Java编程入门：硬件、软件与互联网的融合

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"这是一本全面的Java入门电子书，涵盖了从基础到进阶的内容，并配有相关的扩展练习和习题，旨在帮助读者深入理解和掌握Java编程语言。" 在深入学习Java编程之前，首先需要理解计算机系统的基本构成。硬件和软件是计算机系统的核心组成部分，它们相互协作以完成复杂的任务。硬件包括各种组件，如中央处理器（CPU）、内存（RAM）、硬盘、输入输出设备等，它们各自有特定的功能。CPU执行指令，内存存储临时数据，硬盘用于长期数据存储，而输入输出设备则负责用户与计算机之间的交互。软件方面，主要分为系统软件和应用软件。系统软件如操作系统，管理硬件资源，提供服务给应用软件运行。应用软件则直接服务于用户，例如文本编辑器、游戏、图形处理软件等。此外，还有编程语言，如Java，它是一种面向对象的语言，被广泛应用于企业级应用开发、移动应用以及网页应用等。了解了硬件和软件的基本关系后，本书将探讨计算机网络如何彻底改变了计算机的使用方式。如今，计算机通过网络连接，可以共享信息，协同工作，这在网络编程和分布式系统中显得尤为重要。互联网和万维网（World Wide Web）的出现，使得全球信息共享成为可能，对软件开发产生了深远影响。接下来，本书会引入Java编程语言，讲解其特点和优势。Java以其“一次编写，到处运行”的特性，跨平台能力以及丰富的类库，深受开发者喜爱。书中会介绍编程的基本步骤，包括编写源代码、编译和执行程序的过程。此外，还会涉及图形学基础知识，以及如何在Java中表示和处理图形。这本书不仅是Java初学者的理想教材，也适合希望巩固计算机基础知识的程序员。通过学习，读者不仅可以掌握Java编程技能，还能了解到计算机硬件、软件、网络和互联网的相关知识，为成为一名全面的软件开发者奠定坚实的基础。

16 CHAPTER 1 computer systems

Tapes are usually used only to store information when it is no longer used fre-

quently, or to provide a backup copy of the information on a disk.

Two other terms are used to describe memory devices: random access memory

(RAM) and read-only memory (ROM). It’s important to understand these terms

because they are used often, and their names can be misleading. The terms RAM

and main memory are basically interchangeable. When contrasted with ROM,

however, the term RAM seems to imply something it shouldn’t. Both RAM and

ROM are direct (or random) access devices. RAM should probably be called

read-write memory, since data can be both written to it and read from it. This fea-

ture distinguishes it from ROM. After information is stored on ROM, it cannot

be altered (as the term “read-only” implies). ROM chips are often embedded into

the main circuit board of a computer and used to provide the preliminary instruc-

tions needed when the computer is initially turned on.

A CD-ROM is a portable secondary memory device. CD stands for compact

disc. It is accurately called ROM because information is stored permanently when

the CD is created and cannot be changed. Like its musical CD coun-

terpart, a CD-ROM stores information in binary format. When the CD

is initially created, a microscopic pit is pressed into the disc to repre-

sent a binary 1, and the disc is left smooth to represent a binary 0. The

bits are read by shining a low-intensity laser beam onto the spinning

disc. The laser beam reflects strongly from a smooth area on the disc

figure 1.12 A hard disk drive with multiple disks and read/write heads

Disks

Read/write

head

The surface of a CD has both

smooth areas and small pits. A

pit represents a binary 1 and a

smooth area represents a

binary 0.

key

concept

1.1 hardware components 17

but weakly from a pitted area. A sensor receiving the reflection determines

whether each bit is a 1 or a 0 accordingly. A typical CD-ROM’s storage capacity

is approximately 650 MB.

Variations on basic CD technology have emerged quickly. It is now common

for a home computer to be equipped with a CD-Recordable (CD-R) drive. A

CD-R can be used to create a CD for music or for general computer storage. Once

created, you can use a CD-R disc in a standard CD player, but you can’t change

the information on a CD-R disc once it has been “burned.” Music CDs that you

buy in a store are pressed from a mold, whereas CD-Rs are burned with a laser.

A CD-Rewritable (CD-RW) disc can be erased and reused. They can

be reused because the pits and flat surfaces of a normal CD are simu-

lated on a CD-RW by coating the surface of the disc with a material

that, when heated to one temperature becomes amorphous (and there-

fore non-reflective) and when heated to a different temperature

becomes crystalline (and therefore reflective). The CD-RW media

doesn’t work in all players, but CD-Rewritable drives can create both

CD-R and CD-RW discs.

CDs were initially a popular format for music; they later evolved to be used as

a general computer storage device. Similarly, the DVD format was originally cre-

ated for video and is now making headway as a general format for computer

data. DVD once stood for digital video disc or digital versatile disc, but now the

acronym generally stands on its own. A DVD has a tighter format (more bits per

square inch) than a CD and can therefore store much more information. It is

likely that DVD-ROMs eventually will replace CD-ROMs completely because

there is a compatible migration path, meaning that a DVD drive can read a CD-

ROM. There are currently six different formats for recordable DVDs; some of

these are essentially in competition with each other. The market will decide which

formats will dominate.

The speed of a CD drive is expressed in multiples of x, which represents a data

transfer speed of 153,600 bytes of data per second. The CD-RW drive described

in Fig. 1.8 is characterized as having 24x/10x/40x maximum speed, which means

it can write data onto CD-R discs at 24x, it can write data onto CD-RW discs at

10x, and it reads data from a disc at 40x.

The capacity of storage devices changes continually as technology improves. A

general rule in the computer industry suggests that storage capacity approx-

imately doubles every 18 months. However, this progress eventually will slow

down as capacities approach absolute physical limits.

A rewritable CD simulates the

pits and smooth areas of a

regular CD using a coating

that can be made amorphous

or crystalline as needed.

key

concept

program counter is incremented at this point to prepare for the next

cycle. Then the instruction is decoded electronically to determine

which operation to carry out. Finally, the control unit activates the cor-

rect circuitry to carry out the instruction, which may load a data value

into a register or add two values together, for example.

The CPU is constructed on a chip called a microprocessor, a device that is part

of the main circuit board of the computer. This board also contains ROM chips

and communication sockets to which device controllers, such as the controller

that manages the video display, can be connected.

Another crucial component of the main circuit board is the system clock. The

clock generates an electronic pulse at regular intervals, which synchronizes the

events of the CPU. The rate at which the pulses occur is called the clock speed,

and it varies depending on the processor. The computer described in Fig. 1.8

includes a Pentium 4 processor that runs at a clock speed of 950 megahertz

(MHz), or approximately 950 million pulses per second. The speed of

the system clock provides a rough measure of how fast the CPU exe-

cutes instructions. Similar to storage capacities, the speed of processors

is constantly increasing with advances in technology, approximately

doubling every 18 months.

1.2 networks

A single computer can accomplish a great deal, but connecting several computers

together into networks can dramatically increase productivity and facilitate the

sharing of information. A network is two or more computers connected together

so they can exchange information. Using networks has become the normal mode

1.2 networks 19

figure 1.14

The continuous fetch-decode-execute cycle

Fetch an instruction

from main memory

Execute the instruction

Decode the instruction

and increment program

counter

The von Neumann architecture

and the fetch-decode-execute

cycle form the foundation of

computer processing.

key

concept

The speed of the system clock

indicates how fast the CPU

executes instructions.

key

concept

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