Universidad Carlos III de Madrid

Run the following application:

import java.awt.Dimension;

import javax.swing.JFrame;
import javax.swing.JLabel;

public class HelloWorld extends JFrame {

    public HelloWorld() {
        super("HelloWorld");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        getContentPane().add(new JLabel("Hello World!"));
        setMinimumSize(new Dimension(200,100));
        setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
                public void run() {
                    HelloWorld gui = new HelloWorld();
                }
            });
    }
}

Code an application that shows a calculator similar to the one in the following image:

For now, there is no need for the calculator to react to user's actions (e.g. pressing a key). It just has to always show 0 as result.

In order to obtain a similar aspect to the one showed in the image, you can establish a BorderLayout in the window upper level. Then, you can add:

Besides using the existing components provided by Swing, a graphical application may also draw lines, circles, text and other figures directly over objects of JPanel class. Consider the next example consisting of two classes. The first one extends from JPanel and serves as the drawing canvas:

import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JPanel;


public class Canvas extends JPanel {
    private Dimension preferredSize;

    public Canvas(Dimension preferredSize) {
        this.preferredSize = preferredSize;
    }

    public Dimension getPreferredSize() {
        return preferredSize;
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Dimension size = getSize();
        int width = (int) size.getWidth();
        int height = (int) size.getHeight();
        g.drawLine(0, 0, width - 1, height - 1);
        g.drawLine(width - 1, 0, 0, height - 1);
    }
}

The second class is a window containing an instance of the previous class:

import java.awt.Dimension;

import javax.swing.JFrame;

public class CanvasApplication extends JFrame {

    public CanvasApplication() {
        super("Canvas");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        getContentPane().add(new Canvas(new Dimension(640, 480)));
        pack();
        setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
                public void run() {
                    CanvasApplication gui = new CanvasApplication();
                }
            });
    }
}

Run the application and observe carefully how it works. What happens if you change the window size?

Swing invokes the method paintComponent from Canvas every time the application needs to be redrawn (e.g., when the window size changes). In this method, the object received as parameter, which belongs to Graphics class, is used to draw the lines defining the panel diagonals. Take a closer look at this class API. Notice that you can also draw arcs, ovals, polygons, text, images, etc. It is also possible to establish the drawing color or the text font.

Modify the method paintComponent so that it draws what you want. Use, at least, polygons and circles (only their outline, or adding their filling too), text and different colors. This way you can practice the use of Graphics class. In order to learn how to draw each of the figures mentioned above, check the class API.