Java and Object-Oriented Programming FoundationLesson 7 / 107

Programming and Java Fundamentals

Understanding the First Java Program

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Lesson Overview

In the previous lesson, we created and ran the following Java program:

public class Main {

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

Although the program is small, it contains several fundamental building blocks of a Java program.

In this lesson, we will understand:

  • What a class is
  • What public class Main means
  • The role of the main method
  • The basic meaning of public, static, and void
  • Why String[] args is present
  • What statements and code blocks are
  • What System.out.println() does
  • Where Java program execution begins

Learning Objectives

After completing this lesson, you will be able to:

  • Explain the structure of a basic Java program
  • Identify a class declaration
  • Understand the role of the main method
  • Explain the basic meaning of public, static, and void
  • Identify statements and code blocks
  • Display output using System.out.println()
  • Understand the roles of semicolons and braces
  • Explain the execution order of a basic program

Our First Program

public class Main {

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

Output:

Hello Java

Program structure:

Main class
└── main method
    └── output statement

Now let's look at each part.

public class Main

The first line is:

public class Main {

This is a class declaration.

Here:

  • public — access modifier
  • class — Java keyword
  • Main — class name
  • { — beginning of the class body

What Is a Class?

A class is a fundamental structure in a Java program.

A class can organize data and behavior.

In our first program:

public class Main {
}

Main is a class.

Later, we will create classes such as:

Student
Course
BankAccount
Order
GradeCalculator

When we learn object-oriented programming, we will understand the actual role of classes in much more detail.

For now, remember:

Java code is generally organized inside classes.

The class Keyword

class

is a reserved Java keyword.

It tells the compiler that we are declaring a class.

Example:

class Student {
}

Keywords have meanings defined by the language. Therefore, a keyword cannot normally be used as a variable or class name.

Class Name: Main

Main

is the name of the class.

Main is not a special or mandatory class name.

The following is also valid:

public class Application {

    public static void main(String[] args) {
        System.out.println("Application started");
    }
}

In this case, the source file would be named:

Application.java

The name of a public class and the source file name must match.

In our case:

Class: Main
File: Main.java

Class Naming Convention

Java class names generally follow the PascalCase convention.

Good examples:

Main
Student
Course
BankAccount
GradeCalculator

When a name contains multiple words, each word starts with a capital letter:

GradeCalculator
BankAccount
CourseEnrollment

public

public class Main

Here, public is an access modifier.

Access modifiers help control where a class or member can be accessed from.

For now, remember:

public indicates wider accessibility.

We will study access modifiers in detail when we learn OOP.

Curly Braces { }

The class body is placed inside curly braces:

public class Main {

}

The opening brace:

{

starts a block.

The closing brace:

}

ends a block.

Our program contains two nested blocks:

public class Main {                         // Class block starts

    public static void main(String[] args) { // Method block starts
        System.out.println("Hello Java");
    }                                       // Method block ends

}                                           // Class block ends

Structure:

Class
{
    Method
    {
        Statement
    }
}

The main Method

Now look at the second important line:

public static void main(String[] args) {

This is the declaration of the main method.

A basic Java application usually begins execution from this method.

The method name is:

main

The traditional Java application entry point is:

public static void main(String[] args)

What Is a Method?

A method is a named block of related instructions.

In our program:

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

Method name:

main

Method body:

{
    System.out.println("Hello Java");
}

Later, we will learn how to create our own methods.

Why Is main Important?

When a Java application is started, the runtime looks for a suitable main method and begins execution from there.

Example:

public class Main {

    public static void main(String[] args) {
        System.out.println("First");
        System.out.println("Second");
    }
}

Execution:

Enter main
    ↓
Print First
    ↓
Print Second
    ↓
Exit main

Output:

First
Second

public static void

At the beginning of the main method, we have:

public static void

For now, understanding the basic meaning of each part is enough.

public

public

The method can be accessible from outside when the application starts.

static

static

Because main is static, an object of the Main class does not need to be created before the main method can run.

For now, remember:

The main method is directly available through the class for application startup.

We will understand static in more depth after learning about classes and objects.

void

void

is the return type of the method.

void means that the method does not return a value.

Our main method runs the program but does not return a result to its caller.

String[] args

Inside the parentheses of the main method, we have:

String[] args

This is a parameter used to receive command-line arguments.

Here:

String[]

is the parameter type.

args

is the parameter name.

At this stage, you do not need to use arrays or command-line arguments.

You only need to recognize the standard main method signature:

public static void main(String[] args)

We will learn about String, arrays, and parameters in later lessons.

Parentheses ( )

Parentheses appear after a method name:

main(...)

In our case:

main(String[] args)

Parentheses can contain a method's input or parameter declaration.

Parentheses are also used when calling a method:

println(...)

This concept will become clearer when we learn methods.

Method Body

The instructions of the main method are placed inside curly braces:

public static void main(String[] args) {

}

Our method body is:

{
    System.out.println("Hello Java");
}

When the program executes, the statements inside the body run.

What Is a Statement?

Look at this line:

System.out.println("Hello Java");

This is a statement.

A statement is a complete instruction in a program.

Some other examples of statements are:

int age = 30;

int total = 10 + 20;

We will learn about these in later lessons.

For now, our statement:

System.out.println("Hello Java");

displays a message in the console.

Semicolon ;

At the end of the statement, we have:

;

Complete statement:

System.out.println("Hello Java");

Simple Java statements generally end with a semicolon.

Wrong:

System.out.println("Hello Java")

Correct:

System.out.println("Hello Java");

If the semicolon is missing, a compilation error will occur.

System.out.println()

System.out.println("Hello Java");

This statement sends output to the console.

Output:

Hello Java

At a high level, you can think of it as three parts:

System
    ↓
out
    ↓
println(...)

Now let's look at the basic meaning of each part.

System

System

is a class in the Java standard library.

out

System.out

represents standard output.

In our examples, the output appears in the console.

println

println(...)

is a method that prints a value and then moves to a new line.

Example:

System.out.println("Java");
System.out.println("Foundation");

Output:

Java
Foundation

"Hello Java"

"Hello Java"

is a text value.

In Java, text written inside double quotes is called a String literal.

Examples:

"Java"

"Hello Java"

"Java and OOP Foundation"

If you remove the double quotes, Java will not treat it as text.

Wrong:

System.out.println(Hello Java);

Correct:

System.out.println("Hello Java");

What Is the Dot . Doing?

In this expression:

System.out.println

the dot is used to access one member from another.

Conceptually:

System
└── out
    └── println()

You will see this kind of member access many times in Java code.

For now, just recognize the syntax. Its meaning will become clearer after we learn about objects and methods.

Java Is Case-Sensitive

Java treats uppercase and lowercase letters as different.

Correct:

System.out.println("Hello");

Wrong:

system.out.println("Hello");

Wrong:

System.Out.Println("Hello");

because:

System ≠ system
out ≠ Out
println ≠ Println

Similarly:

Main

and:

main

are not the same identifier.

Indentation

The Java compiler does not determine program structure from indentation, but indentation is extremely important for readable code.

Recommended:

public class Main {

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

Avoid writing code like this:

public class Main {
public static void main(String[] args) {
System.out.println("Hello Java");
}
}

The second version may compile, but it is difficult to read.

Execution Order

Look at the following program:

public class Main {

    public static void main(String[] args) {
        System.out.println("Step 1");
        System.out.println("Step 2");
        System.out.println("Step 3");
    }
}

Statements inside the main method generally execute from top to bottom.

Output:

Step 1
Step 2
Step 3

If the order of the statements changes, the order of the output will also change.

Complete Program Breakdown

Let's look at the complete program again:

public class Main {

    public static void main(String[] args) {
        System.out.println("Hello Java");
    }
}
CodeMeaning
publicAccess modifier
classKeyword used to declare a class
MainClass name
{ }Code block
staticMakes the class-level method available without creating an object
voidThe method does not return a value
mainApplication entry method
String[] argsParameter used to receive command-line arguments
SystemJava standard library class
outStandard output
printlnMethod used to print output
"Hello Java"String literal
;Statement terminator

Common Structural Errors

Missing Semicolon

Wrong:

System.out.println("Hello Java")

Correct:

System.out.println("Hello Java");

Missing Quote

Wrong:

System.out.println("Hello Java);

Correct:

System.out.println("Hello Java");

Missing Brace

Wrong:

public class Main {

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

Correct:

public class Main {

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

Wrong Capitalization

Wrong:

system.out.println("Hello Java");

Correct:

System.out.println("Hello Java");

Incorrect main Name

Wrong:

public static void Main(String[] args) {
}

Correct:

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

Java is case-sensitive.

An Important Point

Not every Java class needs a main method.

Example:

public class Student {
}

This is a valid class.

A main method is needed only in a class that we want to use as an application entry point.

A large application may contain many classes, but not all classes are application entry points.

Important Terms

Class

A structure used to organize Java code and related data or behavior.

Class Declaration

The syntax used to define a class.

public class Main {
}

Keyword

A reserved word in the Java language.

Examples:

public
class
static
void

Method

A named block that contains instructions.

main Method

The entry point of a basic Java application.

Statement

A complete instruction in a program.

Code Block

Code contained inside curly braces { }.

String Literal

A text value written inside double quotes.

Semicolon

Indicates the end of a simple statement.

Standard Output

The normal output destination of a program. In our examples, this is the console.

Practice Exercise 1: Identify the Parts of the Program

Look at the following program:

public class CourseApplication {

    public static void main(String[] args) {
        System.out.println("Course started");
    }
}

Identify:

  1. Class name
  2. class keyword
  3. Method name
  4. Return type
  5. Parameter
  6. Output statement
  7. String literal
  8. Statement terminator

Practice Exercise 2: Predict the Output

Before running the program, write the expected output:

public class Main {

    public static void main(String[] args) {
        System.out.println("Java");
        System.out.println("OOP");
        System.out.println("Foundation");
    }
}

Practice Exercise 3: Fix the Error

Fix the following program:

public class Main {

    public static void main(String[] args) {
        System.out.println("Java")
        System.out.println("Foundation");
    }
}

Expected output:

Java
Foundation

Practice Exercise 4: Fix the Brace Structure

Fix the following code:

public class Main {

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

Practice Exercise 5: Write Your Own Program

Create the following output:

Course: Java and OOP Foundation
Java Version: 21
Status: Learning Java

Write a complete Java program using a Main class and a main method.

Knowledge Check

Question 1

What is a Java class?

Question 2

What does the class keyword do?

Question 3

If the name of a public class is Main, what should the source file be named?

Question 4

Which method is the entry point of a basic Java application?

Question 5

What do you need to remember about static at this stage?

Question 6

What does void mean?

Question 7

What is String[] args used for?

Question 8

What is a statement?

Question 9

Which symbol generally ends a simple Java statement?

Question 10

What does System.out.println() do?

Question 11

What does it mean that Java is case-sensitive?

Question 12

Does every Java class need a main method?

Knowledge Check Answers

Answer 1

A class is a fundamental structure used to organize Java code and related data or behavior.

Answer 2

The class keyword is used to declare a class.

Answer 3

The source file should be named:

Main.java

Answer 4

A basic Java application usually begins execution from:

public static void main(String[] args)

Answer 5

Because main is static, an object of the Main class does not need to be created before the main method can run.

Answer 6

void means that the method does not return a value.

Answer 7

String[] args is a parameter used to receive command-line arguments.

Answer 8

A statement is a complete instruction in a program.

Answer 9

A simple statement generally ends with a semicolon:

;

Answer 10

System.out.println() prints a value to the console and moves to a new line afterward.

Answer 11

Java treats uppercase and lowercase letters as different.

For example:

Main
main

are not the same identifier.

Answer 12

No. Only a suitable entry point is needed to start application execution. Other classes do not need to contain a main method.

Lesson Summary

In this lesson, we learned:

  • Java code is generally organized inside classes
  • The class keyword declares a class
  • The name of a public class and its source file must match
  • Curly braces define class and method bodies
  • A basic Java application begins execution from the main method
  • Because main is static, an object does not need to be created before it runs
  • void means that a method does not return a value
  • String[] args is a parameter for receiving command-line arguments
  • A statement is a complete instruction
  • Simple statements generally end with a semicolon
  • System.out.println() displays output in the console
  • String literals are written inside double quotes
  • Java is case-sensitive
  • Statements inside a method generally execute from top to bottom