input.addActionListener(
            new ActionListener()
            {
                public void actionPerformed( ActionEvent event )
                {
                    if( event.getSource() == input )
                    {
                        radius = Double.parseDouble( event.getActionCommand() );
                        diameter = radius*2;
                        circum = radius*3.14159*2;
                        area = 3.14159*(Math.pow( radius, 2 ));
                        diam = (int)diameter;
                    }
                }
            }
        );
    }
    public void paintComponent( Graphics g )
    {
        super.paintComponent( g );
       
        g.setColor( Color.BLUE );
        g.fillOval( 50, 50, diam, diam );
    }
}

---

Code:

---

import javax.swing.JFrame;

public class circleFrame
{
    public static void main( String args[] )
    {
        JFrame frame = new JFrame( "All you wanted to know about a circle" );
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
       
        circle oval = new circle();
        frame.add( oval );
        frame.setSize( 750, 750 );
        frame.setVisible( true );
    }
}

---

when does the circle method input() ever get called? Does circle need a constructor?

There are a couple of different things that you always want to remember to do that will cause that type of behavior... The one that you will need to do is set the Layout Manager... If you extend JPanel and forget to set the Layout Manager you will always end up with a blank frame...

The second thing that you need to do is to call input()... It seems like you wanted input() to be your constructor, but you really want to use:

Code:

---


    public circle() {

      input();
    }

---

Notice that I called input() in the constructor... This is not necessarily bad practice... A more standard practice is instead of input() you want to call your method initComponents()... (Initiate Components)

The last thing that you want to do is call repaint() in your actionPerformed method... This will repaint the Frame with your oval...

Okay, I was using the word input a little excessively so I changed the method to the name initComponent() and now only the JTextField is called input. I also set a layout manager and made a call to repaint in actionPerformed, but I'm not sure if I did it right:

Code:

---

public void actionPerformed( ActionEvent event )
                {
                    if( event.getSource() == input )
                    {
                        radius = Double.parseDouble( event.getActionCommand() );
                        diameter = radius*2;
                        circum = radius*3.14159*2;
                        area = 3.14159*(Math.pow( radius, 2 ));
                        diam = (int)diameter;
                        repaint();
                    }

---

I'm not sure what you mean by calling input, I tried adding "oval.initComponent()" in class circleFrame but it caused an exception. I thought that creating an object of the class would mean that all the methods in the class would be executed, I'm not sure what you mean by using input as a constructor....

**edit: figured it out and got it running, I realized how stupid I sounded and that I did need to call oval.initComponent() but that there was an exception being created because of me. I was declaring SwingConstants incorrectly in the label which caused the compiler to crash. I've got everything running now, thanks for the help mark and fubar

Quote:

---

I thought that creating an object of the class would mean that all the methods in the class would be executed, right?

---

Heck no. You have to unlearn this. Only a constructor gets executed. Otherwise methods will only get executed if they are called, period.

Mark has put in code what I stated in my post.

Quote:

---

Originally Posted by markw8500

If you extend JPanel and forget to set the Layout Manager you will always end up with a blank frame...

---

I really don't think this is true.

See the JFrame API docs for a discussion of how, by default, JFrame forwards setLayout() calls to the content pane (and how this pane will have a BorderLayout if no other is set.)

pbrockway2: I see what you are saying, but in my experience I have never gotten that to work...

v1nsai: Fubarable is correct... Methods only get called if you call them... You always want your program to do what you want it to do, WHEN you want it to do it... Google "Java Constructors" to get all the details on constructors... But for now you want to call your initComponents() in your constructor like so:

Code:

---

    public circle() {

      initComponents();
    }

---

When you create:

Code:

---


circle oval = new circle();

---

the constructor will be called and it will exicute all the code inside for that constructor... And in our case it will be initComponents()...

Post the code that you have and lets take a look...

...does this

Code:

---

Double.parseDouble( event.getActionCommand() );

---

return the text for a JTextfield?

Regardless, the real problem is that you are trying to draw directly onto the JPanel, which is filled with your JTextField, so it's always going to get overpainted and you'll never see the oval.

I suggest creating a new component (by extending JComponent), and drawing your oval there instead.

I get what your saying about the constructors mark, I realized about halfway into the semester that the book I'm learning from just sucks (online course) so I've been half learning from the book and half from google, I'm still missing bits and pieces of things hehe. I still don't understand how you suggest I should implement the initComponent method as a constructor, I simply called it after I created an object of class circle. Would you maybe edit my code to show me what you mean please?

Code:

---

public class circle extends JPanel
{
    public double radius;
    public double diameter;
    public double circum;
    public double area;
    public int diam;
    public int center;
    public JTextField input;
    public JLabel display;
   
    public void initComponent()
    {
        setLayout( new FlowLayout() );
       
        this.setBackground( Color.BLACK );
       
        //input dialog box sits at top of screen
        input = new JTextField( 20 );
        input.setText( "Please input radius" );
        input.selectAll();
        add( input );
       
        //JLabel displays dimensions of drawn circle
        display = new JLabel( "<html>circumference = " + circum +
                              "<br>radius = " + radius +
                              "<br>diameter = " + diameter +
                              "<br>area = " + area + "</html>" );
        display.setForeground( Color.white );
        add( display );                                   
       
        //a listener for input
        input.addActionListener(
            new ActionListener()
            {
                public void actionPerformed( ActionEvent event )
                {
                    if( event.getSource() == input )
                    {
                        //set variables according to input from JTextField
                        radius = Double.parseDouble( event.getActionCommand() );
                        diameter = radius*2;
                        circum = radius*3.14159*2;
                        area = 3.14159*(Math.pow( radius, 2 ));
                       
                        diam = (int)diameter;//creates a usable int for paintComponent
                       
                        //sets JLabel display
                        display.setText( "<html>circumference = " + circum +
                              "<br>radius = " + radius +
                              "<br>diameter = " + diameter +
                              "<br>area = " + area + "</html>" );
                             
                        //sets the draw offset so circle is centered
                        center = 350 - diam/2;
                        repaint();
                    }
                }
            }
        );
    }
    public void paintComponent( Graphics g )
    {
        super.paintComponent( g );
       
        g.setColor( Color.blue );
        g.fillOval( center, center, diam, diam );
    }
}

---

Code:

---

public class circleFrame
{
    public static void main( String args[] )
    {
        JFrame frame = new JFrame( "All you wanted to know about a circle" );
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
       
        circle oval = new circle();
        oval.initComponent();
       
        frame.add( oval );
        frame.setSize( 750, 750 );
        frame.setVisible( true );
    }
}

---

No Prob... The good thing is that you got your program to work...

Quote:

---

I still don't understand how you suggest I should implement the initComponent method as a constructor

---

You don't want to implement initComponent as a constructor, but you want to call it in the constructor...

Code:

---


public class circle extends JPanel {

    private double radius;
    private double diameter;
    private double circum;
    private double area;
    private int diam;
    private JTextField input;
    private JLabel display;

    public circle() {

      initComponents();
    }




    private void initComponents() {
        //input dialog box sits at top of screen
        input = new JTextField(20);
        input.setText("Please input radius");
        input.selectAll();
       
       
        setLayout( new FlowLayout() );

       
        add(input);

        //JLabel displays dimensions of drawn circle
        display = new JLabel("circumference = " + circum +
                "radius = " + radius +
                "diameter = " + diameter +
                "area = " + area);

        //a listener for input
        input.addActionListener(
                new ActionListener() {

                    public void actionPerformed(ActionEvent event) {
                        if (event.getSource() == input) {
                            radius = Double.parseDouble(event.getActionCommand());
                            diameter = radius * 2;
                            circum = radius * 3.14159 * 2;
                            area = 3.14159 * (Math.pow(radius, 2));
                            diam = (int) diameter;

                            System.out.println(event.getActionCommand());
                            repaint();
                        }
                    }
                });
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);

        g.drawOval(200, 200, 200, 200);
        g.setColor(Color.BLUE);
        g.fillOval(200, 200, diam, diam);
       
    }
}

---

Code:

---


public class Main {

    /**
    * @param args the command line arguments
    */
    public static void main(String[] args) {
        // TODO code application logic here

        JFrame frame = new JFrame( "All you wanted to know about a circle" );
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

        circle oval = new circle();
       
        frame.add( oval );
        frame.setSize( 750, 750 );
        frame.setVisible( true );


    }

}

---

Whenever you create your object circle oval = new circle();... circle() is your constructor... This will call everything inside... Which in our case it is initComponents()...

So if you wanted to set an initial radius of your oval in case the user leaves the text box empty you could add this constructor:

Code:

---


    public circle(double radius) {
 
      this.radius=radius;

      initComponents();
    }

---

and down in your actionPerformed method you would do:

Code:

---


        input.addActionListener(
                new ActionListener() {

                    public void actionPerformed(ActionEvent event) {
                   
                        String s="";
 
                        if (event.getSource() == input) {
                            //set variables according to input from JTextField

                            if (!event.getActionCommand().isEmpty()) {
                                radius = Double.parseDouble(event.getActionCommand());
                                s="";

                            } else {


                                s = "Although you did not enter a radius I did one for you...<br>";

                            }


                            //sets JLabel display
                            display.setText("<html>"+s+"circumference = " + circum +
                                    "<br>radius = " + radius +
                                    "<br>diameter = " + diameter +
                                    "<br>area = " + area + "</html>");


                            diameter = radius * 2;
                            circum = radius * 3.14159 * 2;
                            area = 3.14159 * (Math.pow(radius, 2));

                            diam = (int) diameter;//creates a usable int for paintComponent



                            //sets the draw offset so circle is centered
                            center = 350 - diam / 2;
                            repaint();
                        }
                    }
                });

---

You can add as many constructors as you want as long as they are all different... I think they call it overloading the constructor... Not sure though...

So now your code will look like:

Code:

---


public class Circle2 extends JPanel {

    private double radius;
    private double diameter;
    private double circum;
    private double area;
    private int diam;
    private int center;
    private JTextField input;
    private JLabel display;
    private String s;

    public Circle2() {

        intiComponents();
    }

    public Circle2(double radius) {

        this.radius = radius;
        intiComponents();

    }

    public void intiComponents() {
        setLayout(new FlowLayout());

        this.setBackground(Color.BLACK);

        //input dialog box sits at top of screen
        input = new JTextField(20);
        input.setText("Please input radius");
        input.selectAll();
        add(input);

        //JLabel displays dimensions of drawn circle
        display = new JLabel("<html>circumference = " + circum +
                "<br>radius = " + radius +
                "<br>diameter = " + diameter +
                "<br>area = " + area + "</html>");
        display.setForeground(Color.white);
        add(display);

        //a listener for input
        input.addActionListener(
                new ActionListener() {

                    public void actionPerformed(ActionEvent event) {
                        if (event.getSource() == input) {
                            //set variables according to input from JTextField

                            if (!event.getActionCommand().isEmpty()) {
                                radius = Double.parseDouble(event.getActionCommand());
                                s = "";

                            } else {


                                s = "Although you did not enter a radius I did one for you...<br>";

                            }


                            //sets JLabel display
                            display.setText("<html>" + s + "circumference = " + circum +
                                    "<br>radius = " + radius +
                                    "<br>diameter = " + diameter +
                                    "<br>area = " + area + "</html>");


                            diameter = radius * 2;
                            circum = radius * 3.14159 * 2;
                            area = 3.14159 * (Math.pow(radius, 2));

                            diam = (int) diameter;//creates a usable int for paintComponent



                            //sets the draw offset so circle is centered
                            center = 350 - diam / 2;
                            repaint();
                        }
                    }
                });
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);

        g.setColor(Color.blue);
        g.fillOval(center, center, diam, diam);
    }
}

---

Another good practice is staying away from making your variables public... Unless you intentionally want other classes to access them... Notice that I also made initComponents private... The reason that I was told is that if there are two or three people working on a project, one of the other people might accidentally change a variable that is not supposed to be changed... Like so:

Code:

---


public static void main(String[] args) {
        // TODO code application logic here

        JFrame frame = new JFrame( "All you wanted to know about a circle" );
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

        Circle2 oval = new Circle2(25);
        oval.radius=200;
       
        frame.add( oval );
        frame.setSize( 750, 750 );
        frame.setVisible( true );


    }

}

---

So now what will happen is that if the user leaves the text box empty the initial radius will be 200 and not 25...

cool, I had no idea you could call constructors like that, my stupid book definitely never mentioned that. Thanks a bunch :)

No prob...

Nice little program. Thanks.

Anytime...