Understand the idea

A package is a namespace for related Java types. An import lets you use a type name without spelling its fully qualified name each time; it does not install a library. Multi-file package layouts are easiest to practice locally. The single-file browser example below demonstrates importing a standard-library class.

Explore the details

Java Packages & API

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:

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:

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:

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:

Common Built-in Java Packages

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:

To create a package, use the package keyword:

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

Example

C:\Users\Your Name>javac MyPackageClass.java

Then compile the package:

Example

C:\Users\Your Name>javac -d . MyPackageClass.java

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:

Example

C:\Users\Your Name>java mypack.MyPackageClass

The output will be:

Creating Your Own Package — Quick Reference

// Step 1 — Declare the package at the TOP of the file
package com.myapp.utils;

// Step 2 — Define your class
public class StringHelper {
    public static String shout(String s) {
        return s.toUpperCase() + "!";
    }
}

// Step 3 — Use it from another file
import com.myapp.utils.StringHelper;

public class Main {
    public static void main(String[] args) {
        System.out.println(StringHelper.shout("hello"));
        // Output: HELLO!
    }
}

A complete runnable example

This complete Main.java program focuses on the lesson idea. Reference fragments above may demonstrate individual statements; this example includes the imports, entry point, and supporting types it needs.

import java.util.ArrayList;
public class Main {
    public static void main(String[] args) throws Exception {
        ArrayList<String> topics = new ArrayList<>();
        topics.add("Packages");
        System.out.println(topics.get(0));
    }
}

Packages

Change it. Run it. Understand it.

Load this lesson example into the editor. Change one value or statement, predict the result, then press RUN to execute Java.

Make it your own

Your learning checkpoint

• Explain the example without reading the output first.
• Make the change requested in the practice task.
• Describe why the result changed—or why compilation failed.

Ready to move on? Follow the next lesson in course order, or return to the curriculum to review a prerequisite.