关于JAVA领域的外文翻译

原文

Java

2

Micro

Edition

and

the

World

of

Java

1

Introduction

The

computer

revolution

of

the

1970s

increased

the

demand

for

sophisticated

computersoftware

to

take

advantage

of

the

ever

-increasing

capacity

of

computers

to

process

C

program

ming

language

became

the

linchpin

that

enabled

programmers

to

buildsoftware

that

was

just

as

robust

as

the

computer

it

ran

on.

As

the

1980s

approached,

programmers

were

witnessing

anot

her

spurt

in

the

evolutionof

programming

language.

Computer

tec

hnology

advanced

beyond

the

capabilities

of

the

C

programming

l

anguage.

The

problem

wasn’t

new.

It

occurred

previously

and

ca

used

the

demise

of

generations

of

programming

languages.

The

problem

was

thatprograms

were

becoming

too

complicated

to

des

ign,

write,

and

manage

to

keep

up

with

the

capabilities

of

compu

ters.

It

was

around

this

time

that

a

design

concept

based

on

Sim

ula

67

and

Smalltalk

(from

the

late

1960s)

moved

programming

to

the

next

evolutionary

step.

This

was

the

period

when

object-or

iented

programming

(OOP),

and

with

it

a

new

programming

lang

uage

called

C++,

took

programmers

by

storm.

In

1979,

Bjarne

Stroustrup

of

Bell

Laboratories

in

New

Jersey

enhanced

the

C

programming

language

to

include

object-oriente

d

features.

He

called

the

language

C++.

(The

++

is

the

increme

ntal

operator

in

the

C

programming

language.)

C++

is

truly

an

e

nhancement

of

the

C

programming

language,

and

it

began

as

a

preprocessor

language

that

was

translated

into

C

syntax

before

t

he

program

was

processed

by

the

compiler.

Stroustrup

built

on

the

concept

of

a

class

(taken

from

Simula

67

and

Smalltalk),

from

which

instances

of

objects

are

created.

A

cl

ass

contains

data

members

and

member

functions

that

define

an

object’s

data

and

functionality.

He

also

introduced

the

concept

o

f

inheritance,

which

enabled

a

class

to

inherit

some

or

all

data

members

and

member

functions

from

one

or

more

other

classes

—

all

of

which

complements

the

concepts

of

object-oriented

progr

amming.

By

1988,

ANSI

officials

standardized

Stroustrup’s

C++

specific

ation.

2

Enter

Java

Just

as

C++

was

becoming

the

language

of

choice

for

buildin

g

industrial-strength

applications,

another

growth

spurt

in

the

evo

lution

of

programming

language

was

budding,

fertilized

by

the

lat

est

disruptive

technology

—

the

World

Wide

Web.

The

Internet

had

been

a

well-kept

secret

for

decades

before

the

National

Science

Foundation

(who

oversaw

the

Internet)

removed

barriers

that

pre

vented

commercialization.

Until

1991

when

it

was

opened

to

com

merce,

the

Internet

was

the

almost

exclusive

domain

of

governm

ent

agencies

and

the

academic

community.

Once

the

barrier

to

c

ommercialization

was

lifted,

the

World

Wide

Web

—

one

of

several

services

offered

on

the

Internet

—

became

a

virtual

community

ce

nter

where

visitors

could

get

free

information

about

practically

an

ything

and

browse

through

thousands

of

virtual

stores.

Browsers

power

the

World

Wide

Web.A

browser

translates

AS

CII

text

files

written

in

HTML

into

an

interactive

display

that

can

be

interpreted

on

any

machine.

As

long

as

the

browser

is

compat

ible

with

the

correct

version

of

HTML

and

HTTP

implementation,

any

computer

running

the

browser

can

use

the

same

HTML

docu

ment

without

having

to

modify

it

for

a

particular

type

of

compute

r,

which

was

something

unheard

of

at

the

time.

Programs

written

in

C

or

C++

are

machine

dependent

and

cannot

run

on

a

differ

ent

machine

unless

the

program

is

recompiled.

The

success

of

the

Internet

gave

renewed

focus

to

developin

g

a

machine-independent

programming

language.

And

the

same

year

the

Internet

was

commercialized,

five

technologists

at

Sun

Microsystems

set

out

to

do

just

that.

James

Gosling,

Patrick

Nau

ghton,

ChrisWarth,

Ed

Frank,

and

Mike

Sheridan

spent

18

months

developing

the

programming

language

they

called

Oak,

which

w

as

renamed

Java

when

this

new

language

made

its

debut

in

199

5.

Java

went

through

numerous

iterations

between

1991

and

199

5,

during

which

time

many

other

technologists

at

Sun

made

subs

tantial

contributions

to

the

language.

These

included

Bill

Joy,

Arth

ur

van

Hoff,

Jonathan

Payne,

Frank

Yelin,

and

Tim

Lindholm.

Although

Java

is

closely

associated

with

the

Internet,

it

was

d

eveloped

as

a

language

for

programming

software

that

could

be

embedded

into

electronic

devices

regardless

of

the

type

of

CPU

u

sed

by

the

device.

This

is

known

as

the

EmbeddedJava

platform

and

is

in

continuous

use

today

for

closed

systems.

The

Java

team

from

Sun

succeeded

in

creating

a

portable

pro

gramming

language,

something

that

had

eluded

programmers

sin

ce

computers

were

first

programmed.

Their

success,

however,

wa

s

far

beyond

their

wildest

dreams.

The

same

concept

used

to

ma

ke

Java

programs

portable

to

electronic

devices

also

could

be

use

d

to

make

Java

programs

run

on

computers

running

Microsoft

Wi

ndows,

UNIX,

and

Macintosh.

Timing

was

perfect.

The

Internet/in

tranet

had

whetted

corporate

America’s

appetite

for

cost-effective,

portable

programs

that

could

replace

mission-critical

applications

within

the

corporation.

And

Java

had

proven

itself

as

a

program

ming

language

used

to

successfully

develop

machine-independent

applications.

3

Java

Virtual

Machine

Writing

Java

programs

is

similar

to

writing

C++

programs

in

that

the

programmer

writes

source

code

that

contains

instruction

s

into

an

editor,

or

in

an

integrated

development

environment,

and

then

the

source

code

is

compiled.

However,

th

at’s

where

Java

and

C++

part

ways.

The

compiling

and

linking

process

of

a

C++

program

results

in

an

executable

that

can

be

run

on

an

appropriate

machine.

In

contrast,

the

Java

compiler

converts

Java

source

code

into

bytecode

that

is

executed

by

the

Java

Virtual

M

achine

(JVM).

Machine-specific

instructions

are

not

included

in

bytecode.

Ins

tead,

they

already

reside

in

the

JVM,

which

is

machine

specific.

T

his

means

that

the

bytecode

might

contain

fewer

instructions

tha

t

need

to

be

translated

than

a

comparable

C++

program.

A

lthough

the

Java

compiler

generates

bytecode

that

must

be

interpreted

by

the

JVM

at

run

time,

the

number

of

instructions

that

need

translation

are

usually

minimal

and

have

already

been

optimized

by

the

Java

compiler.

4

Back

to

the

Future:

J2ME

Remember

that

Java

began

as

a

programming

language

to

cr

eate

programs

for

embedded

syst ems

—

microcomputers

found

in

consumer

and

industrial

products

such

as

those

used

to

control

a

utomobiles

and

appliances.

The

development

team

at

Sun

worked

on

Java

in

the

early

1990s

to

address

the

programming

needs

o

f

the

fledgling

embedded

computer

market,

but

that

effort

was

si

detracked

by

more

compelling

opportunities

presented

by

the

Int

ernet.

As

those

opportunities

were

addressed,

a

new

breed

of

porta

ble

communications

devices

opened

other

opportunities

at

the

tur

n

of

the

century.

Cell

phones

expanded

J

2

M

E

:

T

h

e

C

o

m

p

l

e

t

e

R

e

f

e

r

e

n

c

e

from

voice

communications

devices

to

voice

and

text

communications

devices.

Pocket

electronic

telepho

ne

directories

evolved

into

personal

digital

assistants.

Chipmakers

were

releasing

new

products

at

this

time

that

were

designed

to

transfer

computing

power

from

a

desktop

computer

into

mobile

s

mall

computers

that

controlled

gas

pumps,

cable

television

boxes,

and

an

assortment

of

other

appliances.

The

time

was

right

for

the

next

evolution

of

Java.

However,

i

nstead

of

beefing

up

Java

with

additional

APIs,

the

team

at

Sun,

along

with

the

Java

Community

Process

Program,

dismantled

bot

h

the

Java

programming

language

and

the

Java

Virtual

Machine.

They

stripped

down

Java

APIs

and

the

JVM

to

the

minimum

coding

req

uired

to

provide

intelligence

to

embedded

systems

and

microcomputer

devices.

Thi

s

was

necessary

because

of

resource

constraints

imposed

upon

the

hardware

desig

n

of

these

devices.

The

result

of

their

efforts

is

J2ME.

J2ME

is

a

reduced

version

of

the

J

ava

API

and

Java

Virtual

Machine

that

is

designed

to

operate

wit

hin

the

sparse

resources

available

in

the

new

breed

of

embedded

computers

and

microcomputers.

5

How

J2ME

Is

Organized

Traditional

computing

devices

use

fairly

standard

hardware

co

nfigurations

such

as

a

display,

keyboard,mouse,

and

large

amoun

ts

of

memory

and

permanent

storage.

However,

the

new

breed

o

f

computing

devices

lacks

hardware

configuration

continuity

amon

g

devices.

Some

de

vices

don’t

have

a

display,

permanent

storage,

keyboard,

or

mouse.

And

memory

availability

is

inconsistent

am

ong

small

computing

devices.

The

lack

of

uniform

hardware

confi

guration

among

the

small

computing

devices

poses

a

formidable

challenge

for

the

Java

Community

Process

Program,

which

is

char

ged

with

developing

standards

for

the

JVM

and

the

J2ME

for

sma

ll

computing

devices.

J2ME

must

service

many

different

kinds

of

small

computing

d

evices,

including

screenphones,

digital

set-top

boxes

used

for

cabl

e

television,

cell

phones,

and

personal

digital

assistants.

The

chall

enge

for

the

Java

Community

Process

Program

is

to

develop

a

Ja

va

standard

that

can

be

implemented

on

small

computing

devices

that

have

nonstandard

hardware

configurations.

The

Java

Community

Process

Program

h

as

used

a

twofold

approach

to

addressing

the

needs

of

small

co

mputing

devices.

First,

they

defined

the

Java

run-time

environme

nt

and

core

classes

that

operate

on

each

device.

This

is

referred

to

as

the

configuration.

A

configuration

defines

the

Java

Virtual

M

achine

for

a

particular

small

computing

device.

There

are

two

con

figurations,

one

for

handheld

devices

and

the

other

for

plug-in

de

vices.

Next,

the

Java

Community

Process

Program

defined

a

profi

le

for

categories

of

small

computing

devices.

A

profile

consists

of

classes

that

enable

developers

to

implement

features

found

on

a

related

group

of

small

computing

devices.

6

J2ME

configurations

There

are

two

configurations

for

J2ME

as

of

this

writing.

Thes

e

are

Connected

Limited

Device

Configuration

(CLDC)

and

the

Co

nnected

Device

Configuration

(CDC).

The

CLDC

is

designed

for

16

-bit

or

32-bit

small

computing

devices

with

limited

amounts

of

m

emory.

CLDC

devices

usually

have

between

160KB

and

512KB

of

avai

lable

memory

and

are

battery

powered.

They

also

use

an

inconsi

stent,

small-bandwidth

network

wireless

connection

and

may

not

have

a

user

interface.

CLDC

devices

use

the

KJava

Virtual

Machine

(KVM)

implementation,

which

is

a

stripped- down

version

of

the

JVM.

CLDC

devices

include

pagers,

personal

digital

assistants,

cell

phones,

de

dicated

terminals,

and

handheld

consumer

devices

with

between

128KB

and

512KB

of

m

emory.

CDC

devices

use

a

32-bit

architecture,

have

at

least

two

megaby

tes

of

memory

available,

and

implement

a

complete

functional

JVM.

CDC

devices

include

digital

set-top

boxes,

home

appliances,

navigation

syste

ms,

point- of-sale

terminals,

and

smart

phones.

7

J2ME

Profiles

A

profile

consists

of

Java

classes

that

enable

implementation

of

features

for

either

a

particular

small

computing

device

or

for

a

class

of

small

computing

devices.

Small

computing

technology

co

ntinues

to

evolve,

and

with

that,

there

is

an

ongoing

process

of

defining

J2ME

profiles.

Seven

profiles

have

been

defined

as

of

thi

s

writing.

These

are

the

Foundation

Profile,

Game

Profile,

Mobile

Information

Device

Profile,

PDA

Profile,

Personal

Profile,

Personal

Basis

Profile,

and

RMI

Profile.

■

The

Foundation

Profile

is

used

with

the

CDC

configuration

and

is

the

core

for

nearly

all

other

profiles

used

with

the

CDC

configu

ration

because

the

Foundation

Profile

contains

core

Java

classes.

■

The

Game

Profile

is

also

used

with

the

CDC

configuration

and

contains

the

necessary

classes

for

developing

game

applications

f

or

any

small

computing

device

that

uses

the

CDC

configuration.

■

The

Mobile

Information

Device

Profile

(MIDP)

is

used

with

the

CLDC

configuration

and

contains

classes

that

provide

local

storag

e,

a

user

interface,

and

networking

capabilities

to

an

application

t

hat

runs

on

a

mobile

computing

device

such

as

Palm

OS

devices.

MIDP

is

used

with

wireless

Java

applications.

■

The

PDAProfile

(PDAP)

is

used

with

the

CLDC

configuration

an

d

contains

classes

that

utilize

sophisticated

resources

found

on

pe

rsonal

digital

assistants.

These

features

include

better

displays

an

d

larger

memory

than

similar

resources

found

on

MIDP

mobile

de

vices

(such

as

cell

phones).

■

The

Personal

Profile

is

used

with

the

CDC

configuration

and

th

e

Foundation

Profile

and

contains

classes

to

implement

a

complex

user

interface.

The

Foundation

Profile

provides

core

classes,

and

the

Personal

Profiles

provide

classes

to

implement

a

sophisticate

d

user

interface,

which

is

a

user

interface

that

is

capable

of

displ

aying

multiple

windows

at

a

time.