Q. Write the method to define the packages in java programme. Explain.
Ans. Package: When we work on a project we have to break our programme in several classes. To organize our classes we use packages. The package is both a naming and visibility control mechanism. Package is a collection of classes and interfaces which are interrelated. With the help of package, it is possible to give same names to more than one class provided; they are defined in different packages. A single package cannot contain two classes with same name. Programmes are organized as sets of packages. Each package has its own sets of sub packages, which helps to prevent name confections. A top level package is accessible outside the package is hierarchal. The members of a package are class and interface type and sub classes. A package in java is an encapsulation mechanism that can be used for to group related class, interface and sub packages. For small programmes, a package can be unnamed or have a simple name, but if code is to be widely distributed, unique package names should be chose. This can prevent to the conflicts that would otherwise occur if two development groups happened to pick the same package name and these packages were later to be used in a single programme. If we do specify a package for a java class then that class will be the part of the default package of java called as unnamed package. But this requires that every class must have a unique name to avoid collision.
How to define a package
To create a package, include a package command as the first statement in a java source file. Any class declared within that file will belong to the specified package. The package statement defines a name space in which classes are stored. If one omit the package statement, the class name are put into the default package, which has no name and is called as unnamed package. The package statement has the following syntax:
Package package _ name
Java uses file system directories to store packages. Remember that the case is significant, and directory name must match the package name exactly. More than one file can include the same package name exactly. More
Package p 1; // package declaration
Class A // class declaration
................. // body of class A
.....................
}
Package is a keyword; p 1 is the package name in which class A is to be stored.
Example 1:
Package p 1;
Class example _ 1
{
Public static void main (string args [])
{
System. out . print ln ("first package programme")
}
} we have assumed that the current folder is (c:\java >)
Output
First package programme
Note: If we compile the above programme with the folder p 1 will be created automatically and the class file of the above programme will be saved in this folder. But if we do not compile with d option this it is our duty to create the p 1 folder and place the class file in that folder (.) directs the compiler the to create the p 1 folder in the current folder. We have created two files (Box. Java and box _ demo. Java ). The box _ demo. Java contains the main programme, so we will return with this class name.
Step 1
We have to create the java files in the c:\ java & box _ demo. Java)
Note 1: we can't run a java programme directly through class name if it is a part of a package. To run this programme we must specify the (pl. Java box _ demo)
If we run the above programme from any other folder than the current folder then it will not run. By default the java run time system uses the current run time directory as its starting point. Thus if our package is in the current directory, as its starting point or a subdirectory of the current directory, it will be found. But to run the above programme from any other folder we have to set the class path using the command SET CLASSPATH = % CLASSPATH %; C : TEST on command promote. CLASSPATH is a environment variable. We have to type this command every time when we start the computer for the first time. But if we we put this command in auto exec, bat file in the root directory then there is no need to type this command again & again. We can also run the programme without setting the CLASSPATH environment variable to do this type.