Java - import Keyword

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Java import keyboard is used in context of packages. This import keyword is to use components of a package where packages are used in Java in order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations easier, etc.

A Package can be defined as a grouping of related types (classes, interfaces, enumerations and annotations ) providing access protection and namespace management.

Some of the existing packages in Java are −

• java.lang − bundles the fundamental classes

• java.io − classes for input , output functions are bundled in this package

Programmers can define their own packages to bundle group of classes/interfaces, etc. It is a good practice to group related classes implemented by you so that a programmer can easily determine that the classes, interfaces, enumerations, and annotations are related.

Since the package creates a new namespace there won't be any name conflicts with names in other packages. Using packages, it is easier to provide access control and it is also easier to locate the related classes.

If a class wants to use another class in the same package, the package name need not be used. Classes in the same package find each other without any special syntax.

Example

Here, a class named Boss is added to the payroll package that already contains Employee. The Boss can then refer to the Employee class without using the payroll prefix, as demonstrated by the following Boss class.

package payroll;
public class Boss {
   public void payEmployee(Employee e) {
      e.mailCheck();
   }
}

What happens if the Employee class is not in the payroll package? The Boss class must then use one of the following techniques for referring to a class in a different package.

• The fully qualified name of the class can be used. For example −

payroll.Employee

• The package can be imported using the import keyword and the wild card (*). For example −

import payroll.*;

• The class itself can be imported using the import keyword. For example −

import payroll.Employee;

Example

package payroll;

public class Employee {
   public void mailCheck() {
      System.out.println("Pay received.");    
   }
}

Example

package payroll;

import payroll.Employee;

public class Boss {
   public void payEmployee(Employee e) {
      e.mailCheck();
   }
   
   public static void main(String[] args) {
      Boss boss = new Boss();
	  Employee e = new Employee();
      boss.payEmployee(e);
   }
}

Output

Pay received.

Note − A class file can contain any number of import statements. The import statements must appear after the package statement and before the class declaration.

The Directory Structure of Packages

Two major results occur when a class is placed in a package −

• The name of the package becomes a part of the name of the class, as we just discussed in the previous section.

• The name of the package must match the directory structure where the corresponding bytecode resides.

Here is simple way of managing your files in Java −

Put the source code for a class, interface, enumeration, or annotation type in a text file whose name is the simple name of the type and whose extension is .java.

For example −

// File Name :  Car.java
package vehicle;

public class Car {
   // Class implementation.   
}

Now, put the source file in a directory whose name reflects the name of the package to which the class belongs −

....\vehicle\Car.java

Now, the qualified class name and pathname would be as follows −

• Class name → vehicle.Car
• Path name → vehicle\Car.java (in windows)

In general, a company uses its reversed Internet domain name for its package names.

Example − A company's Internet domain name is apple.com, then all its package names would start with com.apple. Each component of the package name corresponds to a subdirectory.

Example − The company had a com.apple.computers package that contained a Dell.java source file, it would be contained in a series of subdirectories like this −

....\com\apple\computers\Dell.java

At the time of compilation, the compiler creates a different output file for each class, interface and enumeration defined in it. The base name of the output file is the name of the type, and its extension is .class.

For example −

// File Name: Dell.java
package com.apple.computers;

public class Dell {
}

class Ups {
}

Now, compile this file as follows using -d option −

$javac -d . Dell.java

The files will be compiled as follows −

.\com\apple\computers\Dell.class
.\com\apple\computers\Ups.class

You can import all the classes or interfaces defined in \com\apple\computers\ as follows −

import com.apple.computers.*;