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JAVA
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WHAT IS JAVA?
Java is a programming language designed to deliver executable content over networks. A user or programmer should know what kinds of interaction Java can make possible and what its true potential can be: enlivening the Web, enriching the display of information in the form of animation and interactive applications.
Java enriches the interactivity possible on the Web. Rather than making just informational content possible, Java can support interactive content in the form of software that can be downloaded and run on any computer host with the Java interpretation environment installed.
Java developed from ideas about platform-independent executable code. Sun Microsystems researchers have developed Java to be a powerful programming and information delivery system for use with the Web.
Java makes animation, interaction, computation, distributed applications, and new forms of communication possible. Through protocol and content handlers, Java has the potential to make new formats and new protocols available for use on the Web.
Java transforms the Web into a software delivery system where users have things to do rather than just places to go. Java may change the surfing behavior of Web users into playing and learning behavior in new interactive environments.
Characteristics of Java as a Programming Language
While users may want to have some awareness of how Java fits into online communication, programmers need to understand more specific technical characteristics of Java. According to the information provided by Sun Microsystems, Java is a “…simple, object-oriented, distributed, interpreted, robust, secure, architecture neutral, portable, high-performance, multithreaded, and dynamic language.”This characterization identifies the key technical features of Java as shown in the following sections.
Simple
The developers of Java based it on the C++ programming language, but removed many of the language features that are rarely used or often used poorly. C++ is a language for object-oriented programming and offers very powerful features. However, as is the case with many languages designed to have power, some features often cause problems. Specifically, Java differs from C++ (and C) in these ways:
I. Java does not support the struct, union, and pointer data types.
II. Java does not support typedef or #define.III. Java differs in its handling of certain operators and does not permit operator
overloading.IV. Java does not support multiple inheritance.V. Java handles command-line arguments differently than C or C++.VI. Java has a String class as part of the java.lang package. This differs from the null-
terminated array of characters as used in C and C++.VII. Java has an automatic system for allocating and freeing memory (garbage
collection), so it is unnecessary to use memory allocation and de-allocation functions as in C and C++.
Object-Oriented
Like C++, Java can support an object-oriented approach to writing software. Ideally, object-oriented design can permit the creation of software components that can be reused.Object-oriented programming is based upon modeling the world in terms of software components called objects. An object consists of data and operations that can be performed on that data called methods. These methods can encapsulate, or protect, an object’s data because programmers can create objects in which the methods are the only way to change the state of the data.Another quality of object-orientation is inheritance. Objects can use characteristics of other objects without having to reproduce the functionality in those objects that supports those characteristics. Inheritance thus helps in software re-use, because programmers can create methods that do a specific job exactly once.Technically, Java’s object-oriented features are those of C++ with extensions from Objective C for dynamic method resolution.
Distributed
Unlike the languages C++ and C, Java is specifically designed to work within a networked environment. Java has a large library of classes for communicating using the Internet’s TCP/IP protocol suite, including protocols such as HTTP and FTP. Java code can manipulate resources via URLs as easily as programmers are used to accessing a local file system using C or C++.
Interpreted
When the Java compiler translates a Java class source file to bytecodes, this bytecode class file can be run on any machine that runs a Java interpreter or Java-enabled browser. This allows the Java code to be written independently of the users’ platforms.
Interpretation also eliminates the compile and run cycle for the client because the bytecodes are not specific to a given machine but interpreted.Robust
Robust software doesn’t “break” easily because of programming bugs or logic errors in it. A programming language that encourages robust software often places more restrictions on the programmer when he or she is writing the source code. These restrictions include those on data types and the use of pointers. The C programming language is notoriously lax in its checking of compatible data types during compilation and runtime. C++ was designed to be more strongly typed than C; however, C++ retains some of C’s approach toward typing. In Java, typing is more rigorous: a programmer cannot turn an arbitrary integer into a pointer by casting, for example. Also, Java does not support pointer arithmetic but has arrays instead. These simplifications eliminate some of the “tricks” that C programmers could use to access arbitrary areas of memory. In particular, Java does not allow the programmer to overwrite memory and corrupt other data through pointers. In contrast, a C programmer often can accidentally (or deliberately) overwrite or corrupt data.
Secure
Because Java works in networked environments, the issue of security is one that should be of concern to developers. Plans are in the works for Java to use public-key encryption techniques to authenticate data. In its present form, Java puts limits on pointers so that developers cannot forge access to memory where not permitted. These aspects of Java enable a more secure software environment. The last section of this chapter outlines the layers of Java’s security in more detail.
Architecture Neutral
The Java compiler creates bytecodes that are sent to the requesting browser and interpreted on the browser’s host machine, which has the Java interpreter or a Java-enabled browser installed.
Portable
The quality of being architecture neutral allows for a great deal of portability. However, another aspect of portability is how the hardware interprets arithmetic operations. In C and C++, source code may run slightly differently on different hardware platforms because of how these platforms implement arithmetic operations. In Java, this has been simplified. An integer type in Java, int, is a signed, two’s complement 32-bit integer. A real number, float, is always a 32-bit floating-point number defined by the IEEE 754
standard. These consistencies make it possible to have the assurance that any result on one computer with Java can be replicated on another.
High-Performance
Although Java bytecodes are interpreted, the performance sometimes isn’t as fast as direct compilation and execution on a particular hardware platform. Java compilation includes an option to translate the bytecodes into machine code for a particular hardware platform. This can give the same efficiency as a traditional compile and load process. According to Sun Microsystems testing, performance of this bytecode to machine code translation is “almost indistinguishable” from direct compilation from C or C++ programs.
Multithreaded
Java is a language that can be used to create applications in which several things happen at once. Based on a system of routines that allow for multiple “threads” of events based on C. A. R. Hoare’s monitor and condition paradigm, Java presents the programmer with a way to support real-time, interactive behavior in programs.
Dynamic
Unlike C++ code, which often requires complete recompilation if a parent class is changed, Java uses a method of interfaces to relieve this dependency. The result is that Java programs can allow for new methods and instance variables in objects in a library without affecting their dependent client objects
The Java Tools
Of course, in order to write Java applications or applets, we need more than a language-we need the tools that let us write, test, and debug your programs. This section gives a very high-level overview of the various Java tools that come with the Java Developer's Kit.
Compiler
There is, of course, a Java compiler, named java. The Java compiler takes input source code files (these files typically have the extension .java) and converts them into compiled byte code files. The file used for this purpose is "javac: The Java Compiler."
Interpreter
The Java interpreter, known eponymously as java, can be used to execute Java applications. The interpreter translates byte codes directly into program actions.
Debugger
The Java debugger, jdb, enables you to debug your Java classes. Unfortunately, the Java debugger is a throwback to the pre-GUI debugger dark ages of programming. The Java debugger is a command-line debugger that is enough to make you wish for even the 1988 version of CodeView. However, you can use the jdb to set breakpoints, inspect objects and variables, and monitor threads.
Disassembler
One of the basic tenets of object-oriented programming is that programmers unfamiliar with a class need only concern themselves with the public interface of that class. If we want to use a Queue or Stack class, you shouldn't be concerned with (or need to be concerned with) how this class has been written. Whether it uses a linked list, a statically sized array, or a dynamic array shouldn't influence whether and how we use the class.
Because we should be interested only in the public interface of a class, the Java Developer's Kit includes a disassembler, javap, that can be used to display the public interface, both methods and variables, of a class. Additionally, the Java disassembler includes options to display private members or to display the actual byte codes for the class's methods. This last option can be particularly useful if we want to achieve a greater understanding of the byte codes used by the Java interpreter.
Header File Generator
Because Java is a new language and must fit in a world dominated by C and C++, included in Java is the capability to use native C code within a Java class. One of the steps in doing this is using the Java header file generator, javah.
JavaDoc
As programmers, we've fought it in every way possible. Unfortunately, there is no longer any excuse for not documenting our source code. Using the JavaDoc utility provided with the Java Developers Kit, we can easily generate documentation in the form of HTML files. To do this, we embed special comments and tags in our source code and then process our code through JavaDoc. All of the on-line Java API documentation was created with JavaDoc.
Bytecode
Bytecode is computer object code that is processed by a program, usually referred to as a virtual machine, rather than by the "real" computer machine, the hardware processor. The virtual machine converts each generalized machine instruction into a specific machine instruction or instructions that this computer's processor will understand. Bytecode is the result of compiling source code written in a language that supports this approach. Most computer languages, such as C and C++, require a separate compiler for each computer platform - that is, for each computer operating system and the hardware set of instructions that it is built on. Windows and the Intel line of microprocessor architectures are one platform; Apple and the PowerPC processors are another. Using a language that comes with a virtual machine for each platform, your source language statements need to be compiled only once and will then run on any platform.
The best-known language today that uses the bytecode and virtual machine approach is Java. The LISP language, used in artificial intelligence applications, is an earlier language that compiled bytecode. Other languages that use bytecode or a similar approach include Icon and Prolog.
A simple java program
Hello.java
public class Hello { public static void main(String[] args)
{ System.out.println("hello world");}
}
