intoduction to JAVA 2

Java is an object-oriented programming language developed by Sun
Microsystems.
Java is Platform independent, Platform independence means, the ability of a
program to move easily from one computer system to another-is one of the most
significant advantages that Java has over other programming languages,
particularly if your software needs to run on many different platforms.
Java can be used to create two types of programs: applets and stand-alone
applications. An Applet is simply a part of a Web page, just as an image or a line
of text can be. Just as a browser takes care of displaying an image referenced in
an HTML document, a Java-enabled browser locates and runs an Applet . When your
Java-capable Web browser loads the HTML document, the Java applet is also loaded
and executed.
Applets are Java programs that execute within the context of a Web page.
Java arrays
An array is simply a sequence of either objects or primitives, all the same type
and packaged together under one identifier name. Arrays are defined and used with
the square-brackets indexing operator [ ]. To define an array you simply follow
your type name with empty square brackets:
int[] a1;
You can also put the square brackets after the identifier to produce exactly the
same meaning:
int a1[];
int[] a1 = { 1, 2, 3, 4, 5 };
Here are some of the major differences between Java, C, and C++:
• Java has no pointers.
• Java has no global variables and no global functions.
• Java supports method overloading.
• Java has no preprocessor. Thus, there are no macros (#define). so it doesn’t
need #include.
• In Java, instance (nonstatic) methods are bound dynamically by default.
• In Java, a String is an object, and not an array of char.
java.applet
This package contains the classes necessary for creating applets.
java.awt
The extension awt stands for Abstract Windowing Toolkit.
java.awt.image
This package provides classes for performing image processing.
java.io
The java.io package is the Java version of stdio.h (if you use C) or stream.h
(C++). In other words,
here are the classes for performing input and output.
java.lang
This package is automatically imported by the Java compiler.
java.net
The java.net package contains classes for interfacing with the Internet and the
World Wide Web.
java.util
In this package you’ll find declarations of basic data structures that come in
really handy, such as a
stack and a hashtable.
The Java Interpreter
Once you’ve written a Java program, you’ll want to run it! There are few steps
involved. First,
compile the source file with the Java compiler. The compiler produces bytecode,
which is stored in a
class file. Then, invoke the Java interpreter for this class file. You can
explicitly invoke the Java
interpreter from a command-line prompt, or let a Web browser do it for you.
Then, the Java interpreter takes over. It does a few things:
1. The appropriate class file is loaded by the bytecode loader. The class file can
come from the
local file system or from the Internet.
2. Since the bytecode may be of unknown origin, it is checked by the bytecode
verifier. Code
that passes the tests of the verifier can be safely executed. In effect, the
bytecode verifier acts
as a gatekeeper, which prevents nasty, damaging programs from being executed.
3. The successfully verified bytecode is executed by the implementation of the
Java virtual
machine.
These components—loader, verifier, and virtual machine—work in conjunction so that
classes are
dynamically loaded and linked as needed.
Program 1: Your First Java Program
class Fun {
public static void main(String argv[]) {
System.out.println("Java is FUN!");
}}
Store this short program in a file called Fun.java. The name of the file must
match the name of the
class. Now, compile it by typing
% javac Fun.java
The Java compiler, javac, produces a file that’s called Fun.class, which contains
the bytecode. To run
your program, type
% java Fun
Now java, the Java interpreter, locates the file Fun.class, and starts execution
from the main() method.
The result is
Java is FUN!
The class Fun defines a method called main(). What meaning do the three keywords
in front of main()
have?
• public. This is an access specifier that states that any object can invoke
main().
• static. This means that main() is a class method, and can be invoked even if an
instance of
the class isn’t allocated.
• void. This means that main() doesn’t return anything.
Inside main() is the line
System.out.println("Java is FUN!");
Q: What is the default value of the local variables?
A: The local variables are not initialized to any default value, neither
primitives nor object references. If you try to use these variables without
initializing them explicitly, the java compiler will not compile the code. It will
complain abt the local varaible not being initilized..
Q: How many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();
A: Only 2 objects are created, c1 and c3. The reference c2 is only declared and
not initialized.
Q: What will be the output of the following statement?
System.out.println ("1" + 3);
A: It will print 13.
Q: What happens if you dont initialize an instance variable of any of the
primitive types in Java?
A: Java by default initializes it to the default value for that primitive type.
Thus an int will be initialized to 0, a boolean will be initialized to false.