GUIs in Java

The Java classes for graphical user interfaces are arranged in two packages, known as AWT (java.awt) and Swimg (javax.swing). The model is complicated and powerful and we don't try to explain it in detail here. Instead, we provide boiler-plate examples and common idioms.

Basic graphics

The exercise is to draw a bar chart, looking like this:

from a data file which the user selects using the Open item of the File menu. We will do this in stages.

Frames and Panels

Look at the program BarChartAxes.java, which simply draws the axes and a single block:

Points to note:

The top-level window is a JFrame, to whose "content pane" we add add a JPanel. We draw into the JPanel. This arrangement is typical.
The run-time environment calls our JPanel's paintComponent method, with the Graphics argument.
The Graphics argument has methods drawLine, drawRect, fillRect, etc.
Arguments for drawLine: x1,y1,x2,y2. Arguments for fillRect: topX, topY, width, height.

You can do the first exercise now if you want to.

Events and menus

Look at the program BarChartEvents.java. It adds a File menu to our Frame, and an item Exit.

Points to note:

the menu items are added to the menu; the menu is added to the menubar; the menubar is added to the JFrame.

The event model:

Some objects, such as buttons and menu items, generate events
Other objects are declared to listen to some of the events. In this case, our JPanel p has been declared as the object which will listen to the action events of the exit menu item.
The class of listening objects must implement an appropriate Listener interface, and define an appropriate method which gets called when the event takes place. In this case, BarChartPanel implements ActionListener and defines actionPerformed.

Another example

Look at the program MyClickExample.java. It displays a frame consisting of some tiles. The program detects where the user clicks, and reports which tile it corresponds to.

Points to note:

The abstract class MouseAdapter implements the MouseListener interface, and defines empty methods mouseClicked(), mouseEntered() etc. We extend MouseAdapter, overriding the methods (in this case, MouseClicked) that we are interested in defining.
Our extension of MouseAdapter is an anonymous inner class.