Mastering Java Program Structure, Packages, and Interfaces

Posted on Nov 23, 2025 in Electronic Engineering

Mastering Java Program Structure

The basic structure of a Java program is based on the concept of classes. Every executable Java program must contain at least one class definition.

Essential Structure of a Java Application

A typical Java application follows this hierarchical structure:

Package Statement (Optional)
The very first statement in a Java source file (if present) is the package statement. It organizes classes into logical groups, preventing naming conflicts.
- Syntax: package package_name;
- Example: package com.mycompany.app;
Import Statements (Optional)
These statements are used to include classes from other packages. They allow you to use classes without specifying their full package name every time.
- Syntax: import package.ClassName; or import package.*;
- Example: import java.util.Scanner;
Class Definition (Mandatory)
This is the core of any Java program. All code must reside inside a class. The class serves as the blueprint for objects.
- Syntax: [access_modifier] class ClassName { ... }
- Example: public class HelloWorld { ... }
Fields/Instance Variables (Optional)
These are variables defined within the class but outside any method. They represent the state or properties of the object created from the class.
- Example (inside HelloWorld): private int number;
Methods (Optional, but main is mandatory for execution)
Methods contain the executable code and define the behavior of the class.
The main Method (Entry Point)
For a Java application to run, it must have a special method called main. The Java Virtual Machine (JVM) starts program execution here.
- Syntax: public static void main(String[] args) { ... }
- Explanation of Keywords:
  - public: The method is accessible from outside the class.
  - static: The method belongs to the class, not to a specific object. The JVM can call it without creating an object.
  - void: The method does not return a value.
  - main: The standard name for the entry point.
  - String[] args: An array of strings to accept command-line arguments.

Example Java Program Structure

This example demonstrates the typical components in a single file:

// 1. Package Statement (Optional)
package com.example.greetings;

// 2. Import Statement (Optional)
import java.io.PrintStream;

// 3. Class Definition (Mandatory)
public class SimpleProgram {

    // 4. Field/Instance Variable (Optional)
    String message = "Hello, Java!";

    // 5. Main Method (Mandatory for application execution)
    public static void main(String[] args) {
        
        // Code inside the main method (The program's execution begins here)
        System.out.println("Program started.");
        
        SimpleProgram sp = new SimpleProgram();
        System.out.println(sp.message);
        
        // Other methods can be called here
        sp.displayMessage();
    }

    // 5. Another Method (Optional)
    public void displayMessage() {
        System.out.println("This is a separate method.");
    }
}

Would you like to know more about the keywords used in the main method (like static and public)?

Interfaces and Behavioral Multiple Inheritance

Multiple Inheritance via Interfaces

Java avoids the complexities of multiple inheritance of classes (often called the “deadly diamond of death”) by allowing a class to implement multiple interfaces. An interface is a contract: it specifies what a class must do, but not how it must do it.

When a class implements multiple interfaces, it inherits the abstract method signatures (the “contracts”) from all of them, thereby combining multiple behavioral definitions without inheriting data or concrete implementation from multiple sources. This ensures the class has a definite implementation for all methods it promises to support, achieving a form of multiple inheritance for behavior rather than state.

Example: Combining Contracts with Interfaces

Here is an example demonstrating a class combining the capabilities defined in two different interfaces:

// Interface 1: Defines movement behavior
interface Mover {
    void accelerate();
}

// Interface 2: Defines communication behavior
interface Communicator {
    void sendSignal(String message);
}

// The Robot class implements multiple contracts
public class Robot implements Mover, Communicator {
    private String name = "Unit 734";
    
    // Implementation of Mover's contract
    @Override
    public void accelerate() {
        System.out.println(name + " is moving forward.");
    }

    // Implementation of Communicator's contract
    @Override
    public void sendSignal(String message) {
        System.out.println(name + " transmitting: \"" + message + "\"");
    }
    
    // The main method demonstrates the combined capabilities
    public static void main(String[] args) {
        Robot r = new Robot();
        r.accelerate();       // Uses Mover capability
        r.sendSignal("Status OK"); // Uses Communicator capability
    }
}

The Robot class now possesses the methods from both the Mover and Communicator interfaces, effectively achieving multiple behavioral inheritance.

Managing Code with Java Packages

Understanding Java Packages

A package is a fundamental mechanism in Java for organizing related classes and interfaces into a logical group, acting as a namespace similar to a folder structure on a file system. Packages serve three main purposes:

Preventing naming conflicts (e.g., two companies can have a Widget class if they are in different packages).
Controlling access (using modifiers like protected or default access).
Managing large projects by improving modularity.

The package statement must be the first line of the source file, and packages are typically named using a reverse domain name convention (e.g., com.mycompany.app) to ensure global uniqueness. The import keyword is used to access classes from other packages.

Creating a User-Defined Java Package

Creating a package involves two main actions: a code declaration and a physical directory structure. This process promotes modularity and prevents naming conflicts.

Choose a Name and Declare
Select a unique name for your package, typically using reverse domain name notation (e.g., com.mycompany.utility) with all letters in lowercase. The package statement must be the very first non-comment line in your Java source file.
- Syntax: package package.name;
- Example (in Calculator.java): package com.app.math;
Create the Directory Structure
The package name must precisely match the file system directory structure. For the package com.app.math, you must create a folder hierarchy: com/app/math/ (relative to your project’s source root). The class file must reside inside the final folder.
Compile
To compile a class within a package, you typically use the -d option with javac to specify the destination directory for the compiled .class files, ensuring the package structure is created automatically.
- Command: javac -d . Calculator.java (The . means place the files in the current directory’s package structure).

Accessing Classes from Other Packages

To use a class that is defined in another package, you have three primary ways to access it:

Using the import Statement (Preferred)
This is the most common and readable method. The import statement tells the compiler where to find the external class and allows you to refer to the class by its simple name.
- Importing a Single Class: import package.name.ClassName; (e.g., import com.app.math.Calculator;)
- Importing All Classes (Wildcard): import package.name.*; (Note: This does not import classes from sub-packages, only classes directly in the specified package.)
Using the Fully Qualified Name (FQN)
You can refer to the class directly by its complete name, which includes its package. This is often used when you have two classes with the same name from different packages (to avoid conflict).
```
// No import needed
com.app.math.Calculator calc = new com.app.math.Calculator();
```
Automatic Import (java.lang.*)
The java.lang package, which contains fundamental classes like String and System, is automatically imported into every Java program. You never need to explicitly write import java.lang.String;.

System Packages (Java SE API)

System packages are the built-in collections of classes and interfaces that constitute the Java Standard Edition (Java SE) API. These packages provide all the core, pre-written functionality needed to develop Java applications, effectively serving as the foundation of the language.

The most essential package is java.lang (containing classes like String, Math, and System), which is unique because it’s automatically imported into every Java program. Other crucial system packages include java.util (for collections like ArrayList and HashMap and utility classes like Scanner), java.io (for input/output operations), and java.net (for networking). To utilize the types within any system package other than java.lang, you must explicitly use the import statement.

User-Defined Package Example

This demonstrates defining and using a custom package:

// File Location: com/tools/Logger.java
package com.tools; // Declares the package

public class Logger {
    public void logMessage(String message) {
        System.out.println("LOG: " + message);
    }
}

// To use this in another class (MainApp.java in a different package):
import com.tools.Logger; 

public class MainApp {
    public static void main(String[] args) {
        Logger myLogger = new Logger();
        myLogger.logMessage("Application started.");
    }
}

Mastering Java Program Structure, Packages, and Interfaces