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Why Java?

Why Java?. Needed program portability Program written in a language that would run on various devices / OS’s without rewriting/recompiling the program Java is “cross platform”. Java Applications & Applets. 2 types of Java programs: Application

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Why Java?

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  1. Why Java? Needed program portability Program written in a language that would run on various devices / OS’s without rewriting/recompiling the program Java is “cross platform”

  2. Java Applications & Applets 2 types of Java programs: Application Stand-alone program (run without a web browser) Relaxed security since user runs program locally Applet Small app embedded in a webpage - requires a Java enabled web browser to run app Enhanced security since user goes to a web page & applet runs itself

  3. A Compiler Programmer writes program using high-level progr. lang. (C, C#, COBOL) using text editor or IDE (Integrated Development Environment) source code file = set of progr. lang. statements Compiler translates it to machine language (=executable code: SomeProgram.exe)

  4. A Compiler is a program processes: Input data: source code file Output data: machine language file finds syntax errors ~ spelling, grammar, structure errors that violate rules of that programming language .

  5. A typical Compiler vs. the Java compiler (& the JVM) Most compilers translate source code into executable file containing machine code for a specific CPU / OS Java compiler translates a Java source file into a file containing byte code instructions Byte code instructions are the “machine language” of the Java Virtual Machine (JVM) & can NOT be executed directly by a CPU

  6. Java Virtual Machine JVM = a program that emulates a CPU JVM executes each byte code instruction as it’s read (unlike a compiler) So it’s called an interpreter Java = an interpreted language

  7. Program Development Process Source code (.java) Saves Java statements Is read by Byte code(.class) Java compiler (javac) Produces Is interpreted by Program Execution Java Virtual Machine (java) Results in Text editor (or IDE)

  8. Portability Portable = a program written on one type of computer can run on a wide variety of computers (with little or no modification.) Java byte code runs on the JVM (on a computer), not on any particular CPU So compiled Java programs are highly portable Specific JVMs exist for many platforms: • Windows • Mac • Linux • Unix • BSD • etc.

  9. Portability most programming languages’ programs: portability achieved by compiling program for each different platform/CPU it’ll run on so many different .exe files Java provides a JVM for each platform so no recompiling for different platforms so only one .class (byte code) file Byte code program runs on ANY JVM

  10. Portability Byte code(.class) Java Virtual Machine for Windows Java Virtual Machine for Unix Java Virtual Machine for Linux Java Virtual Machine for Mac

  11. Java Versions JDK (Java Development Kit) software use to write Java programs different editions of JDK: Java SE - Standard Edition. Java EE - Enterprise Edition. Java ME - Micro Edition. Available for download

  12. Compiling a Java Program javac is the Java compiler Java compiler is a command line utility to compile a program: javac SomeProgram.java must use .java file extension IDE automates (& hides) this Called Build (instead of compile)

  13. Programming LanguagesCommon Language Elements Some common concepts Key words Operators Punctuation Programmer-defined identifiers Strict syntactic rules.

  14. Sample Java Program – key words public class HelloWorld { public static void main(String[] args) { String message = "Hello World"; System.out.println(message); } } Key words: public, class, static, void lower case (Java is a case-sensitive) can’t use these as programmer-defined identifiers

  15. Java – lines vs. statements A statement = a complete instruction that causes the computer to perform an action. Semi-colon at end of every statement not at end of every line System.out.println( message); This is 1 statement written on 2 lines

  16. Java Variables Variables store data in a program (in memory) A variable name represents a location in memory Variables also called fields Variables are created by the programmer who specifies 1) name 2) data TYPE 3) starting value (maybe) example: int age = 18; age variable will contain an integer value; it initially stores the value 18

  17. Variables Variable - a name given to a location in memory 8 locations shown below 0x000 0x001 0x002 0x003 0x004 0x005 0x006 0x007

  18. Variables Here’s a declaration of a varaible called length int length = 72; 0x000 0x001 0x002 0x003 72 0x004 The variable called length is a symbolic name for the Memory location 0x003. Programmer doesn’t know it’s in 0x003. 0x005 0x006 0x007 Java Virtual Machine (JVM) (not programmer) decides where in memory the declared variable is stored

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