A package as the name suggests is a pack(group) of classes, interfaces and other packages. In java we use packages to organize our classes and interfaces. We have two types of packages in Java: built-in packages and the packages we can create (also known as user defined package). In this guide we will learn what are packages, what are user-defined packages in java and how to use them.

In java we have several built-in packages, for example when we need user input, we import a package like this:

import java.util.Scanner

Here:
→ java is a top level package
→ util is a sub package
→ and Scanner is a class which is present in the sub package util.

Before we see how to create a user-defined package in java, lets see the advantages of using a package.

Advantages of using a package in Java

These are the reasons why you should use packages in Java:

• Reusability: While developing a project in java, we often feel that there are few things that we are writing again and again in our code. Using packages, you can create such things in form of classes inside a package and whenever you need to perform that same task, just import that package and use the class.
• Better Organization: Again, in large java projects where we have several hundreds of classes, it is always required to group the similar types of classes in a meaningful package name so that you can organize your project better and when you need something you can quickly locate it and use it, which improves the efficiency.
• Name Conflicts: We can define two classes with the same name in different packages so to avoid name collision, we can use packages

Types of packages in Java

As mentioned in the beginning of this guide that we have two types of packages in java.
1) User defined package: The package we create is called user-defined package.
2) Built-in package: The already defined package like java.io.*, java.lang.* etc are known as built-in packages.

We have already discussed built-in packages, lets discuss user-defined packages with the help of examples.

Example 1: Java packages

I have created a class Calculator inside a package name letmecalculate. To create a class inside a package, declare the package name in the first statement in your program. A class can have only one package declaration.
Calculator.java file created inside a package letmecalculate

package letmecalculate;

public class Calculator {
   public int add(int a, int b){
	return a+b;
   }
   public static void main(String args[]){
	Calculator obj = new Calculator();
	System.out.println(obj.add(10, 20));
   }
}

Now lets see how to use this package in another program.

import letmecalculate.Calculator;
public class Demo{
   public static void main(String args[]){
	Calculator obj = new Calculator();
	System.out.println(obj.add(100, 200));
   }
}

To use the class Calculator, I have imported the package letmecalculate. In the above program I have imported the package as letmecalculate.Calculator, this only imports the Calculator class. However if you have several classes inside package letmecalculate then you can import the package like this, to use all the classes of this package.

import letmecalculate.*;

Example 2: Creating a class inside package while importing another package

As we have seen that both package declaration and package import should be the first statement in your java program. Lets see what should be the order when we are creating a class inside a package while importing another package.

//Declaring a package
package anotherpackage;
//importing a package
import letmecalculate.Calculator;
public class Example{
   public static void main(String args[]){
	Calculator obj = new Calculator();
	System.out.println(obj.add(100, 200));
   }
}

So the order in this case should be:
→ package declaration
→ package import