© 2012 Pearson Education, Inc. All rights reserved. 1-1 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”

© 2012 Pearson Education, Inc. All rights reserved. 1-2 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

© 2012 Pearson Education, Inc. All rights reserved. 1-3 A Compiler 1.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 2.Compiler translates it to machine language (=executable code: SomeProgram.exe )

© 2012 Pearson Education, Inc. All rights reserved. 1-4 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.

© 2012 Pearson Education, Inc. All rights reserved. 1-5 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

© 2012 Pearson Education, Inc. All rights reserved. 1-6 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

© 2012 Pearson Education, Inc. All rights reserved. 1-7 Program Development Process Text editor (or IDE) Source code (.java ) Saves Java statements Java compiler ( javac ) Is read by Byte code (.class ) Produces Java Virtual Machine ( java ) Is interpreted by Program Execution Results in

© 2012 Pearson Education, Inc. All rights reserved. 1-8 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 : Unix BSD etc. Windows Mac Linux

© 2012 Pearson Education, Inc. All rights reserved. 1-9 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

© 2012 Pearson Education, Inc. All rights reserved Portability Java Virtual Machine for Windows Byte code (.class) Java Virtual Machine for Linux Java Virtual Machine for Mac Java Virtual Machine for Unix

© 2012 Pearson Education, Inc. All rights reserved 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

© 2012 Pearson Education, Inc. All rights reserved 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)

© 2012 Pearson Education, Inc. All rights reserved Programming Languages Common Language Elements Some common concepts – Key words – Operators – Punctuation – Programmer-defined identifiers – Strict syntactic rules.

© 2012 Pearson Education, Inc. All rights reserved 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

© 2012 Pearson Education, Inc. All rights reserved 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

© 2012 Pearson Education, Inc. All rights reserved 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

© 2012 Pearson Education, Inc. All rights reserved Variables Variable - a name given to a location in memory – 8 locations shown below 0x000 0x001 0x002 0x003 0x004 0x005 0x006 0x007

© 2012 Pearson Education, Inc. All rights reserved Variables 0x000 0x001 0x002 0x003 0x004 0x005 0x006 0x007 Java Virtual Machine (JVM) (not programmer) decides where in memory the declared variable is stored 72 Here’s a declaration of a varaible called length int length = 72; The variable called length is a symbolic name for the Memory location 0x003. Programmer doesn’t know it’s in 0x003.