C语言与Java对比：关键特性和选择理由

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本资源主要聚焦于将C语言与Java这两种编程语言进行比较和介绍，特别关注于C语言对Java程序员可能有帮助的知识点。首先，"AHistoryLesson"部分可能会回顾C语言的历史，强调其在计算机科学发展中的地位，为理解两者的差异提供背景。"HighLevelDifferences"章节详细比较了C语言与Java在代码编译方式（编译 vs. 解释执行）、安全性和健壮性、多线程支持以及API设计的不同。 C++语言被提及，因为它是C语言的扩展，"TenReasonsToUseC++"列出了选择C++而非C的原因。包括C++的广泛使用、模板技术、操作符重载、标准模板库（STL）、资源自动回收、条件编译控制、访问器与修改器的区别、多重继承的实现方式、空间效率优化，以及私有继承的特点。这些特性展示了C++在性能、灵活性和面向对象编程上的优势。 "Chapter1BasicTypesandControlStructures"则涵盖了基础类型（如整数、浮点数、字符和布尔类型）的介绍，以及C语言和Java在语法结构上的不同，比如运算符和表达式、条件语句、循环结构以及C语言的定义明确赋值规则。此外，还介绍了额外的主题，如预处理器和`using`指令，以及如何在初学阶段编写简单的程序并输出结果。通过深入解析这些章节，Java程序员可以更好地理解C语言的特性和用法，以便在需要时能够利用C语言的优势或者在两者之间做出明智的选择。同时，对于C语言开发者来说，了解Java的特性也有助于他们了解现代软件开发中的多样性，并根据项目需求灵活运用。

A History Lesson 3

In parallel with the standardization of C was an effort by Bjarne Stroustrup to create a

more object-oriented approach to using C. Eventually C++ emerged, and then eventually C++

underwent a standardization process.

C++ should be viewed as a completely new language, based largely on C. It includes mod-

ern constructs to support object-oriented programming. Several design issues have greatly influ-

enced C++.

First and foremost, C++ was intended to be upward compatible with C, as much as possi-

ble. Thus legal C programs should be legal C++ programs, though this is not 100% true. (Obvi-

ous exceptions include C programs that use as identifiers any reserved word that was added to

C++, such as class.) As a result, numerous constructs from C that perhaps we could live with-

out are part of C++, as is the entire idea of using pointers to do arrays. Additionally, because

each new reserved word breaks existing code, the C++ community is reluctant to add new

reserved words, preferring instead to overuse existing words, such as const and static. So

whereas Java has throw and throws, in C++ we simply use throw both to throw an excep-

tion and to list exceptions.

The second crucial philosophy of C++ was in part political: C++ programs should be as

fast as C programs if they do the same thing. Otherwise, it was thought, nobody would want to

learn a more complicated language, just to write slower code. As part of this philosophy, in C++,

if you do not use a feature, you should not have to pay a cost a runtime. Among other things, this

still means, no runtime bounds checks, no dynamic dispatch by default, and no garbage collec-

tion.

The third thing to know about C++ is that it has continually evolved. The first widely-used

version of C++ supported classes and inheritance. Templates, discussed in Chapter 7, were

added later. Exceptions were added even later. At another time came namespaces. The templates

got fancier, with templates inside of templates. This has lead to compilers being perennially

behind the official specification, buggy with new features, and especially bug-prone when sev-

eral new features are used at the same time, much to the chagrin of programmers and textbook

authors. Fortunately, C++ has more or less stabilized and the latest compilers are quite good at

meeting (and in some cases doing more than required by) the language specification.

Java was developed by Sun Microsystems, and marketed as a modern alternative to C++.

Thus, cosmetically, there are many similarities. However, the designers of Java had (at least)

two main goals. First, they attempted to allow the programmer to write the same set of programs

that C++ allows, using simpler syntax. In cases where C++ has several ways to do the same

thing, Java attempts to pick the idiom most widely endorsed by the C++ community and support

it with a language feature. In cases where C++ features are abused, Java does not allow the fea-

ture. Thus some C++ features are not found in Java. Second, they have attempted to make it

harder for incorrect code to run, requiring the compiler to detect many sets of errors when it first

compiles the program, and then the Virtual Machine to throw an exception when bad things hap-

pen at runtime. By detecting most occurrences of bad behavior, not only are honest program-

c++book.mif Page 3 Saturday, July 12, 2003 10:53 AM

4 Chapter 0 • Introduction

ming errors avoided, system security against hacking is enhanced, since hacking generally

works by having the system do something it is not designed to do.

The Java designers have several advantages. First, in designing the language, they can

expect the compiler to work hard. We no longer use PDP-11s, and compiler research has

advanced greatly to the point that modern optimizing compilers can do a terrific job of creating

code, without requiring the programmer to resort to various tricks common in the good old days.

Second, Java designers specified most of the language (classes, inheritance, exceptions) at once,

adding only a second minor revision (inner classes), with most changes after Java 1.1 dealing

with the library. In doing so, they were able to have language features that don’t clash.

Although Java is the new kid on the block (unless C# becomes the next new kid), and has

lots of nice features, it is certainly not true that Java is better than C++, Nor would we say that

C++ is better than Java. Instead, a modern programmer should be able to use both languages, as

each language has applications that can make it the logical choice.

0.2 High Level Differences

As we just described, Java and C++ have different views of the world. C++’s main priority is

getting correct programs to run as fast as it can; incorrect programs are on their own. Java’s main

priority is not allowing incorrect programs to run; hopefully correct programs run reasonably

fast, and the language makes it easier to generate correct programs by restricting some bad pro-

gramming constructs.

0.2.1 Compiled vs. Interpreted Code

Java consists of both a compiler and a Virtual Machine. The compiler generates Java bytecode,

and the bytecode is then interpreted at runtime. As a result, not only is Java code portable, the

compiled bytecode is portable and in theory can be run on any platform.

On the other hand, a C++ compiler generates native code. The C++ language specification

provides no guidance on the specifics of what the native code looks like, so not only is the result

of the compilation not transferable to a different type of computer, parts of a single program that

are compiled on the same computer by different compilers are almost always not compatible

with each other.

Originally, the difference between compiled code and interpreted code meant that C++

was as much as 50 times faster than Java. Recent improvements in the Java compiler, and more

importantly, the Virtual Machine have dramatically closed the gap. In some cases, Java code

may actually be faster than C++, but generally speaking, one would expect that since C++ does

less runtime checks than Java, equally skilled compiler writers should be able to generate some-

what faster C++ code than Java code, assuming similar coding styles.

0.2.2 Security and Robustness

We have already seen that Java is very concerned with not allowing unsafe code to compile, and

with throwing exceptions at the first sign of trouble. C++ is unfortunately, somewhat lax in this

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High Level Differences 5

regard. C++ suffers several problems that can never occur in pure Java code. Four that stand out

are the following:

First, it is possible in C++ to have a pointer or reference to an object that has been returned

back to the memory heap, which is a sure disaster. This is because standard C++ does not do gar-

bage collection; instead the programmer must manage memory themselves, and programmers

are surprisingly bad at doing so. However, some C++ systems include garbage collection and

add runtime checks to avoid using stale pointers. These systems are quite close to the Java stan-

dard of avoiding memory problems.

Second, Standard C++ does not check array indexes, and a common hacker attack is to

find an input routine that reads a string, but doesn’t check that there is enough space for a very

very long string. By judiciously passing a huge string, the hacker can overflow the buffer, writ-

ing replacement values onto variables that are stored in memory adjacent to the buffer. Although

this could never happen in Java, since the C++ specification does not disallow bounds checks,

there is no reason that a safe C++ system couldn’t check array bounds. It would simply be

slower (but safer) than a competitor, and so it is not widely done by default.

Third, old C++ typecasts allow type confusion, in which a type is cast to an unrelated type.

This can never happen in Java, but is allowed in C++.

Fourth, in Java, all variables have a definite assigned value prior to use of their value. This

is because variables that are not local to a method by default are initialized to zero for primitives

and null for references. For local variables, an entire chapter of the Java Language Specifica-

tion is devoted to definite assignment, whereby the compiler is required to perform a flow analy-

sis and produce an error message if a local variable cannot be proven (under a long set of rules)

to have been definitely assigned to through all flows of the method. In C++, this behavior is not

required. Rather, a program that uses an uninitialized variable is said to be incorrect, but the

compiler is not required to take any particular action. Most C++ compilers will print warning

messages if uninitialized variables are detected. But the program will still compile. A similar

story occurs in the case of having a flow that fails to return a value in a non-void function.

0.2.3 Multithreading

C++ does not support multithreading as part of the language. Instead, one must use a set of

library routines that are native to the particular platform. Although Java supports multithreading,

the Java memory model and threading specification has recently been discovered to be inade-

quate and is undergoing revision.

0.2.4 API Differences

The Java API is huge. The core library includes, among other things, the Swing package

javax.swing for designing (mostly) portable GUIs, a networking package in java.net,

database connectivity in java.sql, built-in compression, serialization, and other I/O in

java.util.zip and java.io, reflection in java.lang.reflect, and even support for

remote methods, servlets, and XML.

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6 Chapter 0 • Introduction

Standard C++ has a very small API, containing little more than some I/O support, a com-

plex number package, and a Collections API, known as the Standard Template Library (STL).

Of course, compiler vendors augment Standard C++ with huge libraries, but each vendor has

different versions rather than implementing a single standard.

0.3 Ten Reasons To Use C++

The preceding section might leave one to conclude that C++ has little to offer. This is not true;

there are lots of reason why one would want to learn and use C++. In this section we list ten

advantages of using C++, including features found in C++ that are not found in Java. Figure 0-1

summarizes the differences described in both Section 0.2 and this section, as well as a few dif-

ferences that are described later in this text, but not otherwise mentioned in this chapter.

0.3.1 C++ Is Still Widely Used

Perhaps the most important reason to learn C++ is that it is still very-widely used, and since it is

a more complex and difficult language than C++, it is likely that there will always be a strong

market for knowledgeable C++ programmers.

0.3.2 Templates

Perhaps the most requested missing feature in Java is the equivalent of the C++ template. In

C++, templates allow the writing of generic, type-independent code, such as sorting algorithms

and generic data structures that work for any type. In Java, this is done by using inheritance, but

the downside is that many errors are not detected at compile time, but instead linger until run

time. Further, using templates in C++ seems to lead to faster code than the inheritance-based

alternative in Java. Generics are under consideration for Java 1.5. Template are discussed in

Chapter 7.

0.3.3 Operator Overloading

Java does not allow the user to defined operators for class types. C++ does, and this is known as

operator overloading. We discuss operator overloading in Chapter 5.

0.3.4 Standard Template Library

C++ provides a large library, known as the Standard Template Library (STL) for data structures

and generic algorithms. The STL has several advantages compared to the Java Collections API,

and we discuss these differences in Chapter 10.

0.3.5 Automatic Reclamation of Resources

Although Java provides garbage collection, which is a fantastic feature, it is hard to control the

management of other resources. For instance, in Java, the programmer must remember to close

files and database connections when they are no longer in use, dispose of graphics contexts, and

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