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Java Packages

Java Packages & API

The package in Java is used to integrate related classes. Think of it as a folder in a file index. We use packages to avoid word conflicts, and write better coded code. Packages are divided into two categories:

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

Built-in Packages

The Java API is a library of pre-written classes, free to use, integrated into the Java Development Environment.

The library contains components for input management, site editing, and much more. The full list can be found on the Oracles website: https://docs.oracle.com/javase/8/docs/api/.

The library is divided into packages and classrooms. Which means you can import one class (with its methods and features), or a complete package that contains all the classes that are part of the specified package.

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

Syntax

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

Import a Class

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

Example

import java.util.Scanner;
        

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

To use the Scanner class, create a class object and use any of the methods available in the Scanner class. In our example, we will use the following method nextLine(), which is used to read the 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 a Scanner class from the java.util package. This package also contains date and time resources, a random number generator and other help classes.

To import an entire package, complete the sentence with an asterisk (*). The following example will include ALL classes in the java.util package:

Example

import java.util.*;
        

User-defined Packages

To create your own package, you need to understand that Java uses the file system directory to store them. As 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 merge it:

Then assemble the package:

This forces the compiler to create a "mypack" package.

The -d keyword specifies the location where the class file is stored. You can use any reference name, such as c: / user (windows), or, if you want to save a package within the same directory, you can use the dot ".", as in the example above.

Note: The name of the package should be capitalized to avoid conflict with class names.

When we merged the package in the example above, a new folder was created, called "mypack".

To use the MyPackageClass.java file, type the following:

The output will be

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