Java Programming

Java Programming CIS 421 Web-Based Java Programming

Course Text • Text: Computing with Java: Programs, Objects, Graphics Alternate 2nd Edition Art Gittleman Scott Jones Publishers ISBN: 1-57676-074-X

Prerequsites, Tests, Public area • Prerequisites: Cis237, Mat 126, and QPA of 2.90 • Tests: Two midterms, one final. • Public directory: /export/home/public/spiegel/cis421

Turnin script • Programming Projects are submitted electronically using turnin script • See turnin handout • Must setup turnin once • To invoke: • turnin421 <file1> <file2> .... • each item submitted separated by whitespace • no limit to # items • You should receive a confirmation e-mail containing an ls –l listing of files submitted (can include files previously submitted) • Files submitted may be overwritten by subsequent turnin of files with same name

Overview of Java • Java Features • How Java Works • Applications vs Applets • Program-Driven vs Event Driven • Graphical User Interfaces (GUI)

Java Features • Simple, Object-Oriented, Familiar • Robust and Secure • Architecture Neutral and Portable • High Performance • Interpreted, Threaded, and Dynamic

Simple, OO, Familiar • Its simplicity comes from the fact that there are no pointers in Java. Therefore, the programmer does not have to manage pointers and the resultant problems that pointers bring. • All programs in Java are based on objects • Java uses the familiar syntax and fundamental control structures of C/C++

Robust and Secure • Robust programs run without crashing due to programming errors, erroneous input, or failure of external devices. Java has many checks at compile-time and provides run-time exception handling to deal with unexpected events. • Security, especially across the internet, requires careful measures, which are implemented in Java

Architecture Neutral and Portable • Java programs run on a variety of processors using various operating systems • Portability depends not only on architecture but also on implementation. Java specifies the language carefully to reduce implementation dependencies.

High Performance • Java versions continually increase performance capabilities. • In network applications, communication delays usually far exceed performance delays.

Interpreted, Threaded, and Dynamic • Interpreted, not compiled. • Threaded – capable of multi_tasking and concurrent processing, even across the internet • Dynamic linking to library code as it needs it. • Java is ideally suited for general, interactive, and network programming

How Java Works Java Compiler Java Source Code Java Byte Code Java Interpreter For Processor 2 Java Interpreter For Processor 1

Setting up Unix for Java • Make a directory called java in your home directory. • Put this java directory in your CLASSPATH in .cshrc with the command (this is one long line) setenv CLASSPATH “.:$home/java:whatever_is_there_already“ • this allows packages starting from your java subdirectory • Put the directory /usr/j2sdk1.4.0_03/bin in your path variable in the .login file. • Give the commands source .login and source .cshrc to make the changes effective. • These changes are applied automatically on subsequent logins

Testing Changes acad > java -version java version "1.4.0_03" Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.0_03-b04) Java HotSpot(TM) Client VM (build 1.4.0_03-b04, mixed mode) acad >

Applications vs Applets • Applications run like programs written in other languages. They usually are standalone, running at the command line. • Applets are usually small applications that are embedded in a web-page and run on the internet in a web browser.

Building an Application • Edit the source file either in your favorite editor, or IDE, and save it as <file>.java • The file name must be the same as the one and only public class name in the file • Compile by giving the command javac <file>.java at the command line of a DOS or Unix window. • Run the program by giving the command java <file>

hello.java // File: hello.java // Compiler: Sun javac // Compile: javac hello.java // Executable: hello.class // Execute: java hello // // Purpose: Prints usual first // program

hello.java public class hello { public static void main(String[] args) { int x = 3; System.out.println(“Hello, World!” + “\nx = “ + (x) ); } }

Application Programs • The file name must be the same as the one and only public class in the file. • This public class must have a static method called main (Why static??) public static void main( String[] args) { local declarations and statements }

Exercise • Setup your Unix account to compile and run a Java Application program. Test it with the HelloWorld program hello.java. • The example is in the java/hello subdirectory in the instructor’s public area

Basic Java Programming • The C/C++ component • Comments – C/C++ style • Identifiers and keywords • Types, variables, expressions • Control structures • Functions • System output • Console input • Packages

Identifiers and Keywords • A Java identifier must start with a letter, followed by 0 or more letters and/or digits. Java is case-sensitive. • Keywords cannot be used as user-identifiers. See text for a list of keywords. • Style – recommended & preferred; consistency is a must! • Class names begin with a capital letter • Variable names begin with a lower case letter • Function names begin with a verb which is lowercased. • Constants are all upper case. • Multiple word names are lower case except for the beginning of each word component.

Examples • Request would be a class name • myRequest would be a variable name • getRequest() would be a function (method) name • THE_REQUEST would be a constant. • The Java standard style convention should be followed in our programming.

Standard Types • char – ascii or unicode • boolean – true or false • Numerical types – various sizes of numbers

Numerical Types • Standard numerical types in Java are type size least value greatest value ________________________________________ byte 8 -128 127 short 16 -32768 32767 int 32 -2147483648 2147483647 long 64 -263 263-1 float* 32 ~ -3.4 x 1038 ~ 3.4 x 1038 double* 64 ~ -1.7 x 10308 ~ 1.7 x 10308 * 7 and 15 digit accuracy respectively

Variables and Expressions • Java follows the syntax of C/C++ for expressions and assignment. • The operators for the standard types are the same as those for C/C++ • Remember that = is assignment and == is equal relational operator. • You should NOT use = in a cascading manner.

Control Structures The control structures are the same as C/C++ • if • switch • for • while • do – while Note: unlike C/C++, where the expression can evaluate to int, the test expression MUST be of type boolean

Functions (Methods) • In Java there are no independent functions • A function (method) is always a member function of some class. • The syntax is very similar. modifier(s) resulttype name( <params>) { local declarations and statements } // the modifier is public, private, or protected, and can also be prefaced static

System Output • Output is generated by using streams. The stream classes are defined in the standard Java package java.io. • The class System in the package java.lang contains three different streams for use in Java programs: System.in the keyboard System.out the screen System.err the screen • System.out.println( any string)

Examples of Output • To print an object, the object should have overloaded the method toString that is inherited from the Class Object. • Standard types have this method. • System.out.println(“The value of x = “ + x ); • The + is the concatenation operator for strings.

System Input • System input is quite complicated, so many authors provide a package of IO functions for the standard types. • We prefer to avoid these. A class InputReader is in the public java directory for text input. • More fundamental IO will be discussed later • Java is made for GUIs, particularly components such as TextFields, Menus. etc;

Examples of input • helloYou.java • helloInt.java • java/hello directory in public area

Things that are different • String concatenation is the operator + • It takes two operands which are “stringable”, that is any operand that is a string or has the toString method overloaded for that type. • System.out.println(76 + “trombones”);

Packages • Java organizes code into packages which correspond to directories in the file system. • Each Java class is contained in a package. • The default package is . (the current directory) • The System class is found in java.lang • The Applet class is found in java.applet

Comments • When calling methods of the same class, we do not need to use the class name as a prefix. • When calling methods of another class, we use the class name or object of that class as a prefix.

Objects • Still declared as a class • No separate sections • Each declaration specified with access • public, private, protected • static declarations part of class, but not object • non-static declarations part of instantiated objects only

Java Programming