JAVA-1

What is Java?
Java (with a capital J) is a high-level, third generation programming language, like C, Fortran, Smalltalk, Perl, and many others. You can use Java to write computer applications that crunch numbers, process words, play games, store data or do any of the thousands of other things computer software can do.
Compared to other programming languages, Java is most similar to C. However although Java shares much of C's syntax, it is not C. Knowing how to program in C or, better yet, C++, will certainly help you to learn Java more quickly, but you don't need to know C to learn Java. Unlike C++ Java is not a superset of C. A Java compiler won't compile C code, and most large C programs need to be changed substantially before they can become Java programs.
What's most special about Java in relation to other programming languages is that it lets you write special programs called applets that can be downloaded from the Internet and played safely within a web browser. Traditional computer programs have far too much access to your system to be downloaded and executed willy-nilly. Although you generally trust the maintainers of various ftp archives and bulletin boards to do basic virus checking and not to post destructive software, a lot still slips through the cracks. Even more dangerous software would be promulgated if any web page you visited could run programs on your system. You have no way of checking these programs for bugs or for out-and-out malicious behavior before downloading and running them.
Java solves this problem by severely restricting what an applet can do. A Java applet cannot write to your hard disk without your permission. It cannot write to arbitrary addresses in memory and thereby introduce a virus into your computer. It should not crash your system.

Java is a Platform:-
Java (with a capital J) is a platform for application development. A platform is a loosely defined computer industry buzzword that typically means some combination of hardware and system software that will mostly run all the same software. For instance PowerMacs running Mac OS 9.2 would be one platform. DEC Alphas running Windows NT would be another.
There's another problem with distributing executable programs from web pages. Computer programs are very closely tied to the specific hardware and operating system they run. A Windows program will not run on a computer that only runs DOS. A Mac application can't run on a Unix workstation. VMS code can't be executed on an IBM mainframe, and so on. Therefore major commercial applications like Microsoft Word or Netscape have to be written almost independently for all the different platforms they run on. Netscape is one of the most cross-platform of major applications, and it still only runs on a minority of platforms.
Java solves the problem of platform-independence by using byte code. The Java compiler does not produce native executable code for a particular machine like a C compiler would. Instead it produces a special format called byte code. Java byte code written in hexadecimal, byte by byte, looks like this:CA FE BA BE 00 03 00 2D 00 3E 08 00 3B 08 00 01 08 00 20 08
This looks a lot like machine language, but unlike machine language Java byte code is exactly the same on every platform. This byte code fragment means the same thing on a Solaris workstation as it does on a Macintosh PowerBook. Java programs that have been compiled into byte code still need an interpreter to execute them on any given platform. The interpreter reads the byte code and translates it into the native language of the host machine on the fly. The most common such interpreter is Sun's program java (with a little j). Since the byte code is completely platform independent, only the interpreter and a few native libraries need to be ported to get Java to run on a new computer or operating system. The rest of the runtime environment including the compiler and most of the class libraries are written in Java.
All these pieces, the javac compiler, the java interpreter, the Java programming language, and more are collectively referred to as Java.

Java is Simple:-
Java was designed to make it much easier to write bug free code. According to Sun's Bill Joy, shipping C code has, on average, one bug per 55 lines of code. The most important part of helping programmers write bug-free code is keeping the language simple.
Java has the bare bones functionality needed to implement its rich feature set. It does not add lots of syntactic sugar or unnecessary features. Despite its simplicity Java has considerably more functionality than C, primarily because of the large class library.
Because Java is simple, it is easy to read and write. Obfuscated Java isn't nearly as common as obfuscated C. There aren't a lot of special cases or tricks that will confuse beginners.
About half of the bugs in C and C++ programs are related to memory allocation and deallocation. Therefore the second important addition Java makes to providing bug-free code is automatic memory allocation and deallocation. The C library memory allocation functions malloc() and free() are gone as are C++'s destructors.
Java is an excellent teaching language, and an excellent choice with which to learn programming. The language is small so it's easy to become fluent. The language is interpreted so the compile-run-link cycle is much shorter. The runtime environment provides automatic memory allocation and garbage collection so there's less for the programmer to think about. Java is object-oriented unlike Basic so the beginning programmer doesn't have to unlearn bad programming habits when moving into real world projects. Finally, it's very difficult (if not quite impossible) to write a Java program that will crash your system, something that you can't say about any other language.

Java is Object-Oriented:-
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.

Java is Platform Independent:-
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 .

Java is Safe:-
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