Introduction To Java - MFC 158 G

Week 2 Lecture notes - Fall 2000

• Evolution of Operating systems
• Early computers performed one task at a time - 1950's
• Task requests run in batch - took hours sometimes - punched cards
• Early Operating Systems were developed to speed up time between jobs - help operators
• Allowed smooth transition between jobs - faster throughput
• Timesharing allowed terminal access to multiusers - each user getting a slice of time - 1960's (paper terminals)
• Takes advantage of idle users
• Gives illusion of dedicated resourses - appear to be running simultaneous

• Personal, Distributed and Client/Server computing
• 1977 Apple computers were the first 'popular' home computer
• 1981 IBM PC was introduced - first 'business' computer
• Started out using floppies between machines - later connected with Local Area Networks
• LANs eventually led to distributed computing - work done on several systems
• File servers - client computers (workstations) led to the concept of 'Client-server'
• UNIX, Linux, OS/2 MacOS and Windows (95/98/NT) have Client/Server capabilities

• Computer Languages - 3 types (generation languages)

• Machine language - First Generation (late 40s - early 50s)
• Directly understandable by the system (natural language of 'a' computer)
• Consist of strings (reduced to 1's and 0's) for instructing the computer
• Programmer entered 0s/1s directly into memory - error prone/hard to modify code
• Machine dependant - not portable to other computers!

• Assembly languages - Second Generation (early 50s - present)
• English like abbreviations to represent basic operations of the computer
• Easier to understand than ML
• Allows faster development than ML
• Translator programs called 'assemblers' converted code into ML for runtime
• Not portable to various hardware - each computer has its own assembly language
• Many instructions required for simple tasks
• Still used for programs needing optimized performance (speed/memory)

• High Level languages - Third Generation (middle 50s - present)
• Single statements could now do substantial work
• Higher level of abstraction from source code to actual task
• More 'English like' than lower level languages - COBOL
• Translator programs called compilers convert high level code into ML
• C/C++ are commonly used in client/server environments
• Java is becoming the popular choice for Internet-based applications

• Comparison of code

· Machine code:

§ +1300042774

§ +1400593419

§ +1200274027 (adds overtime pay to base pay and stores in gross pay)

· Assembly code:

• LOAD BASEPAY
• ADD OVERPAY
• STORE GROSSPAY (adds overtime pay to base pay and stores in gross pay)

· High level language code:

• grossPay = basePay + overTimePay

• Fourth Generation Languages
• Code generator software development tools
• Design a screen for accessing data - code is generated
• Microsoft Access
• Ideal language - generates COBOL-like code

• History of C++
• Evolved from C, which evolved from B
• B was used to create early versions of UNIX (1970)- typeless (no int/string concept)
• C now included data types (1972) - used for developing various Operating systems
• C can be Hardware independent - if written portably
• C++ developed in 1980s
• Contained some features to extend C
• Capabilities of OOP
• Usage of objects - reusable software components

• History of Java (James Gosling - Oak)
• Originally researched/created for use with 'intelligent' consumer devices (1991)
• Development in devices was slow - no use for it yet
• WWW popularity was growing (1993)
• Sun initially saw potential to use Java for dynamic web content
• Sun announced Java at a conference (1995) - much interest generated
• Utilized C/C++ syntax, yet eliminated features that caused programming errors

• Java usage's today
• Used to create pages with dynamic/interactive content
• Develop large-scale enterprise applications
• Enhance functionality of WWW Servers
• Applications for consumer devices (cell phones, pagers, etc.)

• The Java Environment and phases of development
• Windows / Solaris OS
• Windows àWindows
• Solaris à Solaris
• Windows à Solaris (Xterm emulator - like exceed)

• Phases for execution
• Edit source code
• Compile source code
• Load the compiled bytecode
• Bytecode verification (prior to execution)
• Execution of code

• Edit source code
• Observe case sensitivity (in source and external filenames)
• Save source with the .java extension
• Compile source code
• translates source code into java bytecode
• the language understood by the Java interpreter in execution phase
• use the javac comand
• creates a file with source name and .class extension
• Loading the Bytecode
• Program gets placed in memory by the class loader
• Applets are loaded from a remote computer to the browser
• Applications are loaded using the Java interpreter (using the java command)
• Browsers supporting java have built in java interpreters
• Bytecode verification (prior to execution)
• Checks to assure that the bytecodes are valid
• Checks that the bytecodes don't violate Java security
• Writing to hard disk
• Certain system changes
• Execution of code
• 'interprets' the program one bytecode at a time
• performs the actions specified by the program

• Distinction between compiler and interpreter
• Compiler creates 'machine independent' bytecode output (.class)
• Java interpreter (machine dependant) interprets the bytecode to interact with it's particular hardware

• JIT (Just in Time) Compiler
• Compiles the code as the interpreter runs
• Produces compiled code and executes the program in machine language
• more efficient, though not as efficient as a full compiler
• Full compilers currently under development

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Webopedia’s rather concise definition of Java:

http://webopedia.internet.com/TERM/J/Java.html

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Comparison of High level vs. Low level program source code:

Sample of a 'hello' program, written in C, assembly and in machine code (represented as octal using the UNIX utility {od -b}

-rw-r--r-- 1 fineberg 75 Sep 7 16:32 hello.c

-rw-r--r-- 1 fineberg 1008 Sep 7 16:33 hello.s

-rwxr-xr-x 1 fineberg 678756 Sep 7 16:32 hello.exe

----Third generation language - C++----

#include <iostream.h>

int main()

{

cout << "hello";

return 0;

}

----------------------------------------

-----Second generation language - Assembly language----

.file "hello.c"

gcc2_compiled.:

.global _Q_qtod

.global __throw

.section ".rodata"

.align 8

.LLC0:

.asciz "hello"

.section ".text"

.align 4

.global main

.type main,#function

.proc 04

main:

.LLFB1:

!#PROLOGUE# 0

save %sp,-112,%sp

.LLCFI0:

!#PROLOGUE# 1

sethi %hi(cout),%o1

or %o1,%lo(cout),%o0

sethi %hi(.LLC0),%o2

or %o2,%lo(.LLC0),%o1

call __ls__7ostreamPCc,0

nop

mov 0,%i0

b .LL281

nop

mov 0,%i0

b .LL281

nop

.LL281:

ret

restore

.LLFE1:

.LLfe1:

.size main,.LLfe1-main

.section ".eh_frame",#alloc,#write

__FRAME_BEGIN__:

.uaword .LLECIE1-.LLSCIE1

.LLSCIE1:

.uaword 0x0

.byte 0x1

.byte 0x0

.byte 0x1

.byte 0x7c

.byte 0x65

.byte 0xc

.byte 0xe

.byte 0x0

.byte 0x9

.byte 0x65

.byte 0xf

.align 4

.LLECIE1:

.uaword .LLEFDE1-.LLSFDE1

.LLSFDE1:

.uaword .LLSFDE1-__FRAME_BEGIN__

.uaword .LLFB1

.uaword .LLFE1-.LLFB1

.byte 0x4

.uaword .LLCFI0-.LLFB1

.byte 0xd

.byte 0x1e

.byte 0x2d

.byte 0x9

.byte 0x65

.byte 0x1f

.align 4

.LLEFDE1:

.ident "GCC: (GNU) 2.8.1"

-----------------------------------------

-----First generation language - Machine code language---

Portion of (c program) Hello executable

using od -b Interpret bytes in octal

0000000 177 105 114 106 001 002 001 000 000 000 000 000 000 000 000 000

0000020 000 002 000 002 000 000 000 001 000 001 234 144 000 000 000 064

0000040 000 012 122 124 000 000 000 000 000 064 000 040 000 005 000 050

0000060 000 072 000 067 000 000 000 006 000 000 000 064 000 001 000 064

0000100 000 000 000 000 000 000 000 240 000 000 000 240 000 000 000 005

0000120 000 000 000 000 000 000 000 003 000 000 000 324 000 000 000 000

0000140 000 000 000 000 000 000 000 021 000 000 000 000 000 000 000 004

0000160 000 000 000 000 000 000 000 001 000 000 000 000 000 001 000 000

...

0002040 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000

0002060 000 000 000 000 000 000 000 202 000 000 000 000 000 000 001 234

0002100 000 000 000 000 000 000 002 057 000 000 001 076 000 000 000 000

0002120 000 000 000 000 000 000 000 000 000 000 000 000 000 000 001 005

0002140 000 000 000 174 000 000 000 231 000 000 002 152 000 000 001 210

---end of sample----