Introduction to Graphical User Interfaces (GUIs)

Basic Applet Code – Draws A Rectangle

import java.applet.Applet;

import java.awt.*;

public class rect extends Applet

{

    public void paint (Graphics g)

    {

        //Program code goes here

        g.setColor (Color.blue);

        g.drawRect (200, 200, 100, 100);

    }

}

Some Graphics Methods To Put in void Paint

g.setColor(Color.red);

• Changes the current color to whatever is specified.

g.drawstring("Hello World", 75, 100);

• Writes Hello World in the current colour at position 75, 100 on the screen.

g.drawRect(1,1, 200, 400);

• Draw a rectangle starting at position 1, 1. It goes on for 200 pixels right and 400 pixels down. It is not filled and it is drawn in the current colour.

g.fillRect(1,1,200,400);

• As above but it fills in the rectangle with the current colour.

g.drawOval(50,50, 20, 30);

• Draws an oval that in a box that starts at 50,50 and that has goes in the x direction 20 pixels and the y direction 20 pixels. It is drawn in the current colour.

g.fillOval(50,50,20,30);

• As above but it fills in the oval with the current colour.

g.drawPolygon(Xarray, Yarray, NumPoints);

g.fillPolygon(Xarray, Yarray, NumPoints);

• Makes a polygon based on the co-ordinates in Xarray and Yarray. See triangle example below.

Void Main (Text Based User Interface)

You already know this mode, we used it in unit 1:

• import java.io* when you are using it to pull in the methods that handle input and output

• Use System.out.print(“hi”) or System.out.println(“hello”) to print. The first one stays on the line to add more, the second line prints its message and moves to a new line.

• Put the Keyin class in the same folder as the file that you are working with. It has code written to handle input. We use the Keyin class instead of writing it ourselves because Java’s input is a little awkward. Whenever you input something, it can throw an IOException and that is still something we aren’t good at handling.

• To Input Strings (prompt is in the brackets):

String c = IBIO.inputString(“Please enter your name”);

•  To Input Integers (prompt is in the brackets):

int j = IBIO.inputInt(“Please enter the year”);

•  There is only text in this mode. We cannot clear the screen, we cannot draw pictures.

Applets (GUI)

Applets are ‘little applications’. You can post them on your webpage. 

To get one:

• (Blue Jay) Click on the New Class Button and choose the Applet class type radio button

• (Ready to Program) File -> New -> Applet Boilerplate

Note these differences from the text based programs we wrote before:

• import java.awt.* to get widgets (buttons, textfields, etc)

• import java.swing.* to get the code for applets.

Applets need extends Applet on the end of the class declaration line.

Applets have certain predefined methods that you should use. 

•  public void init() gets the interface set up and ready to draw. Init is called once when the program starts running. 

• public void paint (Graphics g) handles the drawing of pictures and shapes. 

• public void actionPerformed (Event e) is called once every time the user does something, like click on a button.