UCLA PIC 20A PIC 20 A – Course Organization Java Programming zInstructor: Ivo Dinov, z http://www.stat.ucla.edu/~dinov/courses_students.html Asst. Prof. In Statistics, Neurology and z Program in Computing Summer2002\HTMLs\PIC20A.html z Teaching Assistant: Yon Seo Kim, PIC zJava Demo: C:\Ivo.dir\LONI_Viz\LONI_Viz_MAP_demo\run.bat
University of California, Los Angeles, Summer 2002 http://www.stat.ucla.edu/~dinov/
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Chapter 1 – Getting Started Chapter 1 – Getting Started
z What is the Java platform and what it can do? z Java is a high-level language that is characterized by:-
z How to compile and run two simple programs on the • Simple • Robust • High performance Windows, the UNIX/Linux or the Mac OS X platforms. • Object oriented • Secure • Multithreaded z Software Development Kits (SDKs) provided by Sun • Distributed • Architecture neutral • Dynamic Microsystems include a minimal set of tools to let you run and compile your programs. • Interpreted • Portable z Advanced developers are encouraged to use a z Most programming languages, you either compile or professional Integrated Development Environment interpret a program to run it on your computer. In (IDE). Java a program is both compiled and interpreted.
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Chapter 1 – Getting Started Java Platform z Bytecodes is the machine code instructions for the Java z A platform is the hardware or software environment in Virtual Machine (Java VM). Every Java interpreter is an which a program runs (W2K, Linux, Solaris, MacOS). implementation of the Java VM. Most platforms can be described as a combination of the operating system and hardware. z Write once, run anywhere. As long as a computer has a Java VM, the same Java program should run on z Java platform is software-only platform that runs on top Windows, Solaris, or Mac. of other, hardware-based platforms and has these 2 components: TThe Java Virtual Machine (Java VM) TThe Java Application Programming Interface (API)
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1 Java Platform Java VM & Java API
TThe Java Virtual Machine (Java VM) z The Java VM is the base for the Java platform and is TThe Java Application Programming Interface ported onto various hardware-based platforms. (API) z The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets. The Java API is grouped into libraries of related classes and interfaces; these libraries are known as packages.
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Native vs. Portable Code Java Applets & Applications z Native code is code that, after you compile it, runs on a z The most common types of programs written in the Java specific hardware platform. programming language are applets and applications. z An applet is a program that adheres to certain conventions that z As a platform-independent (portable) environment, the allow it to run within a Java-enabled web browser. Java platform can be a bit slower than native code. z Example: C:/Ivo.dir/UCLA_Classes/Applets.dir/DiceApplet.htm However, smart compilers, well-tuned interpreters, and z An application is a standalone program that runs directly on the just-in-time bytecode compilers can bring performance Java platform. A special kind of application known as a server close to that of native code without threatening serves and supports clients on a network. portability. z Servers Ex: Web servers, Mail servers, Print servers. z Servlets, similar to applets, are run-time extensions of applications. Instead of working in browsers, though, servlets run within Java Web servers, configuring or tailoring the server.
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Java 2 Platform SE v 1.4 First Java Program – HelloWorldApp
z Your first program, HelloWorldApp , will simply display the greeting “Hello World!” To create this program, you complete each of the following steps. z Create a source file. A source file contains text, written in the Java programming language, that you and other programmers can understand. You can use any text editor to create and to edit source files. z Compile the source file into a bytecode file. The compiler takes your source file and translates the text into instructions that the Java VM can understand. The compiler converts these instructions into a bytecode file.
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2 First Java Program – HelloWorldApp First Java Applet – HelloWorld z /** z Applet called HelloWorld , displaying the greeting “Hello z * The HelloWorldApp class implements an application World!” Unlike HelloWorldApp , however, the applet runs in a Java-enabled Web browser, such as the HotJava TM z * that displays "Hello World!"to the standard output. browser, Netscape Navigator, or Microsoft Internet z */ Case Explorer. z public class HelloWorldApp { Sensitive z public static void main(String [] args)) { z Create a source file, compile the source file, and run the z program. However, unlike for an application, you must //Display "Hello World!" also create an HTML file. z System.out.println("Hello World!"); z } Do this example in a DOS shell or Explorer z } javac HelloWorldApp.java java HelloWorldApp
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First Java Applet – HelloWorld.java First Java Applet – HelloWorld
z HTML file to accompany your applet. HelloWorld.html z z import java.applet.Applet; z z
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Running Java Applet – HelloWorld Exceptions … z Compiling: z Exception in thread “main ” z java.lang.NoClassDefFoundError:HelloWorldApp javac HelloWorld.java z If you receive this error, the interpreter cannot find your bytecode z The compiler should generate a Java bytecode file, file, HelloWorldApp.class . HelloWorld.class z One of the places java tries to find your bytecode file is your current directory. So, if your bytecode file is in C drive, you should change z Although you can use a Web browser to view your applets, your current directory to that. To change your directory, type the you may find it easier to test your applets by using the following command at the prompt and press Enter : simple appletviewer application that comes with the Java z cd c: platform. To view the elloWorld applet using appletviewer z The prompt should change to C:. If you enter dir at the prompt, you enter at the prompt: should see your .java and .class files. Now again enter zappletviewer HelloWorld.html z java HelloWorldApp znetscape HelloWorld.html z Still have problems, change your CLASSPATH variable. z set CLASSPATH=
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3 First Java Application Program First Java Applet z HelloWorldApp , displays the greeting “Hello World!” z HelloWorldA, displays the greeting Hello World! in a To createthis program: java enabled browser z Create a source file – text written in the Java that others z Create a source file – notepad HelloWorld.java can understand. Use any text editor to generate/edit source files – *.java. notepad Hello.html. z z Compile the source file into a bytecode file – javac Compile the source – javac HelloWorld.java translates the source/text into instructions that the Java makes – HelloWorld.class. Virtual Machine can understand, bytecode file – *.class. z Run the program – appletviewer Hello.html. z Run the program contained in the bytecode file via Java interpreter – carries out the instructions by translating them z E.g.,C:\Ivo.dir\UCLA_Classes\Summer2002\PIC20A\Examples\JavaTutorialExa into instructions that your computer can understand. mples\getStarted\applet\example\HelloWorld.java && Hello.html z E.g.,C:\Ivo.dir\UCLA_Classes\Summer2002\PIC20A\Examples\JavaTutorialExa mples\getStarted\application\example\HelloWorldApp.java
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HelloWorld – Program description HelloWorld – Program description zKinds of Comments: zDefining a Class: z /* text */ z class HelloWorldApp { z Compiler ignores everything from opening /* to closing */. public static void main(String [] args)){ z /** documentation */ System.out.println("Hello World!"); // Display the string z This style indicates a documentation comment (doc comment, for } short). Compiler ignores all the text within the comment. The SDK } javadoc tool uses doc comments to automatically generate docs. z A class is the basic building block of an object-oriented language, z Example:C:\Ivo.dir\UCLA_Classes\Summer2002\PIC20A\Examples\JavaTutor ialExamples\getStarted\applet\example\doc\index.html such as Java. A class is a blueprint that describes the state and the behavior associated with instances of that class. When you z // text instantiate a class, you create an object that has the same states z The compiler ignores everything from the //to the end of the line. and behaviors as other instances of the same class. z The state associated with a class or an object is stored in member variables.
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HelloWorld – Program description HelloWorld – Program description zDefining a Class: zMain method of a Class: z The behavior associated with a class or an object is implemented z The entry point of every Java application is its main method. The with methods, which are similar to the functions or procedures in interpreter first invokes the main method defined in that class. The procedural languages, such as C/FORTRAN/Pascal. main method controls the initial flow of the program. z Ex. Of a class is a recipe—for making a cake. It’s a blueprint for z /** The HelloWorldApp class implements an application that making a specific instance of the recipe. Your attempt to make a * simply displays "Hello World!"to the standard output. cake is one instance of the recipe, and someone else’s attempt to bake the same cake is another instance. */ class HelloWorldApp { z Ex. of a class that represents a rectangle: The class defines variables for the origin, width, and height. The class might also public static void main(String [] args)){ define a method that calculates its area/circumference. System.out.println("Hello World!"); // Display the string. An instance of the rectangle class, a rectangle object, contains the } information for a specific rectangle, such as the dimensions of the } floor of your office or the dimensions of this page.
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4 HelloWorld – Program description HelloWorld – Program description zMain method of a Class: zMain method of a Class: z Every application must contain a main method declared like this: z The main method accepts a single argument: an array of public static void main(String [] args)) elements of type String , like this: z 3 modifiers: public static void main( String [ ] args ) z public : Allows any class to call the main method z This array is the mechanism through which the Java z static : Means that the main method is associated with the Virtual Machine passes information to your application. HelloWorldApp class as a whole instead of operating on an instance of the class Each String in the array is called a command-line z argument. It let users affect the operation of the void : Indicates that the main method does not return a value application at run-time, without recompiling it. z Each application must contain a main method. When invoked, the z interpreter starts by calling the class’s main method, which then calls The HelloWorldApp application, is very simple and all the other methods. ignores its command-line arguments.
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HelloWorld – Program description HelloWorld – Program description zUsing Classes & Objects: zUsing Classes & Objects: z Because HelloWorldApp is such a simple program, it z System.out.println("Hello World!"); doesn’t need to define any classes except HelloWorldApp . z Ex. of a class variable and an instance method. However, the application does another class, use System.out .println("Hello World!"); System, that is part of the Java API. The System class z provides system-independent access to system-dependent The construct System.out is the full name of the out functionality. variable in the System class. The application never z instantiates the System class but instead refers to out The System class contains standard output stream—a directly through the class. The reason is that out is a class place to send text, usually refers to the terminal window variable—a variable associated with a class rather than the Java interpreter runs. with an object. z Caution! Using the standard output stream isn’t z The Java Virtual Machine allocates a class variable once recommended in 100% Pure Java programs. However, it’s per class, no matter how many instances of that class exist. fine to use during the development cycle.
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HelloWorld – Program description HelloWorld – Program description zUsing Classes & Objects: zUsing Classes & Objects: z System.out.println("Hello World!"); z System’s out variable is a class variable, it refers to an System.out.println("Hello World!"); instance of the Print-Stream class (another Java API- provided class that implements an easy-to-use output stream). When it is loaded into the application, the System z Java also has instance variables. An instance variable is a class instantiates PrintStream and assigns the new member variable associated with an object rather than with PrintStream object to the out class variable. a class. Each time you instantiate a class, the new object z System.out.println ("Hello World!"); gets its own copy of all the instance variables defined in its z Calls one of out’s instance methods. An instance method class. implements behavior specific to a particular object—an z We’ll discuss these further in ch. 4 … instance of a class.
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5 HelloWorld – Applet description HelloWorld – Applet description zImporting Classes & Packages: zImporting Classes & Packages: z HelloWorld.java begins with two import statements that import import java.applet.Applet; the Applet and Graphics classes: import java.awt.Graphics; import java.applet.Applet; public class HelloWorld extends Applet { import java.awt.Graphics; public void paint(Graphics g) { public class HelloWorld extends Applet { g.drawString("Hello world!",50,25); public void paint(Graphics g) { } g.drawString("Hello world!",50,25); } } z Importing the Applet and Graphics classes lets the } program refer to them later without any prefixes. The z By importing classes or packages, a class can easily refer to classes java.applet. and java.awt. prefixes tell the compiler in other packages. Packages are used to group classes, similar to the which packages it should search for the Applet and way libraries group C functions. Graphics classes.
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HelloWorld – Applet description HelloWorld – Applet description zDefining an Applet Sub-Class: zImplementing Applet Methods: z z import java.applet.Applet; import java.applet.Applet; import java.awt.Graphics; import java.awt.Graphics; public class HelloWorld extends Applet { public class HelloWorld extends Applet { public void paint(Graphics g){ public void paint(Graphics g){ g.drawString("Hello world!",50,25); g.drawString("Hello world!",50,25); } } z } There’s a single method here: paint . Every applet should implement at least one of the following methods: init , start , or } paint . Applets do not need to implement a main method. z Every applet must define a subclass of the Applet class. Applets z Applets are designed to be included in HTML pages. Using the inherit a great deal of functionality from the Applet class, ranging
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HelloWorld – Applet description zRunning an Applet: z When it encounters an