Package is a collection of related
classes. Each class defines number of
methods. Java packages are classified
into 2 types.
1. Java
API (Application Programming Interface) packages (or) Predefined packages (or)
Built-in packages: These packages are defined by the system. Some of the example for system defined
packages are java.lang, java.util, java.io, java.awt, java.applet etc.,
2. User
defined packages: These packages are defined by the user.
Defining a Package:
To
define a package, place “package” command as the first statement in the Java
source file. So, that any class declared
within that file will belong to the specified package. The syntax of package creation is as follows.
Syntax: package pack-name;
where pack-name is the name of the
package.
Example: package mypack;
public
class number
{
public
void add (int a,int b)
{
System.out.println(“Sum=”+(a+b));
}
}
Ø The
class that is defined in the package must be start with the public access
modifier. So, that it can be accessible
by any another of them. If it is not
public, it is possible to access only in that package.
Ø Java
uses file system directories to store packages.
We save the program with number.java and compile the package is as
javac –d . number.java
Due to this compilation mypack
directory is automatically created and .class file is stored in that directory.
Ø Package
creation has completed. The package
information is now including in our actual program by means of “import”
statement. “import” is a keyword that
links the package with our program. It
is placed before the class definitions.
import
mypack.*;
or
import
mypack.number;
Example 1:
import mypack.number;
class pack
{
public
static void main(String[] args)
{
number
obj=new number();
obj.add(3,4);
}
}
Example 2:
package math1;
public class Mcal
{
public
void square(int x)
{
System.out.println("Square
of "+x+" is"+(x*x));
}
public
void cube(int x)
{
System.out.println("cube
"+x+" is"+(x*x*x));
}
}
import math1.Mcal;
class calc
{
public
static void main(String[] args)
{
Mcal
obj=new Mcal();
obj.square(3);
obj.cube(4);
}
}
Example 3:
package p6;
public class Balance
{
String name;
double bal;
public Balance(String n, double
b)
{
name=n;
bal=b;
}
public void show()
{
if(bal<0)
{
System.out.println(name+"---"+bal);
}
}
}
import p6.Balance;
class Account
{
public
static void main(String[] args)
{
Balance
c[]=new Balance[3];
c[0]=new
Balance("John",123.33);
c[1]=new
Balance("Arun",157.02);
c[2]=new
Balance("Siri",-12.33);
for(int
i=0;i<3;i++)
c[i].show();
}
}
Note: 1. Java source file contains only one public class and
any number of non-pubic classes. The
filename should be same as the name of the public class with .java extension.
2. Package contain any number of
public and default classes.
Example: package p1;
public
class x
{
//
body of x this class is
accessible
}
class
y
{
//
body of y not accessible
due to non-use
} of
public access modifier
Let
we want to access all the classes of information then the classes are stored in
different files with the same package name.
Example:
package mypack;
public
class number
{
public
void add (int a,int b)
{
System.out.println(“Sum=”+(a+b));
}
}
package
mypack;
public
class number1
{
public
void sub(int a,int b)
{
System.out.println("sub="+(a-b));
}
}
import
mypack.number;
import
mypack.number1;
class
pack
{
public static void main(String[]
args)
{
number obj=new number();
obj.add(3,4);
number1 obj1=new
number1();
obj1.sub(6,2);
}
}
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