Isabella Flores
Painting over an existing screen in Eclipse Java can be achieved through various methods, such as utilizing the paintComponent method, which is commonly used for custom painting. This method is part of the SwingUtilities invokeLater method, which is essential for placing Swing components on the Event Dispatch Thread (EDT). Additionally, the Graphics2D class can be used to paint graphics and draw strings. To avoid screen flicker, the style bit SWT.NO_BACKGROUND can be used when creating the Canvas. This prevents the native background from being drawn but requires the program to draw every pixel of the client area. By understanding these techniques and tools, developers can effectively paint over existing screens in Eclipse Java.
| Characteristics | Values |
|---|---|
| How to paint over existing screen | Use the paintComponent method to paint over a JPanel |
| How to avoid screen flicker | Use the style bit SWT.NO_BACKGROUND when creating the Canvas |
| How to draw on a Control | Use a GC to draw onto a Control, but be aware that your changes will be overwritten when the operating system draws the Control |
| How to draw lines, text, and fill shapes | Use a GC and pass in a drawable such as an Image or a Control using the paintEvent callback |
| How to set the background color | Use the setBackground method and set the color to the desired value |
| How to set the font | Use the setFont method and pass in the desired font as a parameter |
| How to set the preferred size | Use the setPreferredSize method and pass in the desired dimensions |
| How to create a complex game | Follow the model/view/controller pattern and create separate classes for each component |
| How to paint the background | Invoke super.paintComponent before painting the contents of the component |
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What You'll Learn
Drawing with the paintComponent method
The paintComponent method is used for drawing on a JPanel in Java. The paintComponent method is called whenever Swing requests a repaint. The Java 2D API provides extensive support for implementing line widths and styles, as well as patterns for use in filling and stroking shapes.
To paint a background, you can invoke super.paintComponent before painting the contents of your component. You can also paint the background yourself using the following code at the top of a custom component's paintComponent method:
> g.setColor(getBackground);
> g.fillRect(0, 0, getWidth(), getHeight());
> g.setColor(getForeground());
If you are using the paint method, you will need to provide a Graphics argument. If you are using the paintComponent method, you will need to use a super declaration to add something to this method and take Graphics objects as parameters.
The paintComponent method can be used to draw lines, text, and fill shapes. For example, to draw text, you define its top-left corner, width, and height. To draw a line, you use the drawLine method. To fill a rectangle, you use the fillRect method.
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Using GC to draw onto a Control
The class org.eclipse.swt.widgets.Control is a Drawable, so you can draw onto a Control in the same way you would draw onto an Image. This involves passing the Control as the argument of GC to draw onto it. However, unlike drawing on an image, which permanently changes the data that makes up the graphic, using a GC to draw onto a control is not permanent. This is because when the operating system draws the control, it will overwrite your changes.
To draw onto a control, you should add a listener to its paint event. The listener class is org.eclipse.swt.events.PaintListener, and the callback method argument is an instance of org.eclipse.swt.events.PaintEvent. The PaintEvent includes a GC that you can send messages to, which is already prepared to draw onto the control and includes the damaged area.
The API of a GC allows lines and shapes to be drawn in its foreground colour and filled in its background colour. Text can also be drawn onto a GC, with the glyphs drawn in the GC's foreground colour and font, and the area it occupies drawn in the GC's background colour. To draw text, you must define its top left corner, width and height. There are two sets of methods to draw text, the first set of which have 'Text' in their method name, and will handle line delimiters and tabs and are used to implement an emulated Label. The second set of API methods have 'String' in their method name, and no tab expansion or carriage return processing occurs, and are used for more sophisticated controls like the StyledText used by the Eclipse Java editor.
When drawing occurs on a GC, you are altering its pixel values that make up the graphic on the Drawable surface. If you set the GC's XOR mode to true, then for each pixel, the red, green and blue values of the source being drawn are XOR'd with the existing values, and the result becomes the new destination pixel.
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Filling shapes and setting background colours
To fill shapes, you can use the setPaint() method of Graphics2D, which sets the current colour in the graphics context. This method accepts any object that implements the Paint interface, including solid colours, colour gradients, and textures. The Java.awt.Color class handles colour in Java, allowing you to create arbitrary colours by specifying red, green, and blue values.
When drawing on a GC, you are altering its pixel values. By setting the GC's XOR mode to true, you can combine the red, green, and blue values of the source and destination pixels to create new colours. For example, filling a blue rectangle with white will result in a yellow rectangle.
To set the background colour of a component, you can use the setBackground() method. However, if your component is not opaque or does not have custom painting code, the background colour may not be painted. In this case, you need to invoke setOpaque(true) to ensure that the background is painted.
Additionally, the paintComponent method is crucial for painting in Java. This method is called whenever Swing requests a repaint, and it is where you can set the background colour and fill shapes. By invoking super.paintComponent before painting the contents of your component, you can paint the background yourself.
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Avoiding screen flicker
When drawing on a Graphics Control (GC), you are altering its pixel values, which can create screen flicker. The default behaviour for a Canvas is that before it paints itself, the entire client area is filled with the current background colour. This can cause screen flicker because if the paintEvent also draws onto the GC, the user sees a flash between the original background being filled and the drawing occurring.
To avoid this, use the style bit SWT.NO_BACKGROUND when creating the Canvas. This prevents the native background from being drawn, but it means the program must be responsible for drawing every pixel of the client area. When a widget is resized, a paint event occurs, which can also create screen flicker as the client area is repeatedly repainted. This flicker is also known as flash, and the style bit SWT.NO_REDRAW_RESIZE can be used to reduce this. When NO_REDRAW_RESIZE is used and the control's size is reduced, no paint event is generated.
Another way to avoid screen flicker is to use double buffering, which is provided by default in JPanel and JComponent. Do not call repaint inside any paint method, and do not load your images in the paint method.
If you are using Ubuntu 16.04 and Eclipse Neon, you can try switching to GTK2 to solve the problem of screen flicker when pressing enter. However, this can cause issues with the Dark theme, such as scroll bar and button colouring.
If you are using macOS, you may not experience screen flicker at all.
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Custom painting with Swing components
Custom painting in Swing is similar to custom painting in AWT, but it is not recommended to write applications entirely with AWT. The paintComponent method is where all of the custom painting takes place. This method is defined by javax.swing.JComponent and then overridden by subclasses to provide their custom behaviour. Its sole parameter, a java.awt.Graphics object, exposes a number of methods for drawing 2D shapes and obtaining information about the application's graphics environment.
The Java 2D API provides extensive support for implementing line widths and styles, as well as patterns for use in filling and stroking shapes. The paintComponent method is called whenever Swing requests a repaint. The SwingUtilities invokeLater method expects a Runnable as a parameter, so it is easier to make the JFrame class implement Runnable. All Swing applications should use the SwingUtilities invokeLater method to put the Swing components on the Event Dispatch thread (EDT).
If your custom component extends JPanel or a more specialized JComponent descendant, you can paint the background by invoking super.paintComponent before painting the contents of your component. You can paint the background yourself using this code at the top of a custom component's paintComponent method: g.setColor(getBackground); g.fillRect(0, 0, getWidth(), getHeight()); g.setColor(getForeground());
To create something as complex as a game, you should follow the model/view/controller pattern when creating your Java Swing GUI. You should create model classes for the game board and pieces, a view class to draw the board, and another to create the main window.
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Frequently asked questions
You can use the paintComponent method to paint over an existing screen in Eclipse Java. You can also use the paintEvent callback to draw onto a Control in the same way that you draw onto an Image.
Some common issues include:
- The stuff I paint doesn't show up.
- The edges of a particular component look odd.
- I used setBackground to set my component's background color, but it seemed to have no effect.
- I'm using the exact same code as a tutorial example, but it doesn't work.
One way to avoid screen flicker is to use the style bit SWT.NO_BACKGROUND when creating the Canvas. This prevents the native background from being drawn, but it means that the program must be responsible for drawing every pixel of the client area.
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