© 2012 Pearson Education, Inc. All rights reserved. 1-1 2 types of Java programs Application –...

© 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


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/OSwithout rewriting/recompiling program


Normal Compiler1. 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 intomachine language(=executable code: SomeProgram.exe)


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

.


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 intoa file containing byte code instructions

• Byte code instructions arethe “machine language” of the Java Virtual Machine (JVM) & can NOT be executed directly by a CPU


Java Virtual Machine (JVM)

• JVM = a program that emulates a CPU

• JVM executes each byte code instructionas it’s read (unlike a compiler)

– So it’s called an interpreter (vs. a compiler)

• Java = an interpreted language


Program Development Process

Text editor(or IDE)

Source code(.java)

Saves Java statements

Java compiler(javac)

Is read by

Byte code(.class)

Produces

JavaVirtual Machine

(java)

Is interpreted by

ProgramExecution

Results in


Portability

• Portable = program written for 1 type of computerruns 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


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


Portability

Java VirtualMachine for Windows

Byte code(.class)

Java VirtualMachine for Linux

Java VirtualMachine for Mac

Java VirtualMachine for Unix


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


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


Programming LanguagesCommon Language Elements (all languages)

• Some common concepts– Key words– Operators– Punctuation– Programmer-defined identifiers– Strict syntactic (grammar) rules


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


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


Variables

• store data in a program (in memory)• variable name represents a location in memory• also called fields• variables created by programmer – specify: 1)

name2) data TYPE3) starting value (optional)

int age = 18;String name;


Variables

Variable = a name given to a location in memory– 8 locations below

0x0000x0010x0020x0030x0040x0050x0060x007


Variables

0x0000x0010x0020x0030x0040x0050x0060x007

JVM(not programmer)decides wherein memory thedeclared variableis stored

72

int length = 72;

Variable called lengthis a symbolic name for memory location 0x003.

Programmer does NOTknow or careit’s in 0x003.

© 2012 Pearson Education, Inc. All rights reserved. 1-1 2 types of Java programs Application –...

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