Develop Reference

Program with JFrame won't start

I was trying to write a program, where you put some numbers in a JTextField an then it does something with it. I tried figuring out how to even make an input possible. But the way I'm trying it doesn't work despite Eclipse showing no errors. And yes, I know there is no way to stop this program but this is just a test.
import javax.swing.*;
public class NotenEingabe extends JFrame {
public static void main(String[]args) {
JFrame frame = new JFrame();
JPanel panel = new JPanel();
JLabel label = new JLabel("text");
JTextField field = new JTextField("text");
panel.add(label);
panel.add(field);
frame.setTitle("Grade input");
frame.pack();
frame.add(panel);
frame.setVisible(true);
}
}
I hope it is a real problem and not simply my tiredness.

frame.setVisible(true);
frame.add(panel);
Components should be added to the frame BEFORE the frame is made visible.
The layout manager is not invoked so the components have a size of (0, 0) which means there is nothing to paint.
frame.setTitle("Grade input");
frame.add(panel);
//frame.setSize(700, 700);
frame.pack();
frame.setVisible(true);

First of all when you extend your class from jframe you dont need to instantiate new jFrame just make a new instance of your class then access jframe with that or just delete extend Jframe and use functional approach.
The you didn't set de layout of pane
Test this one then you can choose from veriety of layouts
panel.setLayout(newFlowLayout());

Get the real size of a JFrame content (NEW)

I am writing a Game in Java, and I don't want to use a layout manager for my JFrame. My class extends JFrame, and looks something like this:
//class field
static JPanel contentPane;
//in the class constructor
this.contentPane = new JPanel();
this.contentPane.setLayout(null);
this.setContentPane(contentPane);
I want my JPanel be exactly 600x600 px, but when I set the size of my JFrame by calling the this.setSize(600,600) method, the JPanel size is less than the 600x600 px because the border of the JFrame window is included too.
How can I set the size of the JPanel to be exactly 600x600 px?
P.S. I have seen all of the previous post and none of them work for me.
For example:
Get the real size of a JFrame content does not work for me.
What else can I do?

How can I set the size of the JPanel to be exactly 600x600 px?
Override the getPreferredSize() method of your custom game panel to return the size you want the panel to be.
#Override Dimension getPreferredSize()
{
return new Dimension(600, 600);
}
Then the basic code to create your frame will be:
GamePanel panel = new GamePanel();
frame.add(panel, BorderLayout.CENTER);
frame.pack();
frame.setVisible(true);
Now all your game logic will be contained in the GamePanel class.

If you don't need the Frame-Decorations (icon, title, min/max/close-buttons and the border), you can make a Frame undecorated, then it has exactly the size you gave it
java.awt.Frame.setUndecorated(boolean)

JButton added to JPanel doesn't show

I can't seem to add a JButton to a JPanel.
I have a PropWindow (JFrame) that has a PropView (JPanel) in it. the PropView-JPanel seems to be added correctly because I can draw shapes on it with paint().
But when I use this to try adding a button it just won't show up att all :/
JButton testButton;
public PropView(int width, int height) {
super(true);
setLayout(null);
setSize(width, height);
//TestButton
testButton = new JButton("Test");
testButton.setLocation(10,10);
testButton.setSize(100, 50);
testButton.setVisible(true);
add(testButton);
setFocusable(true);
setVisible(true);
}
The JFrame and the JPanel are both 250x600 px.

I can't tell from the code snippet you posted but just in case: make sure you call pack () on the frame after you have added the panel or any other components.
Also, it's usually discouraged to extend a JPanel or JFrame, unless you have a good reason to do it, just a heads up.
Here you have a short tutorial about displaying frames:
And some sample code in it that might help:
//1. Create the frame.
JFrame frame = new JFrame("FrameDemo");
//2. Optional: What happens when the frame closes?
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//3. Create components and put them in the frame.
//...create emptyLabel...
frame.getContentPane().add(emptyLabel, BorderLayout.CENTER);
//4. Size the frame.
frame.pack();
//5. Show it.
frame.setVisible(true);

Make sure you added PropPanel to PropWindow using myPropWindow.getContentPane().add(myPropPanel), not just myPropWindow.add(myPropPanel).

JTextField not appearing on top of JFrame

How do I get a textbox to appear on this JFrame? Also is it good practice to build everything on top of the JFrame itself? Or is it better to overlay a JPanel and build everything on top of that?
Thanks in advance!
public class GUI {
private static JFrame frame = new JFrame("FrameDemo");
public GUI() {
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
BufferedImage myImage = null;
try {
myImage = ImageIO.read(new File("C:/Users/Desktop/background.jpg"));
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
frame.setContentPane(new ImageFrame(myImage));
JTextField field = new JTextField(10);
frame.add(field, BorderLayout.SOUTH);
Dimension dimension = new Dimension();
dimension.setSize(950, 800);
frame.setSize(dimension);
frame.setVisible(true);
}
}

You have replaced the default content pane with you own content pane. I would guess your content pane does not use a layout manager so the text field is never displayed.
Try something like:
//frame.setContentPane(new ImageFrame(myImage));
ImageFrame content = new ImageFrame(myImage));
content.setLayout( new BorderLayout() );
frame.setContentPane(content);
Now you text field should be added to the south of your image panel.
Also is it good practice to build everything on top of the JFrame itself? Or is it better to overlay a JPanel and build everything on top of that?
The content pane of a JFrame is a JPanel, so it doesn't really matter what you do since your will be using a panel either way. The key is to manage the layout manager of your content pane.

How can I display a BufferedImage in a JFrame?

I want to display variations of the same image in the same JFrame, for example display an image in JFrame, then replace it with gray scale of the same image.

To build on camickr's solution (for the lazy like me who want quick code to copy/paste) here's a code illustration:
JFrame frame = new JFrame();
frame.getContentPane().setLayout(new FlowLayout());
frame.getContentPane().add(new JLabel(new ImageIcon(img)));
frame.getContentPane().add(new JLabel(new ImageIcon(img2)));
frame.getContentPane().add(new JLabel(new ImageIcon(img3)));
frame.pack();
frame.setVisible(true);
//frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // if you want the X button to close the app

You will have to repaint the JFrame whenever you update the image.
Here is what a simple google on the topic brings up: (I use those tutorials for all my Java coding)
Java Tutorial: Drawing an Image

Just incase life's to short too read the official docs here's a dirty way to get it done multiple times over
private static JFrame frame;
private static JLabel label;
public static void display(BufferedImage image){
if(frame==null){
frame=new JFrame();
frame.setTitle("stained_image");
frame.setSize(image.getWidth(), image.getHeight());
frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
label=new JLabel();
label.setIcon(new ImageIcon(image));
frame.getContentPane().add(label,BorderLayout.CENTER);
frame.setLocationRelativeTo(null);
frame.pack();
frame.setVisible(true);
}else label.setIcon(new ImageIcon(image));
}

I'm not really sure what you question is but if you have a BufferedImage then you simply create an ImageIcon using the image, then you add the icon to a JLabel and add the label to the GUI like any other component.
If you question is about how to create a gray scale, the I suggest you search the web using those terms as the search keywords, I'm sure you will find examples out there.