© 2012 pearson education, inc. all rights reserved. 1-1 why java? needed program portability –...

© 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”


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


A Compiler

1. Programmer writes program– using high-level progr. lang. (C, C#, COBOL)

– using text editoror IDE (Integrated Development Environment)

source code file = set of progr. lang. statements

2. Compiler translates it to machine language(=executable code: SomeProgram.exe)


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

.


A typical Compiler vs. the Java compiler (& the JVM)

• Most compilers translate source code intoexecutable file containing machine codefor a specific CPU / OS

• Java compiler translates a 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 = a program that emulates a CPU

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

– So it’s called an interpreter

• 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 = 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


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


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 use to write Java programs

• different editions of JDK:

– Java SE - Standard Edition.

– Java EE - Enterprise Edition.

– Java ME - Micro Edition.

• Available for download


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)


Programming LanguagesCommon Language Elements

• Some common concepts– Key words

– Operators

– Punctuation

– Programmer-defined identifiers

– Strict syntactic 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 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


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 specifies1) 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


Variables

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

0x0000x0010x0020x0030x0040x0050x0060x007


Variables

0x0000x0010x0020x0030x0040x0050x0060x007

Java VirtualMachine (JVM)(not programmer)decides wherein memory thedeclared variableis stored

72

Here’s a declaration ofa varaible called length

int length = 72;

The variable called lengthis a symbolic name for theMemory location 0x003.

Programmer doesn’t knowit’s in 0x003.

© 2012 pearson education, inc. all rights reserved. 1-1 why java? needed program portability –...

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