Your code has several problems, including:
- Your class extends JFrame: There's no reason to extend a class if you're not changing the innate behavior of that class, in other words, if you're not overriding methods of the class.
- Also, extending JFrame forces your code to create JFrames, and that limits the flexibility of the code, since sometimes you will want to use the same GUI code in other ways, such as nested within another JFrame or JPanel or in a dialog window such as a JDialog or JOptionPane.
- Even though you extend JFrame in your "test" class, you never create an instance of this class, your main method has no call to
new test(), and so that JFrame class, while defined, is never utilized.
- You are using
setBounds(...) to artificially place a component in your Swing GUI. For this to work, you would need to change the container's layout (here the "test" JFrame) to null, something that you don't do.
- Also, while null layouts and
setBounds() might seem to Swing newbies like the easiest and best way to create complex GUI's, the more Swing GUI'S you create the more serious difficulties you will run into when using them. They won't resize your components when the GUI resizes, they are a royal witch to enhance or maintain, they fail completely when placed in scrollpanes, they look gawd-awful when viewed on all platforms or screen resolutions that are different from the original one.
- You are calling
setSize(...) on the JFrame that you do create, artificially constraining your top-level window (the JFrame) to some size, a size that may not be optimal for that GUI on all platforms.
Instead, I suggest that you:
- Don't extend JFrame, don't extend anything unless needed, and in that situation, you would usually extend a JPanel and not a JFrame. You would usually do this when you desire to override one of the JPanel's methods such as its
paintComponent(Graphics g) method to allow it to paint an image or drawing on it.
- Nest JPanels each using its own layout manager. This will require that you first learn about how to use the Swing layout managers: Layout Manager Tutorial
- If you do need to create a larger GUI, then override the
public Dimension getPreferredSize() method in a smart way.
- Create your JFrame, just as you are doing, in the main method
- But do so in a Swing thread-safe way, on the Swing event thread, using
SwingUtilities.invokeLater(...).
- Study and learn from the Swing tutorials. You can find links to the Swing tutorials and to other Swing resources here: Swing Info.
- Ignore Bala Yokesh Mani's suggestions and code. While he may mean well, he is offering bad examples, I'm afraid.
- Avoid over-use of static fields and methods (as incorrectly shown in Bala Yokesh Mani's code) and instead do most work in the instance (non-static) domain.
- Learn and use Java naming conventions. Variable names should all begin with a lower letter while class names with an upper case letter. Learning this and following this will allow us to better understand your code, and would allow you to better understand the code of others.
For example, your code could look something like:
import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.FlowLayout;
import javax.swing.*;
@SuppressWarnings("serial")
// not really needed to extend JPanel here, but doing it
// for simplicity's sake
public class TestPanel1 extends JPanel {
public static final int PREF_W = 640;
public static final int PREF_H = 480;
private JButton button1 = new JButton("B1");
public TestPanel1() {
JPanel topPanel = new JPanel(new FlowLayout(FlowLayout.LEADING));
topPanel.add(button1);
// this is not the cleanest way to do this, but it is simple:
setPreferredSize(new Dimension(PREF_W, PREF_H));
setLayout(new BorderLayout());
add(topPanel, BorderLayout.PAGE_START);
}
public static void main(String[] args) {
// use SwingUtilities to start the GUI on the event thread
SwingUtilities.invokeLater(() -> {
// create instance of our class
TestPanel1 mainPanel = new TestPanel1();
// create JFrame
JFrame frame = new JFrame("Test 1");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
// add our instance to the JFrame
frame.add(mainPanel);
// pack the JFrame to activate layouts and to
// set sizes of all components and containers
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
});
}
}
Then later if you wanted to add a background image, you could do so easily:
import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.*;
@SuppressWarnings("serial")
public class TestPanel2 extends JPanel {
public static final String IMG_PATH = "https://upload.wikimedia.org/"
+ "wikipedia/commons/thumb/e/ef/"
+ "Mesurupetala%2C_dragonfly%2C_Late_Late_Jurassic%2C_Tithonian_Age"
+ "%2C_Solnhofen_Lithographic_Limestone%2C_Solnhofen%2C_Bavaria"
+ "%2C_Germany_-_Houston_Museum_of_Natural_Science_-_DSC01817.JPG/"
+ "640px-thumbnail.jpg";
private JButton button1 = new JButton("B1");
private BufferedImage backgroundImg = null;
public TestPanel2(BufferedImage bkgImg) {
this.backgroundImg = bkgImg;
JPanel topPanel = new JPanel(new FlowLayout(FlowLayout.LEADING));
topPanel.setOpaque(false);
topPanel.add(button1);
setLayout(new BorderLayout());
add(topPanel, BorderLayout.PAGE_START);
}
@Override
public Dimension getPreferredSize() {
Dimension originalSize = super.getPreferredSize();
if (backgroundImg != null) {
int w = Math.max(backgroundImg.getWidth(), originalSize.width);
int h = Math.max(backgroundImg.getHeight(), originalSize.height);
return new Dimension(w, h);
} else {
return super.getPreferredSize();
}
}
@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
if (backgroundImg != null) {
g.drawImage(backgroundImg, 0, 0, null);
}
}
public static void main(String[] args) {
SwingUtilities.invokeLater(() -> {
BufferedImage img = null;
try {
URL imgUrl = new URL(IMG_PATH);
img = ImageIO.read(imgUrl);
} catch (IOException e) {
e.printStackTrace();
System.exit(-1);
}
TestPanel2 mainPanel = new TestPanel2(img);
JFrame frame = new JFrame("Test");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(mainPanel);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
});
}
}
