The Graphics Class

J McQuillan SE204: 2004/2005: Lecture 4 slide 1

The Graphics Class

• Used when we need to draw to the screen

• Two graphics classes

– Graphics

– Graphics2D


Graphics and Graphics2D

• Graphics– java.awt.Graphics is an abstract class– It provides a limited range of drawing operations

• Graphics2D– Graphics2D is an extension of Graphics– It enables far more powerful drawing operations

Graphics context means the java.awt.Graphics

object that is currently being used. This is mostly the

current drawing space within a window.


An instance of the java.awt.Graphics class can only be

obtained from another graphics element such as an image or an

already existing graphics object.

How do we obtain an instance of a graphics object?

– It is passed to the paint() and paintComponent() methods of the components

– It can be copied from an existing Graphics object using the Graphics method create()

– It can be gotten through the components method getGraphics()


So,

Graphics g = new Graphics();

is invalid!


A Simple Example

import java.awt.*;

import javax.swing.*;

class drawPanel extends JPanel{

drawPanel(){

setPreferredSize(new Dimension(200, 300));

}

public void paintComponent(Graphics g){

super.paintComponent(g);

g.drawLine(10, 10, 100, 100);

}

}


When we want to show graphics on a frame, we will not

directly draw on the surface of the frame.

Instead, we will draw the graphics onto a JPanel and add

the JPanel to the frame.


public class drawFrame extends JFrame {drawFrame(){

setSize(300, 400);drawPanel p = new drawPanel();getContentPane().add(p);setVisible(true);

}

public static void main(String args[]){new drawFrame();

}}



Methods in the Graphics class

void drawLine(int startX, int startY, int endX, int endY)

void drawRect(int x, int y, int width, int height)

void drawOval(int x, int y, int width, int height)

Look up the rest of these methods!


Another Example

import java.awt.*;


class drawPanel extends JPanel{

drawPanel(){

setPreferredSize(new Dimension(200, 300));

}



g.drawLine(10, 10, 100, 100);

g.drawRect(30, 35, 60, 70);

}

}


public class drawFrame extends JFrame {drawFrame(){

setSize(300, 400);drawPanel p = new drawPanel();getContentPane().add(p);setVisible(true);

}


}}



The above methods drew the outline of the shapes. What if

we want to fill the shape with a particular colour?

Use the following methods

fillRect(int x, int y, int width, int height)


Using the Graphics2D class

A graphics object is passed to the paint() and

paintComponent() methods.

To use a Graphics2D object we cast it.


super.paintComponenet(g);

Graphics2D g2 = (Graphics2D)g;

//draw using Graphics2D

}


Using Graphics2D

There are several Graphics2D classes.

These include – Line2D– Rectangle2D– Ellipse2D– Arc2D

All of these implement the shape interface.


Drawing Shapes using Graphics2D

To draw a particular shape using Graphics2D,

1. Create an instance of the shape

2. Call the draw method on the Graphics2D object and pass in the shape

Rectangle r = new Rectangle(5, 10, 20, 30);

g2.draw(r);


import java.awt.*;


public class drawPanel2 extends JPanel{

drawPanel2(){

//constructor code

}

paintComponent(Graphics g){


Graphics2D g2 = (Graphics2D)g;

Rectangle r = new Rectangle(0,0,5,6);

g2.draw(r);

}

}


How do we fill these shapes?

Call the fill method instead of draw

Rectangle r = new Rectangle(0,0,5,7);

g2.fill(r);


Colour

setColor(Color c) - used to set the colour for drawing

public void paintComponent(Graphics g){super.paintComponent(g);g.setColor(Color.red);g.drawLine(10, 10, 100, 100);

}

Can use the brighter() and darker() methods tochange the colour

Color.red.darker()


Text

Text is drawn using the drawString() method

drawString(String s, int x, int y)

x,y coordinates determine the far left position of the baseline



g. drawString(“Hello”, 50, 50);

}



Fonts

Various fonts can be used when drawing text on the screen.

It is possible to to create a font

Font myFont = new Font(“Fixed”, Font.BOLD, 12);

Pass the Font object to the Graphics setFont() method

g.setFont(myFont);


import java.awt.*;import javax.swing.*;class drawPanel extends JPanel{

drawPanel(){setPreferredSize(new Dimension(200, 300));

}public void paintComponent(Graphics g){

super.paintComponent(g);setBackground(Color.white);Font myFont = new Font(“Fixed”, Font.BOLD, 20);g.setColor(Color.red);g.setFont(myFont);g.drawString("Hello", 50, 50);

}}public class drawFrame extends JFrame {

drawFrame(){setSize(300, 400);drawPanel p = new drawPanel();getContentPane().add(p);setVisible(true);

}


}}



Images

• java.awt.Image is an abstract class

• java.awt.image is a package; it contains many classes for image manipulation

• GIF and JPEG are only supported


Displaying Images

To display an image

1. Load the Image

2. Draw the image in the components paint() method or the JPanels paintComponent() method


Loading an Image

We never create an instance of the Image class. We load them

from a file.

String fname = “myImage.gif”;

Image i = Toolkit.getDefaultToolkit.getImage(fname);

We can also load an Image from a URL

String fname = “myImage.gif”;

Image i = getImage(getDocumentBase(), fname);


Drawing the Image

java.awt.Graphics has several methods for drawing an

image.

• g.drawImage(img, x, y, observer);

• g.drawImage(img, x, y, w, h, observer);

An image informs its ImageObserver about any changes

Usually you are loading an image in the paint() method of

a component, so pass this as the ImageObserver

All Components implement ImageObserver

Documents

The Graphics Class