Lesson 7

AppletsJar filesGraphicsA little more Swing

Part I:Very Basic Applets

Programs that run within web browsers

What are applets?

Applets are simply java programs that run in web browsers.

Created a big stir in mid-90’s when Netscape agreed to embed jvm in Navigator web browswer.

Great promise – applications automatically distributed to anyone on any platform!

Reality – non-uniform browswer support, limitations imposed by security, easier ways to accomplish same thing!

What are applets, cont. Still useful in just the right situation

– fancy, full-fledged client– can make some assumptions/have some control over

client technology Also, very good for understanding issues in web

client-server programming Otherwise, server-heavy programming with html

or client-side scripting wins out. Also, Java WebStart new alternative

– can launch full applications remotely without need for host browser

Applet inheritance treeObject

Component

Container

Window

Frame

JFrame

Panel

Applet

JAppletUse to access

Swing Components

Some Hello World Applets

To see how applets work, we start with a couple of versions of a HelloWorld program– One uses the fact that an Applet is a Panel with

a graphics object– The other uses a button label

Soon, we will add full event-handling capabilities and nice graphics. This is just a start.

HelloWorldApplet1

import java.awt.*;import javax.swing.*;

public class HelloWorldApplet1 extends JApplet{ public void init(){ getContentPane().add(new JLabel( “Hello World”, SwingConstants.CENTER)); }}

• Note that all Applets are class that extend either Applet or JApplet• init() is called when the Applet is loaded

HelloWorldApplet2

import java.awt.*;import javax.swing.*;

public class HelloWorldApplet1 extends JApplet{ public void painComponent(Graphics g){ g.drawString(“Hello World”, 50, 50); }}

• Since Applets are Panels, we can override PaintComponent and get a Graphics Object• This will become particularly handy when doing animations

Running Applets To run applets, do the following:

– Compile class file in regular way– Create an html file that includes an applet tag pointing to

the applet <applet code = “Appletname.class” width = 200 height = 50></applet>

(width and height are pixel coords)

– Invoke browser or appletviewer on html file Note that applet tag can include other parameters,

some of which of used by browser automatically (e.g. width).

Also, Java2 plug-in required for browsers to support applets. Good to test with appletviewer first.

Life Cycle of Applet

To write more sophisticated applets, a number of other methods may need to be overwritten. – void init()– void start()– void stop()– void destroy()

We discuss the role of each next.

init() method The browswer calls the init method when the applet is

loaded for the first time. init() behaves much like a constructor. Typical uses:

– parsing param values from html files– opening streams and sockets– creating GUI components– opening database connections

Important: note that the refresh button doesn’t necessarily reload applet. This is browser-dependent. To guarantee reloading, browser needs to be killed and restarted.

start() method

called every time page is located– this can mean moving off and back onto page, hitting

reload, etc.

always called after init() when page is first loaded. Typical uses:

– starting animations

– anything else that needs to start anew with every location of applet

– for simpler things this method can be ignored

stop() method

called whenever user moves off the page where applet sits

always called after start typical uses

– ending animations– stopping other time-consuming system activity

often ignore this for simpler applets

destroy() method

called once when browser shuts down normally

always called after stop() typical uses:

– close database connections– close streams– clean up any other resources

often ignored for simpler applets

More on applet life-cycle

It’s very import to be aware that some other software, ie the host browser, is calling these methods at certain times.

You the program do not call these methods. This is classic OO framework: “don’t call us,

we’ll call you”. You write the class and specialize certain

methods, some other code calls your custorm versions of those methods at specified times.

Other applet issues

Security– Horstmann pg. 499

Pop-up windows– Horstmann pg. 500

Applet tags and attributes– Horstmann pg. 502

Passing info to applets– Horstmann pg. 506

Applets making socket connections

Jar Files

Packaging java applications

What is a jar file?

JAR: Java Archive – typical way of distributing java applications/libraries

JARs are ZIP files containing – java class files

– possibly other resource files (images, audio, text, etc.)

– optional manifest file: describes certain attributes of JAR file.

Applets, beans, applications typically all distributed as single of collection of JAR files.

Creating JAR Files Either use commandline tool like

– jar cvf whatever.jar *.classor API– see java.util.jar docs

No name restrictions for jar file – whatever is legal on a given platform

Can jar directories as well as files. In that case, contents are added recursively

Syntax similar to tar. See http://java.sun.com/docs/books/tutorial/jar

for more details

Self-running JAR files

jars can get quite complex when using with J2EE.– Manifest files can be used to specify security attributes,

versioning, extensions, services, etc.

Also, for better performance multi-class applets can be packaged as jar files.

Course web page contains links to more detailed info on creating sophisticated JARs

Here were present executable JAR files as a simple example.

Steps to making executable jar To make an executable jar, take the following steps:

– create a .jar of all class files in your application using jar cvf Whatever.jar *.class

(packages and nested directories work fine here).– create a manifest file with any name (say manifest.mf).

manifest.mf must at least contain a line pointing to the main class, e.g.:

Main-Class: com/mypackage/MainAppClass– finally, add manifest using the update flag u jar uvfm Whatever.jar manifest.mf– run as “java –jar Whatever.jar”

Gotchas

Manifest file must contain linefeed as last character or else won’t be parsed properly.

You should always use packages with jar files.– be careful to put full pathname to Main-Class.

Other issues

Launching a jar from an icon– System dependent. See Horstmann.

Locating Resources– See URL class

Sealing JARs– Sealed: true attribute in manifest file

Signing JARs– can give trusted permission to trusted parties.– See web link above for more details

Overriding Object Methods

The Object Class Every java class has Object as its superclass and

thus inherits the Object methods. Object is a non-abstract class Many Object methods, however, have

implementations that aren’t particularly useful in general

In most cases it is a good idea to override these methods with more useful versions.

In other cases it is required if you want your objects to correctly work with other class libraries.

Some Object class methods

Object methods of interest:– clone

– equals

– hashcode

– toString

– finalize

Other object methods– getClass

– wait, notify, notifyAll (relevant for threaded programming)

Clone method Recall that the “=“ operator simply copies Object

references. e.g.,>> Student s1 = new Student(“Smith”, Jim, 3.13);>> Student s2 = s1;>> s1.setGPA(3.2);>> System.out.println(s2.getGPA()); 3.2

What if we want to actually make a copy of an Object?

Most elegant way is to use the clone() method inherited from Object. Student s2 = (Student) s1.clone();

Subtleties of clone() method

First, note that the clone method is protected in the Object class.

This means that it is protected for subclasses as well.

Hence, it cannot be called from within an Object of another class and package.

To use the clone method, you must override in your subclass and upgrade visibility to public.

More subtleties of clone

Also, any class that uses clone must implement the Cloneable interface.

This is a bit different from other interfaces that we’ve seen.

There are no methods; rather, it is used just as a marker of your intent.

The method that needs to be implemented is inherited from Object.

More clone() issues

Finally, clone throws a CloneNotSupportedException.

This is thrown if your class is not marked Cloneable.

This is all a little odd but you must handle this in subclass.

Steps for cloning

To reiterate, if you would like objects of class C to support cloning, do the following:– implement the Cloneable interface– override the clone method with public access

privileges– call super.clone()– Handle CloneNotSupported Exception.

This will get you default cloning, but more subtleties still lurk.

Shallow Copies We haven’t yet said what the default clone()

method does. By default, clone makes a shallow copy of all iv’s

in a class. Shallow copy means that all native datatype iv’s

are copied in regular way, but iv’s that are objects are not recursed upon – that is, references are copied.

This is not what you typically want. Must override clone explicitly clone object iv’s!

Immutable Objects

A special class of Objects are called immutable because their state cannot be changed once set.

Common examples are String, Integer, etc. Immutable object simplify programming in certain

instances, such as when writing thread safe code. They also simplify cloning, since an object that

cannot be changed doesn’t really need to be deep-copied.

See ShallowCopy2.java in course examples

Deep Copies

For deep copies that recurse through the object iv’s, you have to do some more work.

super.clone() is first called to clone the first level of iv’s.

Returned cloned object’s object fields are then accessed one by one and clone method is called for each.

See DeepClone.java example

Additional clone() properties

Note that the following are typical, but not strictly required:

– x.clone() != x;– x.clone().getClass() == x.getClass(); – x.clone().equals(x);

Finally, though no one really cares, Object does not support clone();

toString() method The Object method String toString();

is intended to return a readable textual representation of the object upon which it is called. This is great for debugging!

Best way to think of this is using a print statement. If we execute:

System.out.println(someObject);

we would like to see some meaningful info about someObject, such as values of iv’s, etc.

default toString() By default toString() prints total garbage that no

one is interested in getClass().getName() + '@' +

Integer.toHexString(hashCode())

By convention, print simple formatted list of field names and values (or some important subset).

The intent is not to overformat. Typically used for debugging. Always override toString()!

equals() method

Recall that boolean == method compares when applied to object compares references.

That is, two object are the same if the point to the same memory.

Since java does not support operator overloading, you cannot change this operator.

However, the equals method of the Object class gives you a chance to more meaningful compare objects of a given class.

equals method, cont

By default, equals(Object o) does exactly what the == operator does – compare object references.

To override, simply override method with version that does more meaningful test, ie compares iv’s and returns true if equal, false otherwise.

See Equals.java example in course notes.

equals subtleties

As with any method that you override, to do so properly you must obey contracts that go beyond interface matching.

With equals, the extra conditions that must be met are discussed on the next slide:

equals contract It is reflexive: for any reference value x, x.equals(x)

should return true. It is symmetric: for any reference values x and y,

x.equals(y) should return true if and only if y.equals(x) returns true.

It is transitive: for any reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.

It is consistent: for any reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the object is modified.

For any non-null reference value x, x.equals(null) should return false.

hashcode() method

Java provides all objects with the ability to generate a hash code.

By default, the hashing algorithm is typically based on an integer representation of the java address.

This method is supported for use with java.util.Hashtable

Will discuss Hashtable in detail during Collections discussion.

Rules for overriding hashcode Whenever invoked on the same object more than once, the

hashCode method must return the same integer, provided no information used in equals comparisons on the object is modified.

If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.

It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.

finalize() method Called as final step when Object is no longer used,

just before garbage collection Object version does nothing Since java has automatic garbage collection,

finalize() does not need to be overridden reclaim memory.

Can be used to reclaim other resources – close streams, database connections, threads.

However, it is strongly recommended not to rely on this for scarce resources.

Be explicit and create own dispose method.

A little more Swing

MVC Pattern

More graphics

Graphics See course examples

– applets/BrownianMotionApplet.java• shows how to do a simple animation with some basic drawing in the

start() method

– graphics/Mandelbrot.java• shows how to create an image from raw data and draw to an applet

by overriding paintComponent.

– graphics/Shapes.java• A great example of how to architect a simple GUI that seems more

complex when poorly designed.

– graphics/Painter.java• Classic example of how to paint on a Panel

– graphics/Bounce.java• Classic example of why one needs threads.

Miscellaneous

Some how-to snippets

See the following codes in the course examples– GetProperties.java

• How to query various system properties

– ProcessTest.java• An example spawning an OS executable

– NumberFormat.java• An example formatting a double

– TimeTest.java• An example using simple System timers.