Java编程入门指南

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Chapter 1

Overview: The Menta l Lands ca pe

When you begin a journey, it’s a good idea to have a mental map of the terrain you’ll be

passing through. The same is true for an intellectual journey, such as learning to write computer

programs. In this case, you’ll need to know the basics of what comp ute r s are and how they

work. You’ll want to have some idea of what a computer program is and how one is created.

Since you will be writing programs in the Java programming language, you’ll want to know

something about that language in particul ar and ab out the modern computing environment for

which Java is designed.

As you read this chapter, don’t worry if you can’t understand everything in detail. (In fact,

it would be impossible for you to learn all the details from the brief expositions in this chapter.)

Concentrate on learning enough about the big ideas to orie nt yourself, in preparation for the

rest of the book. Most of what is covered in this chapter will be covered in much greater detail

later in the book.

1.1 The Fetch and Execute Cycle: Machine Language

A computer is a complex system consi st i ng of many diﬀerent components. But at the

heart—or the brain, if you want—of the compute r is a single component that does the actual

computing. This is the Central Processing Unit, or CPU. In a modern desktop computer,

the CPU is a single “chip” on the order of one square inch in size. The job of the CPU is to

execute progr ams.

A program is simply a list of unambi guous instructions meant to b e followed mechanically

by a computer. A computer is built to carry out instructions t hat are wr i t te n in a very simple

type of language called machine language. Each type of computer has its own machine

language, and the compute r can directly execute a program onl y if the program is expressed i n

that language. (It can execute programs written in other languages if they are ﬁrst translated

into machine language.)

When the CPU executes a program, that program is stored in the computer’s main mem-

ory (also called the RAM or random access memory). In addition to the progr am, memory

can also hold data that is being used or pr oc e sse d by the program. Main memory consists of a

sequence of locations. These l ocations are numbered, and the sequence number of a locat i on

is called its address. An address pr ovides a way of picking out one particular piece of informa-

tion from among the millions stored in memory. When the CPU needs to ac ce ss the program

instruction or data in a particular location, it sends the address of that information as a sig-

nal to the memory; the memory responds by sending back the data contained in t he speciﬁed

CHAPTER 1. THE MENTAL LANDSCAPE 2

location. The CPU can also store information i n memory by specifying the information to be

stored and the address of the location where it is to be stored.

On the level of machine language, the operation of the CPU is fairly straightforward (al-

though it is very complicated in detail). The CPU executes a program that is stored as a

sequence of machi ne language instructions in main m emor y. It does this by repeatedly reading,

or fetching, an instruction from memory and then carrying out, or execu ting , that instruc-

tion. This process—fetch an instruction, execute it, fetch another instruction, execute it, and so

on forever—is called the fetch-and-execute cycle. With one exception, which will be covered

in the next section, this is all that the CPU ever does.

The details of the fetch-and-execute cycle are not terribly important, but there are a few

basic things you should know. The CPU contains a few internal registers, which are small

memory units capable of holding a single number or machine language instru ct i on. The CPU

uses one of these registers—the program counter, or PC—to keep track of where i t is in the

program it is e x ec ut i ng. The PC simply stores the memory address of the next instruction that

the CPU should execute. At the beginning of each fetch-and-execute cycle, the CPU checks the

PC to see which instructi on it should fetch. During the course of the fetch-and-execute cycle,

the number in the PC is updated to indicate the instruction that is to be e x e cu te d in the next

cycle. (Usually, but not always, this is just the instruction that sequentially follows the current

instruction in the program.)

∗ ∗ ∗

A computer executes machine language programs mechanically—that is without under-

standing them or thinking about them—simply because of the way it is physical l y put together.

This is not an easy concept. A computer is a machin e built of millions of tiny switches calle d

transistors, which have the property that they can be wired together in such a way that an

output from one switch can turn another switch on or oﬀ. As a computer comp ute s, these

switches turn e ach other on or oﬀ in a pattern determined both by the way they are wired

together and by the program that the computer is executing.

Machine language instructions are expressed as binary numbers. A binary number is made

up of just two possible digits, ze r o and one. Each zero or one is called a bit. So, a machine

language instruction is just a sequence of zeros and ones. Each particular se q uen ce encodes

some particular inst r uc ti on. The data that the computer manipulates is also encoded as binary

numbers. In modern computers, each memory location holds a byte, which is a sequence of

eight bits. (A machine language instruction or a piece of data generally consists of several byt e s,

stored in consecutive memory locations.)

A compute r can work directly with binary numbers because switches can readily r e pr e se nt

such numbers: Turn the switch on to represent a one; turn it oﬀ to repres ent a zero. Machine

language instructions are store d in memory as patterns of switches turne d on or oﬀ. When a

machine language instruction is loaded into the CPU, all that happens is that certain switches

are turned on or oﬀ in the pattern that encodes that instruction. The CPU is built to respond

to this pattern by executing the instruction it encode s; it does this simply because of the way

all the other switches in the CPU are wired toget he r .

So, you should understand this much about how computers work: Main memory holds

machine language programs and data. These are encoded as binary numbers. The CPU fetches

machine language instructions from memory one after another and execute s the m. It does

this mechanically, without thinking about or understanding what it does—and therefore the

program it executes must be perfect, complete in all details, and unambiguous because th e CPU

can do nothing but execute it exactly as written. Here is a schematic view of this ﬁrst-stage

CHAPTER 1. THE MENTAL LANDSCAPE 3

understanding of the computer:

10001010 (Location 0)

00110100 (Location 1)

01110111 (Location 2)

10100100 (Location 3)

11010010 (Location 4)

10000110 (Location 5)

01001111 (Location 6)

10100000 (Location 7)

00000010 (Location 8)

10100010 (Location 9)

00010100 (Location 10)

Data to Memory

Data from Memory

Address for

reading/writing

data

Program

counter:

0010110111001000

CPU

Memory

1.2 Asynchronous Events: Polling Loops and Interrupts

The CPU spends almost all of its time fetching instructions from memory and executi ng

them. However, the CPU and main memory are only two out of many components in a real

computer sy st em. A complete system contains other devices such as :

• A hard disk or solid state drive for storing programs and data ﬁles. (Note that main

memory holds only a comparatively small amount of inform ati on, and holds it only as

long as the power is turned on. A hard disk or solid state drive is use d for permanent

storage of larger amounts of information, but programs have to be loaded from there into

main memory before they can actually be executed. A hard disk stor es data on a spinning

magnetic di sk , wh i l e a solid state drive is a purely electronic device with no moving parts.)

• A keyboard and mouse for user input.

• A monitor and printer which can be used to d i spl ay the computer’s output.

• An audio output device that allows the compute r to play sounds.

• A network interface that allows the computer to communicate with other computers

that are connected to it on a network, either wirelessly or by wire.

• A scanner that converts images into coded binary numbe r s that can be stored and

manipulated on the computer.

The list of devices is entirely open ended, and computer systems are built so that they can

easily be expanded by adding new devices. Somehow the CPU has to communicate wit h and

control all these devices. The CPU c an only do this by ex e cu ti ng machine language inst ru ct i ons

(which is all it can do, period). The way thi s works is that for each d ev i c e in a system, there

is a device driver, which consists of software that the CPU executes when it has to deal

with the device . Installing a new device on a system generally has two steps: plugging the

device physically into the computer, an d installing the device driver software. Without the

device driver, the actual physical device would be useless, since the CPU would not be able to

communicate wi t h it.

CHAPTER 1. THE MENTAL LANDSCAPE 4

∗ ∗ ∗

A computer system consisting of many devices is typically organized by connecting those

devices to one or more busses. A bus is a set of wire s that carry various sorts of information

between the devices connected to those wires. The wires carry data, addresses, and control

signals. An addre ss directs the data to a particular device and perhaps to a part i cu l ar register

or location within that device. Control signals can be used, for example, by one device to alert

another that data is available for it on the data bus. A fairly simple computer syste m might

be organized like this:

CPU

Input/

Output

Controller

Data

Address

Control

Empty Slot

for future

Expansion

Memory

Disk Drive

Display

Keyboard

Network

Interface

Now, devices such as keyboard, mouse, and network interface can produce input that needs

to be pro c es se d by the CPU. How does the CPU know that the data is there? One simple idea,

which turns out to be not very satisfactory, is for the CPU to keep checking for incoming data

over and over. Whe ne ver i t ﬁnds data, it processes it. This method is called polling, since

the CPU polls the input devices continually to see whether they have any input data to report.

Unfortunately, although polling is very simple, it is also very ineﬃcient. The CPU can waste

an awful lot of ti me just waiting for in put .

To avoid this ineﬃciency, inter r upts are generally used instead of polling. An interrupt

is a signal sent by another device to the CPU. The CPU re sponds to an interrupt signal by

putting aside whatever it is doing in order to respond to the interrupt. Once it has handled

the interrupt, it returns to what it was doing befor e the interrupt occurred. For example, when

you press a key on your computer keyboard, a keyboard interrupt is sent to the CPU. The

CPU responds to this signal by interrupting what it is doing, reading the key that you pressed,

processing i t, and then returning to the task it was perfor mi n g before you pressed the key.

Again, you should understand that this is a purely mechanical process: A device signals an

interrupt simply by turning on a wi r e . The CPU is built so that when that wire is turned on,

the CPU saves enough information about what it is currently doing so that it can r et ur n to

the same state later. This information consists of the contents of important internal registers

such as the program counter. Then the CPU jumps to some predetermine d memory location

and begins ex e cu ti ng the instructions stor ed there. Those instructions make up an interrupt

handler that does the processing necessary to respond to the interrupt. (This interrupt handler

is part of the device driver software for the device that signaled the interrupt.) At the end of

the interrupt handler is an instruction that tells the CPU to jump back to what it was doing;

it does that by restoring its previously saved state.

CHAPTER 1. THE MENTAL LANDSCAPE 5

Interrupts allow the CPU to deal with asynchronous events. In the regular fetch-and-

execute cycle, things happen in a predetermined order; everything that happens is “synchro-

nized” with everything else. Interrupts make it possible for the CPU to deal eﬃciently with

events that happen “asynchronously,” that is, at unpredictable times.

As another ex ampl e of how interr u pts are used, consider what happens when the CPU needs

to access data that is stored on a hard disk. The CPU can access data directly only if it is

in main memory. Data on the disk has to be copied i nto memory before it can be accessed.

Unfortunately, on the scale of speed at which the CPU operates, the disk drive is extremely

slow. When the CPU needs data from the disk, it sends a signal to the disk d ri ve telling it

to locate the data and get it ready. (This signal is sent synchronously, under the control of

a regular program.) Then, instead of just waitin g the long and unpredictable amount of time

that the disk drive will take to do this, the CPU goes on with some other task. When the disk

drive has the data ready, it sends an interrupt signal to the CPU. The interrupt handler can

then read the requested data.

∗ ∗ ∗

Now, you might have noticed th at all this only makes sense if the CPU actually has several

tasks to perform. If it has nothing better to do, it might as well spend its time polling for input

or waiting for disk drive operations to complete. All modern computers use multitasking to

perform several tasks at once. Some computers can be used by several people at once. Since the

CPU is so fast, it can quickly switch its attention from one user to another, devoting a fraction

of a second to e ach user in turn . This application of multitasking i s called timesharing . But a

modern personal computer with just a single user also uses multitasking. For example, the user

might be typing a paper while a clock is continuously displaying the time and a ﬁle is being

downloaded ove r the network.

Each of t he individual tasks that the CP U is working on is called a thread. (Or a process;

there ar e technical diﬀerences between t hr ead s and processes, but they are not important here,

since it is thr e ads that are used in Java.) Many CPUs can literally execute more than one

thread simul t ane ousl y —s uch CPUs contain multiple “cores,” each of which can run a thread—

but there is always a limit on the number of threads that can be executed at the same time.

Since there are often more threads than can be executed simultaneously, the computer has to be

able switch its attention from one thread to anot he r, just as a timesharing computer switches

its attention from one user to another. In general, a thread that is being executed will continue

to run until one of several things happens:

• The thread might voluntarily yield control, to give other threads a chance to run.

• The thread might have to wait for some asynchronous event to occur. For example, the

thread might request some data from the disk drive, or it might wait for the user to press

a key. W hi l e i t is waiting, the thread is said to be blocked, and other threads, if any, have

a chance to run. When the event occurs, an interrupt will “wake up” the thread so that

it can continue running.

• The thread might use up its allotted slice of time and be suspended to allow other threads

to run. Not all computers can “forcibly” susp en d a thread in this way; those that can are

said to us e preemptive multitasking. To do pree mpt i ve multitasking, a computer needs

a special timer device that generates an interrupt at regular intervals, such as 100 times

per second. When a timer interrupt occurs, the CPU has a chance to switch from one

thread to another, whethe r the thread that is currently running likes it or not. All modern

desktop and laptop computers, and even typical smartphones and tablets, use preemptive

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