Teach Yourself Java in 21 Days NOTES

Teach Yourself Java in 21 Days NOTES:

DAY 1:
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Enter Java, and the capability for Web pages of containing Java applets. Applets are small
programs that create animations, multimedia presentations, real-time (video) games, multi-user
networked games, and real interactivity—in fact, most anything a small program can do, Java
applets can. Downloaded over the net and executed inside a Web page by a browser that supports
Java, applets are an enormous step beyond standard Web design.

An applet is a dynamic and interactive program that can run inside a Web page displayed
by a Java-capable browser such as HotJava or Netscape 2.0.

Java is an object-oriented programming language developed by Sun Microsystems, a company
best known for its high-end Unix workstations. Modeled after C++, the Java language was
designed to be small, simple, and portable across platforms and operating systems, both at the
source and at the binary level (more about this later).
Java is often mentioned in the same breath as HotJava, a World Wide Web browser from Sun
like Netscape or Mosaic (see Figure 1.1). What makes HotJava different from most other
browsers is that, in addition to all its basic Web features, it can also download and play applets
on the reader’s system. Applets appear in a Web page much in the same way as images do, but
unlike images, applets are dynamic and interactive. Applets can be used to create animations,
figures, or areas that can respond to input from the reader, games, or other interactive effects on
the same Web pages among the text and graphics.
Although HotJava was the first World Wide Web browser to be able to play Java applets, Java
support is rapidly becoming available in other browsers. Netscape 2.0 provides support for Java
applets, and other browser developers have also announced support for Java in forthcoming
products.

Platform independence is one of the most significant advantages that Java has over other
programming languages, particularly for systems that need to work on many different platforms.
Java is platform-independent at both the source and the binary level.
Platform-independence is a program’s capability of moving easily from one computer
system to another.
At the source level, Java’s primitive data types have consistent sizes across all development
platforms. Java’s foundation class libraries make it easy to write code that can be moved from
platform to platform without the need to rewrite it to work with that platform.
Platform-independence doesn’t stop at the source level, however. Java binary files are also
platform-independent and can run on multiple problems without the need to recompile the
source. How does this work? Java binary files are actually in a form called bytecodes.
Bytecodes are a set of instructions that looks a lot like some machine codes, but that is not
specific to any one processor.
Normally, when you compile a program written in C or in most other languages, the compiler
translates your program into machine codes or processor instructions. Those instructions are
specific to the processor your computer is running—so, for example, if you compile your code
on a Pentium system, the resulting program will run only on other Pentium systems. If you want
to use the same program on another system, you have to go back to your original source, get a
compiler for that system, and recompile your code.

Java applications fall into two main groups: applets and applications.
Applets, as you have learned, are Java programs that are downloaded over the World Wide Web
and executed by a Web browser on the reader’s machine. Applets depend on a Java-capable
browser in order to run (although they can also be viewed using a tool called the appletviewer,
which you’ll learn about later today).
Java applications are more general programs written in the Java language. Java applications don’t
require a browser to run, and in fact, Java can be used to create most other kinds of applications
that you would normally use a more conventional programming language to create. HotJava
itself is a Java application.

The paint() method displays the content of the applet to the screen.
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DAY 2:
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