Packages In Java

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:

1. built-in packages
2. user defined package

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.

Advantages of using a package in Java

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.

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.

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.

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

Example 3: Using fully qualified name while importing a class

You can use fully qualified name to avoid the import statement. Lets see an example to understand this:

Calculator.java

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));
   }
}

Example.java

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

In the Example class, instead of importing the package, I have used the full qualified name such as package_name.class_name to create the object of it. You may also want to read: static import in Java

Sub packages in Java

A package inside another package is known as sub package. For example If I create a package inside letmecalculate package then that will be called sub package.
Lets say I have created another package inside letmecalculate and the sub package name is multiply. So if I create a class in this subpackage it should have this package declaration in the beginning:

package letmecalculate.multiply;
Multiplication.java

package letmecalculate.multiply;
public class Multiplication {
	int product(int a, int b){
		return a*b;
	}
}

Now if I need to use this Multiplication class I have to either import the package like this:

import letmecalculate.multiply;

or I can use fully qualified name like this:

letmecalculate.multiply.Multiplication obj = 
     new letmecalculate.multiply.Multiplication();