Packages

In Java, packages are used to group related classes and interfaces into a single namespace. They are analogous to folders in a file directory and are a crucial mechanism for organizing code, preventing naming conflicts, and improving code maintainability.

Creating a Package

To place a class within a package, you use the package keyword at the very beginning of your Java source file. The package name typically follows a hierarchical naming pattern. For example, to create a package named com.example.myapp, you would have a corresponding directory structure src/com/example/myapp, and the Java files within that directory would start with the package com.example.myapp; declaration.

// This file would be located at src/com/example/myapp/MyClass.java
package com.example.myapp;

public class MyClass {
    // ...
}

Using a Class from Another Package

To use a class that resides in a different package, you must first import it using the import keyword. You have the option to import a single, specific class or all the classes within a package.

Importing a Single Class

To import a specific class, you provide its fully qualified name in the import statement.

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner myScanner = new Scanner(System.in);
        // ...
    }
}

Importing an Entire Package

If you need to use multiple classes from the same package, Java allows to import all of them at once using the * wildcard, although it is considered bad practice.

import java.util.*;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        Scanner myScanner = new Scanner(System.in);
        // ...
    }
}

Built-in Packages

Java provides a rich standard library organized into numerous built-in packages. Some of the most frequently used packages include java.lang, which contains fundamental classes and is automatically imported into every Java program; java.util, which provides the collections framework and other utility classes; java.io for input and output operations; java.net for networking; and java.awt for creating graphical user interfaces.