Packages in java define

Creating a Package

To create a package, you choose a name for the package (naming conventions are discussed in the next section) and put a package statement with that name at the top of every source file that contains the types (classes, interfaces, enumerations, and annotation types) that you want to include in the package.

The package statement (for example, package graphics; ) must be the first line in the source file. There can be only one package statement in each source file, and it applies to all types in the file.

Note: If you put multiple types in a single source file, only one can be public , and it must have the same name as the source file. For example, you can define public class Circle in the file Circle.java , define public interface Draggable in the file Draggable.java , define public enum Day in the file Day.java , and so forth.

You can include non-public types in the same file as a public type (this is strongly discouraged, unless the non-public types are small and closely related to the public type), but only the public type will be accessible from outside of the package. All the top-level, non-public types will be package private.

If you put the graphics interface and classes listed in the preceding section in a package called graphics , you would need six source files, like this:

//in the Draggable.java file package graphics; public interface Draggable < . . . >//in the Graphic.java file package graphics; public abstract class Graphic < . . . >//in the Circle.java file package graphics; public class Circle extends Graphic implements Draggable < . . . >//in the Rectangle.java file package graphics; public class Rectangle extends Graphic implements Draggable < . . . >//in the Point.java file package graphics; public class Point extends Graphic implements Draggable < . . . >//in the Line.java file package graphics; public class Line extends Graphic implements Draggable

If you do not use a package statement, your type ends up in an unnamed package. Generally speaking, an unnamed package is only for small or temporary applications or when you are just beginning the development process. Otherwise, classes and interfaces belong in named packages.

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Java Packages

A package in Java is used to group related classes. Think of it as a folder in a file directory. We use packages to avoid name conflicts, and to write a better maintainable code. Packages are divided into two categories:

• Built-in Packages (packages from the Java API)
• User-defined Packages (create your own packages)

Built-in Packages

The Java API is a library of prewritten classes, that are free to use, included in the Java Development Environment.

The library contains components for managing input, database programming, and much much more. The complete list can be found at Oracles website: https://docs.oracle.com/javase/8/docs/api/.

The library is divided into packages and classes. Meaning you can either import a single class (along with its methods and attributes), or a whole package that contain all the classes that belong to the specified package.

To use a class or a package from the library, you need to use the import keyword:

Syntax

import package.name.Class; // Import a single class import package.name.*; // Import the whole package 

Import a Class

If you find a class you want to use, for example, the Scanner class, which is used to get user input, write the following code:

Example

In the example above, java.util is a package, while Scanner is a class of the java.util package.

To use the Scanner class, create an object of the class and use any of the available methods found in the Scanner class documentation. In our example, we will use the nextLine() method, which is used to read a complete line:

Example

Using the Scanner class to get user input:

import java.util.Scanner; class MyClass < public static void main(String[] args) < Scanner myObj = new Scanner(System.in); System.out.println("Enter username"); String userName = myObj.nextLine(); System.out.println("Username is: " + userName); >> 

Import a Package

There are many packages to choose from. In the previous example, we used the Scanner class from the java.util package. This package also contains date and time facilities, random-number generator and other utility classes.

To import a whole package, end the sentence with an asterisk sign ( * ). The following example will import ALL the classes in the java.util package:

Example

User-defined Packages

To create your own package, you need to understand that Java uses a file system directory to store them. Just like folders on your computer:

Example

└── root └── mypack └── MyPackageClass.java

To create a package, use the package keyword:

MyPackageClass.java

package mypack; class MyPackageClass < public static void main(String[] args) < System.out.println("This is my package!"); >> 

Save the file as MyPackageClass.java, and compile it:

This forces the compiler to create the «mypack» package.

The -d keyword specifies the destination for where to save the class file. You can use any directory name, like c:/user (windows), or, if you want to keep the package within the same directory, you can use the dot sign » . «, like in the example above.

Note: The package name should be written in lower case to avoid conflict with class names.

When we compiled the package in the example above, a new folder was created, called «mypack».

To run the MyPackageClass.java file, write the following:

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