Packages in Java

Packages in Java can be defined as an assortment for grouping various classes and interfaces based on their performance. It is a catalog for holding various java files. They provide a very efficient mechanism for alignment of various files. The files which are related to each other can be stored in the same package, and whenever there is a need to access the related files, we just have to go through one package. A unique namespace is provided by the package. A package can contain the following types of file in java:

• Class
• Interface
• Enumerated types
• Annotations

Advantages of packages

• Two classes in two different packages can have the same name so, name collision issue is not a problem.
• Classes related to each other are stored in the same package so, accessing them is easy.
• Access protection is provided by packages so, no need to worry about the security concerns. The files that are crucial can be hidden successfully.
• Programmer can make their own packages as per their convenience. It is programmer friendly.
• Code reusability is provided.

Types of packages in Java

Built-in packages in java.

1. java.lang: This package contains language support classes. They are by default imported in the java programs.
2. java.util: This package contains language utility classes such as vectors, lists, hash tables, etc.
3. java.awt: This package contains a set of classes for implementing the graphic user interface.
4. java.applet: This package contains a set of classes for using applets in java program.
5. java.net: This package contains a set of network classes.
6. java.io: This package contains classes for input and output support.

User-defined packages

• Select a suitable name for the package.
• Declare the name of the package with the 'package' keyword in the first line of a program. E.g. package firstpack.
• Define a public class inside that package.
• Create a subdirectory in the main directory to store the class. Save it as classname.java.
• Compile the file.

package packagename;
public class classname;
{
.................
}

• By explicitly declaring the class name to be imported: E.g. import firstpackage.secondpackge.classname;
• By importing all the classes in a package. E.g. import packagename.*;

package pkg1; //creating package
public class Calculator
{
int x;
int y;
public int add(int a,int b)
{
x=a;
y=b;
return x+y;
}
public int sub(int c,int d)
{
x=c;
y=d;
return x-y;
}
public int mul(int e,int f)
{
x=e;
y=f;
return x*y;
}
public int div(int g,int h)
{
x=g;
y=h;
return x/y;
}
}

import pkg1.Calculator; //importing package
class Operate
{
public static void main(String args[])
{
Calculator obj1 = new Calculator();
obj1.add(5,6);
Calculator obj2 = new Calculator();
obj2.sub(8,6);
Calculator obj3 = new Calculator();
obj3.mul(5,6);
}
}

Program to import two packages simultaneously.

package pkg1;
public class Calculator
{
int v=45;
public void show()
{
System.out.println("Hy!! value of v is"+v);
}
}

import pkg1.Calculator;
import pkg2.ClassA;
class Test
{
public static void main(String args[])
{
Calculator obj1 = new Calculator();
obj1.add(5,6);
ClassB obj2 = new ClassB();
obj1.show();
}
}

How to add a new class to an existing package?

package p1;
public class One
{
..............
}

• Define the class and use a public modifier.
• Place the package statement at the top of the program.
• Compile the class as done earlier.
• The class is now added to the package.

package p1
public class Two
{
................
}