Java Packages And Programs
Packages are used in Java in order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations easier, etc.
A Package can be defined as a grouping of related types(classes, interfaces, enumerations and annotations ) providing access protection and name space management.
Some of the existing packages in Java are:
• java.lang - bundles the fundamental classes
• java.io - classes for input, output functions are bundled in this package
Programmers can define their own packages to bundle group of classes/interfaces, etc. It is a good practice to group related classes implemented by you so that a programmer can easily determine that the classes, interfaces, enumerations, annotations are related.
Since the package creates a new namespace there won't be any name conflicts with names in other packages. Using packages, it is easier to provide access control and it is also easier to locate the related classes.
Creating a package:
When creating a package, you should choose a name for the package and put a package statement with that name at the top of every source file that contains the classes, interfaces, enumerations, and annotation types that you want to include in the package.
The package statement should be the first line in the source file. There can be only one package statement in each source file, and it applies to all types in the file.
If a package statement is not used then the class, interfaces, enumerations, and annotation types will be put into an unnamed package.
Types of Java programs:
Applications and Applets
Applications - Java programs that run directly on your machine.
I). Applications must have a main().
II). Java applications are compiled using the javac command and run using the java command.
Applets - Java programs that can run over the Internet. The standard client/server model is used when the Applet is executed. The server stores the Java Applet, which is sent to the client machine running the browser, where the Applet is then run.
I). Applets do not require a main(), but in general will have a paint().
II). An Applet also requires an HTML file before it can be executed.
III). Java Applets are also compiled using the javac command, but are are run either with a browser or with the appletviewer command.
Important Features of Java
Java is a "platform-independent language".
A Java program can be compiled once into a Java Bytecode program. The compiled program can then be run on any computer that has an interpreter for the Java virtual machine. Other languages have to be re-compiled for each platform on which they are going to run. The point about Java is that it can be executed on many different types of computers without being recompiled.
Java is an object-oriented programming language.
An object consists of some data together with a set of subroutines that manipulate that data. (An object is a kind of "module," or self-contained entity that communicates with the rest of the world through a well-defined interface. An object should represent some coherent concept or real-world object.)
The Java platform is designed from the ground up to support concurrent programming, with basic concurrency support in the Java programming language and the Java class libraries.
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