Programming a Computer: Java

Transcription

1 Programming a Computer: Java The Java Programming Language A high-level language Java Compiler translates Java program to Java bytecodes Java Virtual Machine (JVM) translates Java bytecodes to machine language for execution on a computer 1

2 Compiling A compiler is a large, complex program, and it is very expensive to produce. Yet for most programming languages, every operating system must have its own compiler. When a C or FORTRAN program is compiled under Windows, it will not run on Linux or MacOS. It must be compiled again (by a different compiler) on each new environment where one wants to run it. 2

3 Compiling The high level version of the program is more or less the same on all platforms The machine language program is specific to each platform. Example: two hypothetical platforms (CPU and OS combinations) sum=16+23; CPU1/OS1: CPU2/OS2:

4 Java is Different The advantage of the Java approach is that it makes compiled Java programs portable. A Java program compiled under Windows can run on Linux or MacOS without the need to recompile it. The JVM interprets each line and passes it down to the machine to run it. The extra step makes Java programs slow. 4

5 The Java Virtual Machine (JVM) The Java virtual machine is a hypothetical machine which has the features that are common to all machines. The JVM is the target for the Java byte code. Thus, translating from byte code to machine language is relatively easy. Each platform has its own Java byte code interpreter, just like each platform has to have its own C compiler. However the interpreter is a much simpler program and can be written quickly. 5

6 Performance vs. Portability Java programs are slow because they are interpreted line by line as they are being run. Thus, Java is not suitable for applications where speed is of the essence. It is a small price to pay for portability, which allows Java programs to be sent over the Web and run automatically on diverse remote machines. 6

7 Java Pros and Cons Advantage of Java Virtual Machine approach Can run compiled Java program on any machine with a Java Virtual Machine installed One compiler will work on all machines. Makes compiled Java programs very portable Disadvantage Takes more time -- Java programs run slower than programs compiled to machine code BUT... Today s machines are so fast that the difference is not noticeable for many programs 7

8 Popular Programming Languages C, C++ Used by engineers and scientists for high performance applications. Perl, Python Used for rapid application development. PHP Used for Web-based applications. C#, Visual Basic (VB.Net) Used for Windows applications. Java General purpose language used for many different applications. 8

9 Scripting Languages A scripting language is a type of programming language which is interpreted, not compiled. That is, each line is converted to machine language and run before the next line is handled. Scripts are relatively slow. Scripting languages tend to be specialpurpose, e.g JavaScript and PHP for the Web, VBScript for Windows applications, ActionScript for Flash Animation. 9

10 Types of Programming Languages 1/3 Hundreds of programming languages No best programming language for all uses Some are better than others for certain uses Four major groups Imperative Declarative Functional Object-oriented 10

11 Types of Programming Languages 2/3 Imperative Languages Programs state explicitly how problem is to be solved. Programs are broken down into named modules of sequential code. e.g., C, Pascal, FORTRAN, COBOL Declarative Languages Programs state what is to be solved engine seeks out solution! Programs are stated as rules. e.g., Prolog, XSL 11

12 Types of Programming Languages 3/3 Functional Languages Programs state how problem is solved by applying and composing functions. Programs are stated as functions. e.g., LISP, Scheme, Mathematica Object-Oriented Languages Special case of imperative languages. Real world is modelled as objects, i.e. data+actions that can be performed on data. e.g., Java, C++, Smalltalk 12

13 Programming Languages: Imperative C Popular with programmers who write system-level code (operating systems, compilers,...) Example /* This C program finds the sum of 16 and 23, then displays the result. */ #include <stdio.h> void main(void) { int sum; sum = ; printf("the sum of 16 and 23 is %d\n", sum); } 13

14 Programming Languages: Imperative Pascal (named after Blaise Pascal) Used extensively to teach programming until the early 1990 s Example PROGRAM AddNums (Input, Output); { This Pascal program finds the sum of 16 and 23, then displays the result. } VAR Sum : Integer; BEGIN Sum := ; writeln ('The sum of 16 and 23 is ', Sum) END. 14

15 Languages: Declarative Prolog Based on formal rules of logic and inference Used widely in artificial intelligence (especially in Europe and Japan) Example /* This Prolog program finds the sum of 16 and 23, then displays the result. */ run :- Sum is , write ("The sum of 16 and 23 is ", Sum), nl. 15

16 Programming Languages: Functional Lisp Most popular functional language Used widely in artificial intelligence (especially in the US) Example ; This Lisp program finds the ; sum of 23 and 16, then displays the result. (progn ) (let (Sum) ) (setq Sum ( )) (princ "The sum of 23 and 16 is ") (princ Sum) 16

17 Languages: Object-oriented C++ Introduced in the mid 1980 s Based on the C language Hybrid language: imperative & object-oriented Example: // This C++ program finds the sum of 16 and 23, // then displays the result. #include <iostream.h> void main(void) { int sum; sum = ; cout << "The sum of 16 and 23 is " << sum << endl; } 17

18 Languages: Object-oriented Java Introduced in 1995 by Sun Microsystems Released version designed for use on the Internet and as a general-purpose programming language Example: // This Java program finds the sum of 16 and 23, // then displays the result. class Sum16And23 { } public static void main(string args[]) { int sum; sum = ; System.out.println("The sum of 16 and 23 is " + sum); } 18

19 Internet The Internet is an international network of interconnected computer networks Consists of millions of networks It carries information and services, e.g. Electronic mail ( ) World Wide Web File Transfer Vast information and communication resource used by Academic institutions Governments Companies Private individuals 19

20 Internet: World Wide Web The World Wide Web is a system of interlinked, hypertext documents accessible over the Internet A browser is used to access the World Wide Web, Mozilla Firefox Internet Explorer A web address, or URL, is needed to access a particular document or website, e.g. Because information on the web is so vast, search engines are used to find information, e.g

21 Internet: (electronic mail) is the most popular form of communication on the internet It allows an individual to send a message to one or more people Each person needs an address to send and receive addresses are in the standard notation, using (at) sign e.g. MNDNEL001@uct.ac.za or gstewart@cs.uct.ac.za 21