Your First Java Program: Hello World Explained Line by Line

The traditional first program prints a greeting. In Java it takes a few more lines than in some languages, and each of them teaches you something about how Java programs are built.

The program#

Save this as HelloWorld.java:

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Compile and run it:

javac HelloWorld.java
java HelloWorld

Hello, World!

Line by line#

Line 1: public class HelloWorld { declares a class named HelloWorld. All Java code lives inside classes. public means it can be used from anywhere, and a public class must be saved in a file with the same name (HelloWorld.java). The opening brace starts the class body.

Line 2: public static void main(String[] args) { declares the main method, where the program starts. Every word matters:

Line 3: System.out.println("Hello, World!"); prints the text and then moves to a new line.

• System.out is the standard output stream, normally your terminal.
• println means "print line".
• "Hello, World!" is a string literal in double quotes.
• The semicolon ends the statement.

Lines 4 and 5 close the method and the class. Every { needs a matching }.

print, println and printf#

public class Main {
    public static void main(String[] args) {
        System.out.print("Java ");
        System.out.print("is ");
        System.out.println("fun.");
        System.out.println("New line.");
        System.out.printf("%s is %d years old.%n", "Asha", 21);
        System.out.println("2 + 3 = " + (2 + 3));
    }
}

Java is fun.
New line.
Asha is 21 years old.
2 + 3 = 5

• print does not add a newline; println does.
• printf formats values into placeholders: %s for text, %d for whole numbers, %.2f for a decimal with 2 places, and %n for a newline.
• + joins strings. The parentheses in (2 + 3) make Java add the numbers first; without them, "2 + 3 = " + 2 + 3 would give 2 + 3 = 23.

Escape sequences#

A backslash inside a string starts an escape sequence:

public class Main {
    public static void main(String[] args) {
        System.out.println("Name:\tAsha");
        System.out.println("She said \"hi\".");
        System.out.println("Path: C:\\Users\\asha");
    }
}

Name:	Asha
She said "hi".
Path: C:\Users\asha

Reading compiler errors#

If something is wrong, javac stops and reports where. For example, without the semicolon:

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!")
    }
}

javac reports:

HelloWorld.java:3: error: ';' expected
        System.out.println("Hello, World!")
                                           ^
1 error

The message gives the file, the line number, what was expected, and a caret pointing at the spot. Fix the first error first; later ones are often caused by it.

Common mistakes#

Practice#

Name:	Asha
City:	Pune
Score:	92.50

public class Profile {
    public static void main(String[] args) {
        System.out.println("Name:\tAsha");
        System.out.println("City:\tPune");
        System.out.printf("Score:\t%.2f%n", 92.5);
    }
}

Name:	Asha
City:	Pune
Score:	92.50

Key takeaway#