Chapter 4: First Java Program

This chapter walks through one small program and explains each moving part and how the JVM starts it.

1. Topic title

Your first Java program: public class, main, System.out.println, execution flow

2. What it means

A Java application starts at a single entry method with this exact shape (for now):

public static void main(String[] args) { ... }

The JVM loads the class you asked to run, finds this method, and executes it from the top of the method body downward (until branches, loops, or calls change order—later chapters).

3. Why it is used

Every standalone Java tool, student exercise, and many servers bootstrap through main. Frameworks like Spring still start from a generated main under the hood.

If you understand main, you understand where your code begins.

4. Real-world example

A Spring Boot service might hide main inside generated boilerplate, but production still launches with Java running a class that contains main. Your tiny Hello class is the same pattern, just without frameworks yet.

5. Java code example

public class FirstProgram {
    public static void main(String[] args) {
        System.out.println("My first Java program");
        if (args.length > 0) {
            System.out.println("First argument: " + args[0]);
        }
    }
}

Run without arguments:

javac FirstProgram.java
java FirstProgram

Output:

My first Java program

Run with an argument:

java FirstProgram Alice

Output:

My first Java program
First argument: Alice

6. Explanation of code

• public class FirstProgram — the class is a template for behavior. The public class name must match the filename FirstProgram.java.
• public static void main(String[] args) — public: JVM can call it from outside the class. static: belongs to the class, not a specific object instance (you have not learned objects deeply yet—treat static as “callable without creating an object”). void: returns nothing. String[] args: command-line arguments.
• System.out.println — writes a line to standard output (usually your terminal).

The JVM executes statements in order until it hits a branch (if) that skips a block.

7. Common mistakes

Filename does not match the public class name. public class FirstProgram must live in FirstProgram.java.

Writing Main instead of main. Java is case-sensitive. The JVM looks for main, not Main.

Running the wrong class name. If your class is FirstProgram, run java FirstProgram, not java FirstProgram.java.

Putting multiple public classes in one file. Only one public top-level class per file (with matching name).

8. Best practices

One class per file while learning. Keep life simple.

Use meaningful class names once you move past FirstProgram—TaxCalculatorDemo beats Test3.

Keep main thin later. For now it is fine to experiment in main. Soon you will call other methods from main so main stays readable.

9. Small practice task

Change the program to print two lines: your name on the first line, today’s date as plain text on the second (hard-coded string is fine). Recompile and run.

Beginner tip

When your IDE shows a green run gutter icon next to main, that is the same as typing java YourClass with the correct classpath. If the IDE run fails but terminal works, compare working directory and JDK settings.