Java Program Structure and First Program

Anatomy of a Java File

A typical Java source file follows this order: an optional package declaration, optional import statements, and one or more class (or interface/enum/record) declarations. Only one public class is allowed per file, and if a public class exists, the file name must exactly match that class's name, including capitalization.

• package declaration (optional, must be the first line if present)
• import statements (optional, bring in classes from other packages)
• Class declaration(s) — exactly one may be public

The main Method

Execution begins at the main method, and the JVM looks for this exact signature: public static void main(String[] args). Each keyword has a specific reason for being there: public so the JVM (outside the class) can call it, static so it can be invoked without creating an object first, void because it returns nothing to the operating system, and String[] args to receive command-line arguments.

Compilation and Execution Flow

Java source code (.java) is compiled by javac into bytecode (.class). The JVM then loads that class, locates the main method, and begins executing instructions line by line, following normal control flow (sequential execution, conditionals, loops, and method calls).

Example

package com.webnest.basics; // optional, groups related classes

public class FirstProgram {          // file must be named FirstProgram.java

    public static void main(String[] args) {
        System.out.println("Hello, Webnest learners!");
        greet("Java");
    }

    static void greet(String language) {
        System.out.println("Welcome to " + language + " programming.");
    }
}

Hello, Webnest learners!
Welcome to Java programming.

Exercise

Write a program with a second method. greet should take a name and return a greeting, and main should print the greeting for two different names.

Expected output: Hello, Asha! then Hello, Ravi! on separate lines.

public class Greeter {

    // TODO: add a static method greet(String name) that returns "Hello, <name>!"

    public static void main(String[] args) {
        // TODO: print the greeting for "Asha" and for "Ravi"
    }
}

public class Greeter {

    static String greet(String name) {
        return "Hello, " + name + "!";
    }

    public static void main(String[] args) {
        System.out.println(greet("Asha"));
        System.out.println(greet("Ravi"));
    }
}

Quick quiz

Interview questions