Of course! Since "Java source code" is a very broad topic, I'll provide you with several examples, starting from the most basic "Hello, World!" and progressing to more complex and practical examples.

Each example will include:

1. The Code: The complete Java source code.
2. Explanation: A breakdown of what the code does.
3. How to Compile and Run: Simple commands to get you started.
4. Key Concepts: A list of important terms and ideas introduced in the example.

The Absolute Basics: "Hello, World!"

This is the traditional first program for any new programmer. It simply prints a line of text to the console.

The Code (HelloWorld.java)

// This is a comment. The compiler ignores it.
// File name must be HelloWorld.java
public class HelloWorld {
    // This is the main method. It's the entry point of any Java application.
    public static void main(String[] args) {
        // This line prints the text "Hello, World!" to the console.
        System.out.println("Hello, World!");
    }
}

Explanation

• A single-line comment.
• public class HelloWorld: Defines a class named HelloWorld. In Java, all code resides inside a class. The file name must match the public class name (HelloWorld.java).
• public static void main(String[] args): This is the main method. The Java Virtual Machine (JVM) looks for this specific method to start executing the program.
  • public: The method can be called from anywhere.
  • static: The method belongs to the class itself, not to a specific instance of the class. You can call it without creating an object.
  • void: The method does not return any value.
  • String[] args: This is an array of strings that can hold command-line arguments.
• System.out.println("..."): This is a statement that prints the given string to the standard output (your console/terminal) and then moves the cursor to a new line.

How to Compile and Run

1. Save the code in a file named HelloWorld.java.
2. Open a terminal or command prompt.
3. Navigate to the directory where you saved the file.
4. Compile: javac HelloWorld.java
   • This command uses the Java compiler (javac) to create a bytecode file named HelloWorld.class.
5. Run: java HelloWorld
   • This command uses the Java Virtual Machine (java) to execute the compiled bytecode. You will see the output: Hello, World!

Key Concepts: Class, Method, main method, System.out.println, Compilation, Execution.

Variables and Basic Operations

This example shows how to declare variables of different types and perform basic arithmetic operations.

The Code (VariablesExample.java)

public class VariablesExample {
    public static void main(String[] args) {
        // --- Variable Declaration and Initialization ---
        // Primitive data types
        int integerVar = 10;
        double doubleVar = 20.5;
        char charVar = 'A';
        boolean booleanVar = true;
        // String is a class, not a primitive type, but it's very commonly used.
        String stringVar = "Hello Java!";
        // --- Basic Operations ---
        int sum = integerVar + 5;
        double product = doubleVar * 2;
        String concatenatedString = stringVar + " How are you?";
        // --- Print Results ---
        System.out.println("Integer Variable: " + integerVar);
        System.out.println("Double Variable: " + doubleVar);
        System.out.println("Character Variable: " + charVar);
        System.out.println("Boolean Variable: " + booleanVar);
        System.out.println("String Variable: " + stringVar);
        System.out.println("-------------------------------------");
        System.out.println("Sum (10 + 5): " + sum);
        System.out.println("Product (20.5 * 2): " + product);
        System.out.println("Concatenated String: " + concatenatedString);
    }
}

Explanation

• Variables: A named storage location in memory. We declare a variable's type and then assign it a value.
• Primitive Types: These are the most basic data types provided by Java.
  • int: For whole numbers (e.g., 10, -50, 0).
  • double: For floating-point numbers (numbers with decimals, e.g., 20.5, -3.14).
  • char: For a single character (e.g., 'A', 'z', '7').
  • boolean: For one of two values: true or false.
• Reference Type: String is a class used to represent a sequence of characters. It's not a primitive type.
• Concatenation: The operator is used for both addition and joining strings together. When you use with a String and another type, Java automatically converts the other type to a string and joins them.

How to Compile and Run

1. Save as VariablesExample.java.
2. Compile: javac VariablesExample.java
3. Run: java VariablesExample

Key Concepts: Variables, Data Types (int, double, char, boolean, String), Initialization, Arithmetic Operators, Concatenation.

Control Flow (if-else and for loop)

This example demonstrates conditional logic (if-else) and looping (for loop).

The Code (ControlFlow.java)

public class ControlFlow {
    public static void main(String[] args) {
        // --- IF-ELSE STATEMENT (Conditional Logic) ---
        int number = 15;
        System.out.println("--- IF-ELSE Example ---");
        if (number > 0) {
            System.out.println("The number is positive.");
        } else if (number < 0) {
            System.out.println("The number is negative.");
        } else {
            System.out.println("The number is zero.");
        }
        // --- FOR LOOP (Iteration) ---
        System.out.println("\n--- FOR Loop Example ---");
        System.out.println("Counting from 1 to 5:");
        for (int i = 1; i <= 5; i++) {
            System.out.println("Current count: " + i);
        }
    }
}

Explanation

• if-else statement: Executes a block of code if a condition is true. The else if and else blocks provide alternative paths if the previous conditions are false.
  • number > 0 is a boolean expression that evaluates to either true or false.
• for loop: Used for repeating a block of code a specific number of times.
  • int i = 1: Initialization. Runs once at the beginning.
  • i <= 5: Condition. The loop continues to run as long as this is true.
  • i++: Update. Runs at the end of each loop iteration. It increments i by 1.

How to Compile and Run

1. Save as ControlFlow.java.
2. Compile: javac ControlFlow.java
3. Run: java ControlFlow

Key Concepts: Conditional logic, if-else, for loop, iteration, comparison operators (>, <).

Object-Oriented Programming (OOP) - A Simple Class

This example introduces the core concepts of OOP: classes, objects, attributes, and methods.

The Code (Dog.java and DogDemo.java)

We need two files for this example.

File 1: Dog.java (The "Blueprint")

// This is the blueprint for a Dog object.
public class Dog {
    // Attributes (also called fields or instance variables)
    String breed;
    String name;
    int age;
    // Constructor: A special method to create and initialize an object.
    public Dog(String breed, String name, int age) {
        this.breed = breed;
        this.name = name;
        this.age = age;
    }
    // Method: A function that belongs to a class.
    public void bark() {
        System.out.println(name + " says: Woof! Woof!");
    }
    // Another method
    public void displayInfo() {
        System.out.println("Name: " + name);
        System.out.println("Breed: " + breed);
        System.out.println("Age: " + age);
    }
}

File 2: DogDemo.java (The "Driver" that uses the blueprint)

public class DogDemo {
    public static void main(String[] args) {
        // Create an instance (an object) of the Dog class
        Dog myDog = new Dog("Golden Retriever", "Buddy", 3);
        // Use the dot (.) operator to access the object's attributes and methods
        System.out.println("--- My Dog's Info ---");
        myDog.displayInfo();
        System.out.println("\n--- Action! ---");
        myDog.bark();
    }
}

Explanation

• Class (Dog.java): A blueprint for creating objects. It defines the attributes (data) and behaviors (methods) that objects of that type will have.
• Object (myDog in DogDemo.java): An instance of a class. It's a concrete entity created from the blueprint, with its own actual data.
• Constructor (public Dog(...)): A special method that is called when you create a new object (new Dog(...)). Its job is to initialize the object's attributes.
• Method (bark(), displayInfo()): Functions defined inside a class that define the behavior of objects.
• this keyword: Refers to the current instance of the class. this.name means "the name attribute of this specific object."
• DogDemo.java: This class contains the main method. It's the entry point and demonstrates how to create and use Dog objects.

How to Compile and Run

1. Save both files (Dog.java and DogDemo.java) in the same directory.
2. You can compile them together: javac *.java or compile them one by one.
3. Run the driver class: java DogDemo

Key Concepts: Object-Oriented Programming (OOP), Class, Object, Attribute/Field, Method, Constructor, this keyword.

Handling User Input with Scanner

This example shows how to get input from the user at the command line.

The Code (UserInput.java)

// Import the Scanner class from the java.util package
import java.util.Scanner;
public class UserInput {
    public static void main(String[] args) {
        // Create a Scanner object to read input from the console
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter your name: ");
        // Read a line of text (String) from the user
        String name = scanner.nextLine();
        System.out.print("Enter your age: ");
        // Read an integer from the user
        int age = scanner.nextInt();
        // Close the scanner to prevent resource leaks (good practice)
        scanner.close();
        // Display the information back to the user
        System.out.println("\n--- User Profile ---");
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Next year, you will be " + (age + 1) + " years old.");
    }
}

Explanation

• import java.util.Scanner;: Java has many built-in packages. This line tells the compiler we want to use the Scanner class, which is in the java.util package.
• Scanner scanner = new Scanner(System.in);: We create a new Scanner object that reads from standard input (System.in), which is the keyboard.
• scanner.nextLine(): Waits for the user to type a line of text and press Enter.
• scanner.nextInt(): Waits for the user to type an integer and press Enter.
• scanner.close(): It's important to close resources like Scanner when you're done with them to free up system resources.

How to Compile and Run

1. Save as UserInput.java.
2. Compile: javac UserInput.java
3. Run: java UserInput

   The program will pause and wait for you to type your name and age.

Key Concepts: Importing classes, Scanner object, reading user input (nextLine(), nextInt()), resource management (close()).

Where to Find More Java Source Code

If you want to explore more complex and real-world examples, you can look at these sources:

• GitHub: Search for projects with languages like "Java". You'll find everything from small utilities to large enterprise applications.
  • Apache Commons: A collection of reusable Java components.
  • Spring Framework: A very popular framework for building enterprise applications.
• Java Tutorials (Oracle): The official Oracle Java tutorials have many more detailed examples.
• Open-Source Projects: Look at the source code of applications you use. Many are open-source.

This should give you a solid foundation to understand and write Java source code. Let me know if you'd like an example on a more specific topic