Learn the Basic Structure of a Java Program

Basic Structure of a Java Program

In this tutorial, we will explore a Java program's basic structure and its includes. By understanding the basics of a Java program, you will be better equipped to write your code and build more advanced programs.

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public class Test
{
    public static void main(String[] args)
    {
        // your program goes here
    }
}

Now we will discuss the various parts of the basic structure of a Java program given above.

Class Declaration

A Java program may contain several class definitions, and classes are an essential part of any Java program. Every program in Java will have at least one class with a main method. The class that contains the main method does not need to be declared public, but if it is declared public, the class name must match the filename in which the program is saved.

Braces (Both Opening and Closing Brace)

The curly braces are used to group all the commands together. To make sure that the commands belong to a class or a method.

Main Method

In Java, the public static void main(String[] args) is treated as the entry point of the program. The main method is called by the Java Runtime Environment (JRE) when the user runs the program.

What is a Compiler

In Java, a compiler (the javac tool) is a special program that translates Java source code (.java files) into an intermediate, platform-independent code called Bytecode (.class files), which can then be executed by the Java Virtual Machine (JVM).

Steps Involved in Compiling and Running a Java Program

Compiling a Java program is like turning your handwritten instructions into something a computer can understand. When you write a Java program and save it with a .java extension, the journey from source code to execution goes through five main steps and they are:

Step 1: Write the Program

You write your Java program (source code) in a text editor or IDE (e.g., Notepad, Eclipse, IntelliJ). Save it with a .java extension. Example: Hello.java.

Step 2: Compilation

In this step, the compiler takes the source code file (for example, Hello.java) and check it for Syntax errors (grammar mistakes in code). It ensures type correctness. If there are no errors, it generates a bytecode file. Example Hello.class.

Step 3: Loading the Class (Class Loader)

When you run the program, the Class Loader inside the JVM loads the .class (bytecode) file into memory. It also loads any required built-in or user-defined classes.

Step 4: Bytecode Verification

JVM’s Bytecode Verifier checks the bytecode for: Security (no illegal memory access), Type safety (data types used correctly) and Format correctness. This prevents malicious or corrupted code from harming the system.

Step 5: Execution by JVM

The Interpreter inside the JVM starts executing bytecode instructions line by line. Frequently used code is optimized by the Just-In-Time (JIT) compiler, which converts bytecode into native machine code for faster execution.