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: They are the ones which are predefined by the java compiler. They can be imported to use by the user.

User-defined packages: They are the ones which are made by the programmer to group related classes, enumerations, annotations and interfaces as required by him.

Built-in packages in java.

There are some built-in packages in java that can be used as it is by the programmer. They are listed below.

Let us take a look at each one of these packages:

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

User-defined packages

Having seen the predefined packages, now it’s time to see how the packages can be created, imported and used by the users.

Creating 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.

E.g.


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

package packagename;

public class classname;

{

.................

}

Importing Packages

After the creation of packages, it can be imported using the ‘import’ keyword. Packages can be imported in two ways.

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.*;

The drawback of using the shortcut approach is that it is difficult to determine from which class a particular member came. While using the shortcut approach, all the classes get imported. If the programmer wishes to hide some of the class then it is advised to use access modifier other then public for that class.

Using Packages

After having seen how packages are created and imported, let us have a look at how they are used. In the program given below, we will see all the steps mentioned.


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;
}
}

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;

}

pack1 is the user-defined package, and ‘Calculator’ is the name of the class. The program is saved with the name Calculator.java.

Once defined, let’s see how the package is used.


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);
}
}

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);
}
}

package pkg1;

public class Calculator

{

int v=45;

public void show()

{

System.out.println("Hy!! value of v is"+v);

}

Now, we will make another class which will import both the packages.


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();
}
}

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();

}

This class uses both the packages simultaneously. We can import as many packages as we desire.

How to add a new class to an existing package?

There is an example given below to add a new class to an existing package.


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

package p1;

public class One

{

..............

}

This is the package in which we wish to add a new class, and it can be done as:

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
{
................
}

package p1

public class Two

{

................

}

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