物件導向程式設計

 

 

第一章、Java語言概論

 

 

授課教師：陳慶瀚

WWW : http://www.miat.ee.isu.edu.tw/java

E-mail : pierre@isu.edu.tw  　

 

1.1 什麼是Java?

  • Java是一個程式設計語言(programming language)

  • Java是一個虛擬機器(virtual machine)

  • Java是一個作業系統(operating system)

  • Java是一個應用平台(application platform)

  • Java也是一種咖啡......

Java提供完整的系統開發平台，包含硬體、軟體、晶片、內嵌式系統、可攜式裝置、虛擬機器、編譯器、軟體開發工具，並支援所有作業系統。

　

Java不是什麼呢？Java不是一個應用軟體(software)，Java不是一種通信協定(protocol)，Java不是一種電腦架構(architecture)，Java也不是一家公司......

　

1.2 Java的特色

1.2.1 Java是簡單的

Java從C/C++語言演化而來，它簡化了C/C++語言中複雜、容易出錯的語法，並且增加了許多先進的程式語言特徵，例如動態記憶體配置採用garbage collection技術，大幅縮小程式設計的困難度。

　

1.2.2 Java是物件導向的

Object oriented programming is the catch phrase of computer programming in the 1990's. Although object oriented programming has been around in one form or another since the Simula language was invented in the 1960's, it's really begun to take hold in modern GUI environments like Windows, Motif and the Mac. In object-oriented programs data is represented by objects. Objects have two sections, fields (instance variables) and methods. Fields tell you what an object is. Methods tell you what an object does. These fields and methods are closely tied to the object's real world characteristics and behavior. When a program is run messages are passed back and forth between objects. When an object receives a message it responds accordingly as defined by its methods.

Object oriented programming is alleged to have a number of advantages including:

  • Simpler, easier to read programs

  • More efficient reuse of code

  • Faster time to market

  • More robust, error-free code

In practice object-oriented programs have been just as slow, expensive and buggy as traditional non-object-oriented programs. In large part this is because the most popular object-oriented language is C++. C++ is a complex, difficult language that shares all the obfuscation of C while sharing none of C's efficiencies. It is possible in practice to write clean, easy-to-read Java code. In C++ this is almost unheard of outside of programming textbooks.

　

1.2.3 Java與作業平台無關

　

Java was designed to not only be cross-platform in source form like C, but also in compiled binary form. Since this is frankly impossible across processor architectures Java is compiled to an intermediate form called byte-code. A Java program never really executes natively on the host machine. Rather a special native program called the Java interpreter reads the byte code and executes the corresponding native machine instructions. Thus to port Java programs to a new platform all that is needed is to port the interpreter and some of the library routines. Even the compiler is written in Java. The byte codes are precisely defined, and remain the same on all platforms.

The second important part of making Java cross-platform is the elimination of undefined or architecture dependent constructs. Integers are always four bytes long, and floating point variables follow the IEEE 754 standard for computer arithmetic exactly. You don't have to worry that the meaning of an integer is going to change if you move from a Pentium to a PowerPC. In Java everything is guaranteed.

However the virtual machine itself and some parts of the class library must be written in native code. These are not always as easy or as quick to port as pure Java programs. This is why for example, there's not yet a version of Java 1.2 for the Mac.

　

1.2.4 Java是安全的

　

Java was designed from the ground up to allow for secure execution of code across a network, even when the source of that code was untrusted and possibly malicious.

This required the elimination of many features of C and C++. Most notably there are no pointers in Java. Java programs cannot access arbitrary addresses in memory. All memory access is handled behind the scenes by the (presumably) trusted runtime environment. Furthermore Java has strong typing. Variables must be declared, and variables do not change types when you aren't looking. Casts are strictly limited to casts between types that make sense. Thus you can cast an int to a long or a byte to a short but not a long to a boolean or an int to a String.

Java implements a robust exception handling mechanism to deal with both expected and unexpected errors. The worst that an applet can do to a host system is bring down the runtime environment. It cannot bring down the entire system.

Most importantly Java applets can be executed in an environment that prohibits them from introducing viruses, deleting or modifying files, or otherwise destroying data and crashing the host computer. A Java enabled web browser checks the byte codes of an applet to verify that it doesn't do anything nasty before it will run the applet.

However the biggest security problem is not hackers. It's not viruses. It's not even insiders erasing their hard drives and quitting your company to go to work for your competitors. No, the biggest security issue in computing today is bugs. Regular, ordinary, non-malicious unintended bugs are responsible for more data loss and lost productivity than all other factors combined. Java, by making it easier to write bug-free code, substantially improves the security of all kinds of programs.

　

1.2.4 Java是高性能的

　

Java byte codes can be compiled on the fly to code that rivals C++ in speed using a "just-in-time compiler." Several companies are also working on native-machine-architecture compilers for Java. These will produce executable code that does not require a separate interpreter, and that is indistinguishable in speed from C++.

While you'll never get that last ounce of speed out of a Java program that you might be able to wring from C or Fortran, the results will be suitable for all but the most demanding applications.

As of May, 1999, the fastest VM, IBM's Java 1.1 VM for Windows, is very close to C++ on CPU-intensive operations that don't involve a lot of disk I/O or GUI work; C++ is itself only a few percent slower than C or Fortran on CPU intensive operations.

It is certainly possible to write large programs in Java. The HotJava browser, the Java Workshop integrated development environment and the javac compiler are large programs that are written entirely in Java.

　

1.2.5 Java是多執行序的(Multi-Threaded)

　

Java is inherently multi-threaded. A single Java program can have many different threads executing independently and continuously. Three Java applets on the same page can run together with each getting equal time from the CPU with very little extra effort on the part of the programmer.

This makes Java very responsive to user input. It also helps to contribute to Java's robustness and provides a mechanism whereby the Java environment can ensure that a malicious applet doesn't steal all of the host's CPU cycles.

Unfortunately multithreading is so tightly integrated with Java, that it makes Java rather difficult to port to architectures like Windows 3.1 or the PowerMac that don't natively support preemptive multi-threading.

There is a cost associated with multi-threading. Multi-threading is to Java what pointer arithmetic is to C, that is, a source of devilishly hard to find bugs. Nonetheless, in simple programs it's possible to leave multi-threading alone and normally be OK.

　

1.3 開始寫Java程式

1.4 實作練習

  • TD一：在網頁上顯示0~35

  • TD二：在console mode下，設計一個計算F=ma的程式(使用者輸入m,a,程式輸出F)

　

　

物件導向程式設計

義守大學電機系 陳慶瀚

2001.10.02