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

2. © 2012 Pearson Education, Inc. All rights reserved. 1-2 Why Java (vs. other languages)? Java is “cross platform” Portability – Program written for 1 type of device/HW/OS runs on ANY OTHER device/HW/OS without rewriting/recompiling program

3. © 2012 Pearson Education, Inc. All rights reserved. 1-3 Normal Compiler 1.Programmer writes program – in high-level progr. lang. (C, C#, COBOL,…) – using text editor or IDE (Integrated Development Environment)  source code file = set of programming language statements 2.Compiler translates source code into machine language (=executable code: SomeProgram.exe )

4. © 2012 Pearson Education, Inc. All rights reserved. 1-4 Compiler = a program IPO (Input / Processing / Output) 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. © 2012 Pearson Education, Inc. All rights reserved. 1-5 A typical Compiler vs. the Java compiler (& the JVM) Most compilers translate source code file into executable file (= machine code for a specific CPU / OS) Java compiler translates 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. © 2012 Pearson Education, Inc. All rights reserved. 1-6 Java Virtual Machine (JVM) 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 (vs. a compiler) Java = an interpreted language

7. © 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

8. © 2012 Pearson Education, Inc. All rights reserved. 1-8 Portability Portable = program written for 1 type of computer runs on a wide variety of computers (with little/no modification) (e.g., applets) Java byte code runs on the JVM (on a computer), not on any particular CPU So compiled Java programs highly portable Specific JVMs exist for many platforms : Unix BSD etc. Windows Mac Linux

9. © 2012 Pearson Education, Inc. All rights reserved. 1-9 Portability most programming languages’ programs: portability by re-compiling program for each different platform/CPU – so, many different.exe files – (what about applets for web?) Java provides a JVM for each platform – so only 1.class (byte code) file for everywhrte – Byte code program runs on ANY JVM

10. © 2012 Pearson Education, Inc. All rights reserved. 1-10 Portability Java Virtual Machine for Windows Byte code (.class) Java Virtual Machine for Linux Java Virtual Machine for Mac Java Virtual Machine for Unix

11. © 2012 Pearson Education, Inc. All rights reserved. 1-11 Java Versions JDK (Java Development Kit) – software used to write Java programs different editions of JDK: – Java SE - Standard Edition. – Java EE - Enterprise Edition. – Java ME - Micro Edition. Available for free download

12. © 2012 Pearson Education, Inc. All rights reserved. 1-12 2 ways to compile Java program 1.In command-prompt (B&W) window – javac is Java compiler (for specific JVM) – to compile: javac SomeProgram.java – must use.java file extension 2.IDE autBomates (& hides) this – called Build (instead of compile) – Automatic build when program is Run

13. © 2012 Pearson Education, Inc. All rights reserved. 1-13 Programming Languages Common Language Elements (all languages) Some common concepts – Key words – Operators – Punctuation – Programmer-defined identifiers – Strict syntactic (grammar) rules

14. © 2012 Pearson Education, Inc. All rights reserved. 1-14 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. © 2012 Pearson Education, Inc. All rights reserved. 1-15 Java – lines vs. statements A statement = a complete instruction that causes 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. © 2012 Pearson Education, Inc. All rights reserved. 1-16 Variables store data in a program (in memory) variable name represents a location in memory also called fields variables created by programmer – specify: 1) name 2) data TYPE 3) starting value (optional) int age = 18; String name;

17. © 2012 Pearson Education, Inc. All rights reserved. 1-17 Variables Variable = a name given to a location in memory – 8 locations below 0x000 0x001 0x002 0x003 0x004 0x005 0x006 0x007

18. © 2012 Pearson Education, Inc. All rights reserved. 1-18 Variables 0x000 0x001 0x002 0x003 0x004 0x005 0x006 0x007 JVM (not programmer) decides where in memory the declared variable is stored 72 int length = 72; Variable called length is a symbolic name for memory location 0x003. Programmer does NOT know or care it’s in 0x003.