CODEWITHSUNDEEP
javaswingjframejpanel
muni
muni

Reputation: 117

Initiate a JPanel from a separate class (beginner)

I try to buid JPanels in a separate class to invoke them separately and add different items on it. Please can you tell me what i am doing wrong?

MyFrame.java

import javax.swing.*;
import java.awt.*;

public class MyFrame extends JFrame {

    public static void main(String[] args) {

        MyFrame frame = new MyFrame();
        frame.setVisible(true);

    }

    public MyFrame() {

       setTitle("MyFrame");
       setSize(300, 200);
       setLocationRelativeTo(null);
       setDefaultCloseOperation(EXIT_ON_CLOSE);

       MyPanel panel = new MyPanel();

       JButton testButton = new JButton("Test");
       panel.add(testButton);

    }

}

MyPanel.java

import javax.swing.*;
import java.awt.*;

class MyPanel extends JPanel {

    public MyPanel() {

        this.setOpaque(true);
        this.setVisible(true); 

    }

}

Upvotes: 1

Views: 898

Answers (1)

Hovercraft Full Of Eels
Hovercraft Full Of Eels

Reputation: 285440

You're not adding your panel variable to your JFrame's contentPane.

Add:

public MyFrame() {
   setTitle("MyFrame");
   // setSize(300, 200); // let's avoid this
   setDefaultCloseOperation(EXIT_ON_CLOSE);

   MyPanel panel = new MyPanel();

   JButton testButton = new JButton("Test");
   panel.add(testButton);
   add(panel);  // *** here
   pack();  // this tells the layout managers to do their thing
   setLocationRelativeTo(null);  
   setVisible(true);
}

As a side note:

public MyPanel() {

    this.setOpaque(true);
    this.setVisible(true); 

}

JPanels are opaque and visible by default, so your method calls within the MyPanel constructor do nothing useful.

Side note 2: I rarely if ever extend JFrame or any other top level window (with the exception of JApplet if I'm forced to use one of these), since I rarely change the innate behavior of the window. Better I think to create my top level windows (i.e., my JFrames) when needed.

Side note 3: Always strive to start your Swing GUI's on the Swing event thread. So do...

public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        @Override
        public void run() {
            MyFrame frame = new MyFrame();
            frame.setVisible(true);
        }
    });
}

Edit
You ask:

You wrote lets avoid setting the size.. can you tell me how i can pack it but set a minimum frame size?

I like to override the JPanel's getPreferredSize() method, and have it return a Dimension that makes sense.

For example, you could do something like this to be sure that your GUI is at lest PREF_W by PREF_H in size:

import java.awt.Dimension;

import javax.swing.*;

public class ShowGetPreferredSize extends JPanel {
   private static final int PREF_W = 400;
   private static final int PREF_H = PREF_W;

   @Override
   public Dimension getPreferredSize() {
      Dimension superSize = super.getPreferredSize();
      if (isPreferredSizeSet()) {
         return superSize;
      }
      int w = Math.max(superSize.width, PREF_W);
      int h = Math.max(superSize.height, PREF_H);
      return new Dimension(w, h);
   }

   private static void createAndShowGui() {
      JFrame frame = new JFrame("ShowGetPreferredSize");
      frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      frame.getContentPane().add(new ShowGetPreferredSize());
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

Upvotes: 5

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