Packages in Java

Package is a way of grouping a variety of classes and interfaces together. In java, package is a way to organize set of related classes and interfaces. In simple words, a package is similar to different folders in our computer. We keep images in one folder, songs in other folder and so on. So, we keep set of related documents in one folder. There can be two classes in two different package can have same name as two files having same name in two different folders in our computer.

In java, we have ability to reuse code by extending classes and implementing interfaces. But if we have to reuse the code written in another program without copying the code. This can be achieved by using packages. Java provides a class library for use in our own application called API or Application Programming Interface. This API provides large number of classes grouped into different packages according to their functionality.

Advantages of Package in Java

(1) While making a large project, we have many classes and interfaces, so it is required to group set of related classes and interfaces into packages. It provides better organisation and improves efficiency.

(2) We can put two classes of same name in two different package, so it avoids name collision.

(3) Java package provides reusability of code by using different classes and interfaces in different packages.

(4) We can't directly access the code of classes and interfaces, so it provides access protection.

Types of Package in Java

Built-in package

Java already provides many different packages like java.lang.*, java.io.* known as built-in packages.

User defined package

We can also create our own package known as user defined package.

Example of Java Package

package showMe;

public class Show{
 public void disp(){
  System.out.println("Hello");
 }
}

import showMe.Show;

public class Demo{
 public static void main(String[] agrs){
  Show obj = new Show();
  obj.disp();
 }
}

Output:

Hello

In this example, first we create a package namely "showme" using package keyword. We define a class called "Show" and this class containing only one method called "disp". While putting a class to a package, the class must be of public access modifier.

We can create another class called "Demo", this class imports "showMe" using import keyword. In Demo class, we create an object of "Show" class of "showMe" package and called a method called "disp".

To Compile: javac -d . Show.java

Then again, javac Demo.java

To run Demo class: java Demo

Example 2:

We can create a class inside a package while importing another package.

package Package2;
import showMe;

class Example{
 public static void main(String[] args){
 Show obj = new Show();
 obj.disp();
 }
}

In this program, first we write package declaration and then we import another package.

It is important to know that if we write package declaration statement in our program, then it must be the first statement of program.

Example 3:

If you don't want to write import statement, then you can use fully qualified name.

Greatest.java

package Package1;
public class Greatest{
  public void checkGreatest(int a, int b){
    if(a>b)
   System.out.println(a+" is greater than "+b);
 else
   System.out.println(b+" is greater than "+a); 
  }
}

Example.java

class Example{
 public static void main(String[] args){
 Package1.Greatest obj = new Package1.Greatest();
 obj.checkGreatest(10, 20);
 }
}

Output:

20 is greater than 10

Sub package in Java

We can create a package inside another package. For example, java has also sub packages like lang, io, net etc.

Example:

Show.java

package Package1.SubPackage1;
public class Show{
 public void disp(){
  System.out.println("Welcome");
 }
}

Example.java

import Package1.SubPackage1.*;
class Example{
 public static void main(String[] args){
 Show obj = new Show();
 obj.disp();
 }
}

Output:

Welcome