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Ch 1.3 Writing Your First Java Program

Let's now learn how to write and execute Java code in practice.

1. Hello.java​

This is the most basic "Hello World" program.

public class Hello {
    public static void main(String[] args) {
        // Output text to the console
        System.out.println("Hello, Java LLC!");
    }
}

🔍 Code Explanation​

  • public class Hello: In Java, all code must reside within a class. The class name (Hello) must exactly match the file name (Hello.java).
  • public static void main(String[] args): This is the application's ** Entry Point**. When a Java program starts, it specifically looks for the main method.
  • System.out.println(...): A command that tells the system to print text to the console.

2. Java Execution Process​

Execution involves two distinct steps:

  1. Compilation: Using the javac Hello.java command, human-readable code is converted into bytecode (.class files) that the JVM understands.
  2. Execution: Using the java Hello command, the JVM executes the compiled bytecode.

3. Comments​

Comments are used to write notes or explanations. They do not affect the program's execution.

  • Single-line comment: Everything following //.
  • Multi-line comment: Everything contained between /* and */.

4. Common Errors for Beginners​

  • Missing Semicolon (;): Every statement in Java must end with a semicolon.
  • Case Sensitivity: Java treats main and Main as different entities.
  • Filename Mismatch: If the class name and the file name don't match, a compilation error will occur.
tip

In IntelliJ IDEA, typing psvm and hitting Tab automatically generates the main method. Similarly, typing sout followed by Tab expands to System.out.println.

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