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.
Advantage of Java Package
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
Simple example of java package
The package keyword is used to create a package in java.
//save as Simple.java package mypack; public class Simple{ public static void main(String args[]){ System.out.println("Welcome to package"); } }
How to compile java package
If you are not using any IDE, you need to follow the syntax given below:
javac -d directory javafilename
For example
javac -d . Simple.java
The -d switch specifies the destination where to put the generated class file. You can use any directory name like /home (in case of Linux), d:/abc (in case of windows) etc. If you want to keep the package within the same directory, you can use . (dot).
Types of packages in Java
As mentioned in the beginning of this guide that we have two types of packages in java.
User defined package: The package we create is called user-defined package.
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.
How to run java package program
You need to use fully qualified name e.g. mypack.Simple etc to run the class.
To Compile: javac -d . Simple.java To Run: java mypack.Simple Output:Welcome to package
The -d is a switch that tells the compiler where to put the class file i.e. it represents destination. The . represents the current folder.
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.*;
How to access package from another package?
There are three ways to access the package from outside the package.
- import package.*;
- import package.classname;
- fully qualified name.
1) Using packagename.*
If you use package.* then all the classes and interfaces of this package will be accessible but not subpackages.
The import keyword is used to make the classes and interface of another package accessible to the current package.
Example of package that import the packagename.*
//save by A.java package pack; public class A{ public void msg(){System.out.println("Hello");} }
//save by B.java package mypack; import pack.*; class B{ public static void main(String args[]){ A obj = new A(); obj.msg(); } }
Output:
Hello
2) Using packagename.classname
If you import package.classname then only declared class of this package will be accessible.
Example of package by import package.classname
//save by A.java package pack; public class A{ public void msg(){System.out.println("Hello");} }
//save by B.java package mypack; import pack.A; class B{ public static void main(String args[]){ A obj = new A(); obj.msg(); } }
Output:
Hello
3) Using fully qualified name
If you use fully qualified name then only declared class of this package will be accessible. Now there is no need to import. But you need to use fully qualified name every time when you are accessing the class or interface.
It is generally used when two packages have same class name e.g. java.util and java.sql packages contain Date class.
Example of package by import fully qualified name
//save by A.java package pack; public class A{ public void msg(){System.out.println("Hello");} }
//save by B.java package mypack; class B{ public static void main(String args[]){ pack.A obj = new pack.A();//using fully qualified name obj.msg(); } }
Output:
Hello
Note: If you import a package, subpackages will not be imported.
If you import a package, all the classes and interface of that package will be imported excluding the classes and interfaces of the subpackages. Hence, you need to import the subpackage as well.
Note: Sequence of the program must be package then import then class.
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
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
Subpackage in java
Package inside the package is called the subpackage. It should be created to categorize the package further.
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:
Let's take an example, Sun Microsystem has definded a package named java that contains many classes like System, String, Reader, Writer, Socket etc. These classes represent a particular group e.g. Reader and Writer classes are for Input/Output operation, Socket and ServerSocket classes are for networking etc and so on. So, Sun has subcategorized the java package into subpackages such as lang, net, io etc. and put the Input/Output related classes in io package, Server and ServerSocket classes in net packages and so on.
The standard of defining package is domain.company.package e.g. com.javatpoint.bean or org.sssit.dao.
Example of Subpackage
package com.javatpoint.core; class Simple{ public static void main(String args[]){ System.out.println("Hello subpackage"); } }
To Compile: javac -d . Simple.java
To Run: java com.javatpoint.core.Simple
Output:
Hello subpackage
How to send the class file to another directory or drive?
There is a scenario, I want to put the class file of A.java source file in classes folder of c: drive. For example:
//save as Simple.java package mypack; public class Simple{ public static void main(String args[]){ System.out.println("Welcome to package"); } }
To Compile:
e:\sources> javac -d c:\classes Simple.java
To Run:
To run this program from e:\source directory, you need to set classpath of the directory where the class file resides.
e:\sources> set classpath=c:\classes;.; e:\sources> java mypack.Simple
Another way to run this program by -classpath switch of java:
The -classpath switch can be used with javac and java tool.
To run this program from e:\source directory, you can use -classpath switch of java that tells where to look for class file. For example:
e:\sources> java -classpath c:\classes mypack.Simple
Welcome to package
Ways to load the class files or jar files
There are two ways to load the class files temporary and permanent.
Temporary
By setting the classpath in the command prompt
By -classpath switch
Permanent
By setting the classpath in the environment variables
By creating the jar file, that contains all the class files, and copying the jar file in the jre/lib/ext folder.
Rule: There can be only one public class in a java source file and it must be saved by the public class name.
//save as C.java otherwise Compilte Time Error class A{} class B{} public class C{}
How to put two public classes in a package?
If you want to put two public classes in a package, have two java source files containing one public class, but keep the package name same.
//save as A.java package javatpoint; public class A{}
//save as B.java package javatpoint; public class B{}
Points to remember:
Sometimes class name conflict may occur. For example: Lets say we have two packages abcpackage and xyzpackage and both the packages have a class with the same name, let it be JavaExample.java. Now suppose a class import both these packages like this:
This will throw compilation error. To avoid such errors you need to use the fully qualified name method that I have shown above. For exampleimport abcpackage.*; import xyzpackage.*;
This way you can avoid the import package statements and avoid that name conflict error.abcpackage.JavaExample obj = new abcpackage.JavaExample(); xyzpackage.JavaExample obj2 = new xyzpackage.JavaExample();I have already discussed this above, let me mention it again here. If we create a class inside a package while importing another package then the package declaration should be the first statement, followed by package import. For example:
package abcpackage; import xyzpackage.*;A class can have only one package declaration but it can have more than one package import statements. For example:
package abcpackage; //This should be one import xyzpackage; import anotherpackage; import anything;The wild card import like package.* should be used carefully when working with subpackages. For example: Lets say: we have a package abc and inside that package we have another package foo, now foo is a subpackage.
So if I import the package abc using wildcard like this:
import abc.*;
To import the classes of subpackage you need to import like this:
So to import all the classes present in package and subpackage, we need to use two import statements like this:import abc.foo.*;
import abc.*; import abc.foo.*;
