Java Crash Course: Package & Classes

In Java, a package is a namespace that organizes a set of related classes and interfaces. Packages help avoid name conflicts, manage code more efficiently, and control access to classes.

Packages

• Definition: A package is a way to group related classes, interfaces, and sub-packages together.

• Syntax: The package statement is the first line in a Java source file (if it's not part of the default package). For example:

  package com.yadavanuj.github;

• Purpose:

  • Namespace Management: Packages prevent name conflicts. For example, you can have two classes named Employee in different packages.

  • Code Organization: Packages help organize large codebases by logically grouping related classes.

  • Access Control: Packages can control the visibility of classes and methods using access modifiers like public, protected, and default (package-private).

Classes

• Definition: A class is a blueprint for creating objects. It encapsulates data (fields) and methods that operate on the data.

• Class Declaration:

  public class MyClass {
      // fields
      private int id;
      private String name;
      
      // constructor
      public MyClass(int id, String name) {
          this.id = id;
          this.name = name;
      }
      
      // methods
      public void displayInfo() {
          System.out.println("ID: " + id + ", Name: " + name);
      }
  }

• Class Access Levels:

  • Public: Accessible from any other class.

  • Default (Package-Private): Accessible only within the same package.

  • Protected: Accessible within the same package and by subclasses.

Using Packages and Classes Together:

• Importing Packages: To use classes from another package, you import them using the import statement:

  import com.yadavanuj.github.MyClass;

• Importing all classes inside a package: To use all or many classes from another package, you can import them all using *:

  import java.security.*;

• Fully Qualified Class Names: Alternatively, you can use the fully qualified name of a class (including its package) without importing:

  com.yadavanuj.github.MyClass obj = new com.yadavanuj.github.MyClass();

Common Java Packages:

• java.lang: Includes fundamental classes like String, System, and Math. This package is automatically imported.

• java.util: Contains utility classes like ArrayList, HashMap, and Date.

• java.io: Provides classes for input and output, like File, InputStream, and OutputStream.

• java.net: Contains classes for networking, such as Socket, URL, and InetAddress.

Organizing Classes into Packages

Let's say you're building an application with different modules, such as users, products, and orders. You might organize your classes into packages like this:

com/yadavanuj/github/
├── users/
│   ├── User.java
│   ├── UserService.java
├── products/
│   ├── Product.java
│   ├── ProductService.java
├── orders/
│   ├── Order.java
│   ├── OrderService.java

Usage in the same package

We do not need to import package when components (Class, Enum Interface, etc.) are in the same package.

User.java

package com.yadavanuj.github.users;

public class User {
    private String username;
    private String email;
    
    // constructor, getters, setters, etc.
}

UserService.java

package com.yadavanuj.github.users;

public class UserService {
    public void registerUser(User user) {
        // registration logic
    }
}

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