Java Quickstart: Your First Java Program - datacouch-io/spark-java GitHub Wiki

Introduction

In Java, every application starts with a class, and this class must match the filename. In this quickstart guide, we will create our first Java file, MyClass.java, which will contain a "Hello World" message. This is a simple yet essential step to get started with Java programming.

Creating Your First Java File

Choose a Text Editor: You can use any text editor to create your Java file. Popular options include Notepad (on Windows), Visual Studio Code, or any code editor of your choice.
Write the Java Code: Open your text editor and create a new file named MyClass.java. In this file, you'll write the following Java code:
```
public class MyClass {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}
```
Here's what this code does:
- public class MyClass: This line defines a class named MyClass. In Java, classes should always start with an uppercase letter.
- public static void main(String[] args): This line defines the main method, which is the entry point of your Java program. It takes an array of strings (String[] args) as its parameter. We'll explain this in more detail shortly.
- System.out.println("Hello World!");: This line uses the println method from the System class to print the "Hello World!" message to the console.
Save the File: Save the file using the class name, MyClass, and add ".java" to the end of the filename. So, save it as MyClass.java.

Running Your Java Program

To run your Java program, follow these steps:

Open a Terminal/Command Prompt: You need to open a terminal or command prompt on your computer.
Navigate to the Directory: Use the cd command to navigate to the directory where your MyClass.java file is located. For example:

cd path/to/your/java/file

Replace path/to/your/java/file with the actual path to your file.

Compile the Java File: To compile your Java file, run the following command:

javac MyClass.java

This command tells the Java compiler (javac) to compile your MyClass.java file. If there are no errors in your code, this command will generate a bytecode file named MyClass.class.

Run the Java Program: After compiling successfully, run your Java program using the following command:

java MyClass

You should see the "Hello World!" message printed to the console:

Hello World!

Important Notes:

Java is Case-Sensitive: Keep in mind that Java is case-sensitive. For example, "MyClass" and "myclass" have different meanings.
Matching File Name and Class Name: The name of the Java file must exactly match the class name defined inside it. In our example, the filename is MyClass.java, and the class is MyClass.
Semicolons: Each code statement in Java must end with a semicolon (;).

Congratulations! You've just created and executed your first Java program. You're now ready to explore more complex Java concepts and build powerful applications.