Graphics in Java
CSE470 Fall 1998
Thanks to Aaron Malenfant for providing the original slides.
The slides have been slightly modified to be used in CSE470.
Fall 1998
CSE 470
1
Simple Windowing
• Frame
– A frame provides a Window (through the Window
class) with a title bar, menubar, and border.
– Inheritence Tree
java.lang.Object
|
+----java.awt.Component
|
+----java.awt.Container
|
+----java.awt.Window
|
+----java.awt.Frame
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Each Class in the Tree
• java.lang.Object
– Everything non-basic type is inherited from Object
• java.awt.Component
– A part of a window from a scrollbar and button to the entire
window
• java.awt.Container
– A container is a component that can contain other components.
• java.awt.Window
– A Window bas no border or menubars. It is used for pop-up
windows.
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3
Simple Frame Example
import java.awt.*;
public class cse470 extends Frame {
Label label1;
// A simple Component
public cse470() {
super("Frame Title");
setLayout(new FlowLayout());
resize(300, 300);
//Size the window to 300,300
label1 = new Label("Simple Frame");
add(label1);
//Add the component
show();
//Show the frame
}
public static void main(String args[]) {
new cse470();
}
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Frame Example Continued
public boolean handleEvent(Event event) {
if (event.id == Event.WINDOW_DESTROY) {
EndApp();
}
return super.handleEvent(event);
}
public void EndApp() {
hide();
// hide the Frame
dispose(); // tell windowing system to free resources
System.exit(0); // exit
}
}
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Simple Components
•
•
•
•
Button
CheckBox
Label
Choice
– Drop-down menu
• List
– Selectable list
• Scrollbar
• TextArea
– An area which a user can enter text. Has both rows and columns.
• TextField
– A field which a user can enter text. Only has columns.
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Using a simple component
• Declare the object
– Button Send;
• Create a new instance of the object
– Send = new Button(“Send”);
• “add” the object to the Frame (or container)
– add(Send);
• handle the event (in action(Event e, Object arg); )
– if(e.target == Send) { do_something; }
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Events and Actions
• Some components that generate action events :
Button
• arg = a String with the text label of the button.
– Checkbox
• arg = a Boolean value which is true if the checkbox is checked.
– TextField
• arg = a String representing the text in the field
– Choice
• arg = a String that represents the selected text in the choice
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Complex component
• Canvas
• Containers
– Frame
– Panel
– Dialog
– Window
– Applet
• Complex components must be “extend”ed.
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Using a complex component
• Create a new class and source file
import java.awt.*;
public class MainPanel extends Panel {
public MainPanel() {
super();
setLayout(new FlowLayout());
label1=new Label("MainPanel", Label.CENTER);
add(label1);
}
public boolean handleEvent(Event event) {
return super.handleEvent(event);
}
Label label1;
}
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Using a complex component, contd.
• In the main class, use the new class just like the
other components.
– Declare the object
• MainPanel mp;
– Create a new instance of the object
• mp = new MainPanel();
– “add” the object to the Frame (or container)
• add(mp);
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Layouts
• Layouts automatically arrange the components in a
container.
• The default is the GridLayout.
• To add a component to a container with the GridLayout:
– add(String pos, Component ToBeAdded);
• The pos can be
–
–
–
–
–
Fall 1998
“North”
“South”
“East”
“West”
“Center”
CSE 470
12
BorderLayout
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Code for GridLayout example
import java.awt.*;
public class BorderTest extends Frame {
public BorderTest() {
super("BorderTest");
resize(300, 300);
add("Center", new Label("Center",Label.CENTER));
add("North", new Label("North", Label.CENTER));
add("South", new Label("South", Label.CENTER));
add("West", new Label("West", Label.CENTER));
add("East", new Label("East", Label.CENTER));
show();
}
}
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Containers within Containers
• One of the benifits of a Container is that other containers can be
inside a container.
Buttonbar Panel
Other
Sidebar
Panel
Sidebar
Panel
Statusbar Panel
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Canvas
•
•
•
•
Canvas is a complex component.
It is NOT a container
You can draw in the Canvas
While you can draw in any complex component, you
should only do it in a canvas.
• Anytime the canvas is redrawn, the paint method is called
to redraw it.
• Overload the paint method.
– void paint(Graphics g) { }
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Graphics object
• Allows an application to draw onto a component.
• Some of the information contained in the graphics
object is:
– The Component to draw on
– The current color
– The current font
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Some methods in Graphics
•
•
•
•
•
•
•
•
drawLine(int x1, int y1, int x2, int y2)
– Draws a line between the coordinates (x1,y1) and (x2,y2).
drawOval(int x, int y, int width, int height)
drawRect(int x, int y, int width, int height)
drawString(String, int, int)
getColor()
– Gets the current color.
getFont()
– Gets the current font.
setColor(Color)
– Sets the current color to the specified color.
setFont(Font)
– Sets the current font to the specified font.
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Canvas Example
– Simple.java -- The Frame
BoxCanvas n,s,e,w,c;
public Simple() {
super("Simple App");
resize(300, 300);
n = new BoxCanvas();
e = new BoxCanvas();
c = new BoxCanvas();
n.resize(0, 50);
w.resize(50,0);
add("South", s);
add("Center", c);
add("West", w);
show();
}
Fall 1998
s = new BoxCanvas();
w = new BoxCanvas();
s.resize(0, 50);
e.resize(50,0);
add("North", n);
add("East", e);
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Canvas Example -- BoxCanvas.java
import java.awt.*;
class BoxCanvas extends Canvas
{
public void paint(Graphics g)
{
g.drawRect(0,0, size().width - 1, size().height - 1);
g.drawOval(0,0, size().width - 1, size().height - 1);
}
}
– size() returns a rectangle with the size of the
component.
– A Rectangle has a height, width, x, and y.
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Output
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Java Applet Basics
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Converting an application to an Applet
• Make an HTML page with the APPLET tag
• Remove the main method, move any code from main to
“init”
• Width and Height must be specified in the HTML page
• Derive from Applet instead of Frame
• Replace the constructor with init
• If the application implicitly uses a border layout, it must be
explicit
– setLayout(new BorderLayout());
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Applet Methods
• Methods to override
– public void init()
• Called by the browser or applet viewer to inform this applet
that it has been loaded into the system.
• All your GUI initialization should go here. All components
should be added here, layouts should be set here.
– public void start()
• Called by the browser or applet viewer to inform this applet
that it should (re)start its execution.
• If the browser leaves the page and returns, the start method will
be called. The init method will not be called again and all data
values will be left intact.
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More Methods to override
– public void stop()
• Called by the browser or applet viewer to inform this applet
that it should stop its execution.
• This happens when the browser leaves the page. The applet is
not destroyed, it should just stop running.
– public void destroy()
• Called when the applet is destroyed. This is the last method
called in the Applet.
– public String getAppletInfo()
• Returns information about this applet. An applet should
override this method to return a String containing information
about the author, version, and copyright of the applet.
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Callable Methods
• Callable Methods
– public boolean isActive()
• Determines if this applet is active. An applet is marked active
just before its start method is called. It becomes inactive
immediately after its stop method is called.
– public void resize(int width, int height)
– public void resize(Dimension d)
• Requests that this applet be resized.
– public void showStatus(String msg)
• Requests that the argument string be displayed in the "status
window". Many browsers and applet viewers provide such a
window, where the application can inform users of its current
state.
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Document Location
– public URL getDocumentBase()
• Gets the document URL. This is the URL of the document in
which the applet is embedded.
– public URL getCodeBase()
• Gets the base URL. This is the URL of the applet itself.
– URL
• Contains protocal, port, hostname, and file.
• Usually passed to other methods
• URL is in java.net.URL so it must be imported.
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Paramaters
– public String getParameter(String name)
• Returns the value of the named parameter in the HTML tag.
• To use paramaters, define them in your HTML file
<applet code="Applet1" width=300 height=300>
<param name=Owner value=”CSE470">
</applet>
• Then a call to getParameter("Owner") returns the value
“CSE470”.
• The name argument is case insensitive.
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AppletContext
– public AppletContext getAppletContext()
• Determines this applet's context, which allows the applet to
query and affect the environment in which it runs.
• This environment of an applet represents the document that
contains the applet.
• You can use the AppletContext to load other pages
URL u = new URL( “http://www.cps.msu.edu/~cps470”)
getAppletContext().showDocument(u);
• or for a new window
getAppletContext().showDocument(u, “_blank”);
• The second parameter could be the name of a frame.
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Images
– public Image getImage(URL url)
• Returns an Image object that can then be painted on the screen.
The url that is passed as an argument must specify an absolute
URL.
– public Image getImage(URL url, String name)
• Returns an Image object that can then be painted on the screen.
The url argument must specify an absolute URL.
• The name argument is a specifier that is relative to the url
argument.
– This method always returns immediately, whether or not the image
exists. When this applet attempts to draw the image on the screen,
the data will be loaded. The graphics primitives that draw the
image will incrementally paint on the screen.
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More Images
• Displaying Images
– Graphics Class
• public abstract boolean drawImage(Image img, int x, int y,
ImageObserver observer)
• public abstract boolean drawImage(Image img, int x, int y, int
width, int height, ImageObserver observer)
– Draws as much of the specified image as is currently available at
the specified coordinate (x, y). This method will return
immediately in all cases, even if the entire image has not yet been
scaled, dithered and converted for the current output device. If the
current output representation is not yet complete then the method
will return false and the indicated ImageObserver object will be
notified as the conversion process progresses.
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31
Events in Java 1.0.2
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32
Simple Action Event
import java.awt.*;
import java.applet.*;
public class SimpleEvent extends Applet {
Button button1 = new Button("Press me");
public void init() {
this.add(button1);
}
public boolean action(Event e, Object obj) {
if(e.target == button1) {
button1.setLabel("Again");
return true;
}
return super.action(e, obj);
}
}
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Handling Events
• While event handling could be done in handleEvent, it is
best to use the convenience functions defined by java.
• action(Event evt, Object arg);
– All action events such as buttons being pressed, Checkboxes being
checked are sent here.
– The Object (arg) will be set to one of the following based the
component that created the action event (evt.target)
•
•
•
•
Fall 1998
Button arg is a String with the text label of the button.
Checkbox arg is a Boolean value -- the state of the checkbox.
TextField arg is a String representing the text in the field
Choice arg is a String that represents the selected text in the choice
CSE 470
34
Example -- TextTest.java
import java.awt.*;
import java.applet.*;
public class TextTest extends Applet
{
private TextField hourField = new TextField("12", 3);
private TextField minuteField = new TextField("00", 3);
private ClockCanvas clock = new ClockCanvas();
private Panel p = new Panel();
private Button tick = new Button("Tick");
private Button set = new Button("Set time");
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Example -- Init method
public void init()
{
setLayout(new BorderLayout());
p.setLayout(new FlowLayout());
p.add(tick);
p.add(set);
p.add(hourField);
p.add(minuteField);
add("South", p);
add("Center", clock);
}
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Example -- action Method
public boolean action(Event evt, Object arg)
{ if (arg.equals("Tick")) clock.tick();
else if (arg.equals("Set time"))
{ int hours = Integer.parseInt(hourField.getText());
int minutes =
Integer.parseInt(minuteField.getText());
clock.setTime(hours, minutes);
}
else return super.action(evt, arg);
return true;
}
}
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Example -- ClockCanvs.java
import java.awt.*;
class ClockCanvas extends Canvas
{
int minutes = 0;
public int getSmaller()
{
if(size().width < size().height) {
return size().width / 2;
} else {
return size().height / 2;
}
}
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38
Example -- paint
public void paint(Graphics g)
{
int sm = getSmaller();
g.drawOval(0, 0, 2*sm - 1, 2*sm - 1);
double hourAngle = 2 * Math.PI *
(minutes - 3 * 60) / (12 * 60);
double minuteAngle = 2 * Math.PI * (minutes - 15) / 60;
g.drawLine(sm, sm,
sm + (int)(sm /2.0 * Math.cos(hourAngle)),
sm + (int)(sm /2.0 * Math.sin(hourAngle)));
g.drawLine(sm, sm,
sm + (int)(sm / 1.1 * Math.cos(minuteAngle)),
sm + (int)(sm / 1.1 * Math.sin(minuteAngle)));
}
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Example -- other methods
public void reset()
{ minutes = 0;
repaint();
}
public void tick()
{ minutes++;
repaint();
}
public void setTime(int h, int m)
{ minutes = h * 60 + m;
repaint();
}
}
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Mouse Events
• mouseDown(Event evt, int x, int y);
– Called when the mouse button is clicked
• mouseDrag(Event evt, int x, int y);
– Called when the mouse moves with the mouse button
depressed
• mouseUp(Event evt, int x, int y);
– Called when the mouse button is released
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More Mouse Events
• mouseMove(Event evt, int x, int y);
– Called when the mouse moves with no mouse button
depressed
• mouseEnter(Event evt, int x, int y);
– Called when the mouse enters the component
• mouseExit(Event evt, int x, int y);
– Called when the mouse leaves the component
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Example
import java.awt.*;
import java.applet.*;
import java.util.Vector;
public class cse470 extends Applet {
Vector down = new Vector();
Vector up = new Vector();
public void init() {
super.init();
}
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Example -- mouse events
public boolean mouseDown(Event evt, int x, int y)
{
down.addElement(new Point(x,y));
repaint();
return true;
}
public boolean mouseUp(Event evt, int x, int y)
{
up.addElement(new Point(x,y));
repaint();
return true;
}
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Example -- paint
public void paint(Graphics g) {
Point c;
g.setColor(Color.red);
for(int i = 0 ; i < down.size();i++) {
c = (Point) down.elementAt(i);
g.drawOval(c.x, c.y, 2, 2);
}
g.setColor(Color.blue);
for(int i = 0 ; i < up.size();i++) {
c = (Point) up.elementAt(i);
g.drawOval(c.x, c.y, 2, 2);
}
}
}
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45
Exceptions
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What is an Exception?
• An exception is an event that occurs that disrupts
the flow of execution.
• An exception could be an array out of bounds, File
not Found, etc.
• A programmer can create their own exceptions.
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Examine the following Code
public class cse470 {
public static void main(String args[]) {
int i = Integer.parseInt("Not an Integer");
System.out.println("End of Program");
}
}
– The output is
• java.lang.NumberFormatException: Not an Integer
– not
• End of Program
– parseInt threw an Exception
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Runtime Exceptions
• Runtime exceptions are generally caused by
programmers.
• Runtime exceptions do not have to be explicitely
handled.
– Integer.parseInt(“Not an Integer”) throws the Runtime
Exception NumberFormatException
• Non-runtime exceptions must be caught.
– MalformedURLException
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Handling Exceptions
• To handle an exception, use a “try” block.
int i;
try {
i = Integer.parseInt("Not an Integer");
} catch (NumberFormatException nfe) {
System.err.println("parseInt only works with
numbers!");
}
• If a NumberFormatException occurs, the catch block will be
executed, otherwise the program will continue.
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Basic Form
• The basic form of exception handling is as
follows:
try {
SomethingThat could Throw an Exception;
SomethingThat could Throw an Exception;
…
} catch (SomeTypeOfException somevar) {
handleTheException;
} catch (SomeOtherTypeOfException somevar) {
handleTheException;
}
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The Exception class
• String getMessage()
– Returns the detail message of this exception object.
• void printStackTrace()
– Prints this Exception and its backtrace to the standard
error stream.
• String toString()
– Returns a short description of this throwable object.
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Using the members of the Exception
class
try { …
} catch (Exception e) {
System.out.println(“Exception “ + e.getMessage() + “
occurred in MyClass.MyMethod “);
e.printStackTrace();
}
– or
} catch (Exception e) {
System.out.println(“Exception “ + e + “ occurred”);
}
– e alone calls e.toString() to output the class e
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Example
public class cse470 {
public static void main(String args[]) {
int i = -1;
try {
i = SomeMethod();
} catch (NumberFormatException nfe) {
System.err.println("Exception " + nfe.getMessage()
+ " occurred");
nfe.printStackTrace();
}
System.out.println("End of Program, i = " + i);
public static int SomeMethod() {
int i = Integer.parseInt("Not an Integer");
return i * 2;
}
}
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Output
• Exception Not an Integer occurred
• java.lang.NumberFormatException: Not an
Integer
• End of Program, i = -1
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Finally block
• The finally block is used at the end of a try…catch block.
• The finally block will always be executed even if there is
an unhandled exception or no exception
try {
something
} catch (Exception e) {
...
} finally {
clean up code
}
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Example
import java.awt.*;
import java.applet.*;
import java.io.*;
public class ExcepTest extends Applet {
public void init() {
super.init();
setLayout(new GridLayout(2,1));
Panel p = new Panel();
p.setLayout(new GridLayout(4, 2));
add(p);
add(tArea);
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Example contd.
CheckboxGroup g = new CheckboxGroup();
p.add(new Checkbox("Divide by zero", g, false));
p.add(new Checkbox("Bad cast", g, false));
p.add(new Checkbox("Array bounds", g, false));
p.add(new Checkbox("Null pointer", g, false));
p.add(new Checkbox("sqrt(-1)", g, false));
p.add(new Checkbox("Overflow", g, false));
p.add(new Checkbox("No such file", g, false));
p.add(new Checkbox("Throw unknown", g, false));
}
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Example -- action
public boolean action(Event evt, Object arg) {
try {
if (evt.target instanceof Checkbox) {
String name = ((Checkbox)evt.target).getLabel();
if (name.equals("Divide by zero")) {
a[1] = a[2] / (a[3] - a[3]);
} else if (name.equals("Bad cast")) {
f = (Frame)arg;
} else if (name.equals("Array bounds")) {
a[1] = a[10];
} else if (name.equals("Null pointer")) {
f = null;
f.resize(200, 200);
} else if (name.equals("sqrt(-1)")) {
a[1] = Math.sqrt(-1);
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Example -- action contd.
} else if (name.equals("Overflow")) {
a[1] = 1000 * 1000 * 1000 * 1000;
int n = (int)a[1];
} else if (name.equals("No such file")) {
FileInputStream is = new FileInputStream(name);
} else if (name.equals("Throw unknown")) {
throw new UnknownError();
}
} else return super.action(evt, arg);
} catch(RuntimeException e) {
tArea.append("Caught RuntimeException: " + e + "\n");
} catch(Exception e) {
tArea.append("Caught Exception: " + e + "\n");
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Example -- action contd.
} finally {
tArea.append("In Finally for " +
((Checkbox)evt.target).getLabel() + "\n");
}
return true;
}
private double[] a = new double[10];
private Frame f = null;
TextArea tArea = new TextArea();
}
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Throwing your own exceptions
• your method must be declared to “throw
Exception”
– public MyMethod throws IOException
– public MyMethod throws Exception
– public MyMethod throws MyExeception
• Then you “throw” the exception
– throw new Exception(“Bad Input”);
– throw new MyException(“Incorrect
Parameter”);
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Example
public class MyClass {
public double FourthRoot(double x) throws Exception {
if ( x < 0) {
throw new Exception(“X must be positive”);
}
return Math.sqrt(Math.sqrt(x));
}
public void MyMethod() {
try {
FourthRoot(2);
FourthRoot(-1);
FourthRoot(3);
} catch (Exception e) {
e.printStackTrace();
}
}
}
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Creating your own Exception types
• Your Exception class must be extended from
Exception (or a child of Exception)
public MyException extends Exception {
public MyNextException extends MyException {
• In the method
public MyClass throws MyException {
public BadMethod {
throw new MyNextException();
}
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Using custom Exceptions
• In the calling program
MyClass mc = new MyClass();
try {
mc.BadMethod()
} catch (MyException e) {
do_something;
}
– MyException includes MyException and
MyNextException since MyNextException is a child of
MyException.
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