Your first program

public class Main {
    public static void main(String[] args) {
        System.out.println("Welcome to Java");
        System.out.println(2 + 3);
        System.out.print("No new line ");
        System.out.println("after print");
    }
}

Line by line

public class Main: all Java code lives inside a class. The file must be named after its public class, so this code goes in Main.java.

public static void main(String[] args): the main method, where the program starts.

• public: it can be called from outside the class (the JVM needs to call it),
• static: it belongs to the class itself, so no object has to be created first,
• void: it returns nothing,
• String[] args: command-line arguments, as an array of strings.

System.out.println(...) prints a value and then a new line. System.out.print(...) prints without the new line.

Braces { } mark the start and end of the class and of the method; semicolons end statements.

Case matters

Java is case-sensitive: system.out.println and Main versus main are different. Conventions:

Comments

// single-line comment
/* multi-line
   comment */
/** Javadoc comment, used to generate documentation */

Joining text

+ joins strings, and converts numbers to text when one side is a string:

public class Main {
    public static void main(String[] args) {
        int apples = 3;
        System.out.println("I have " + apples + " apples");
        System.out.println("Sum: " + 2 + 3);      // "Sum: 23"!
        System.out.println("Sum: " + (2 + 3));    // "Sum: 5"
    }
}

The second line surprises people: Java works left to right, so "Sum: " + 2 becomes text first, then 3 is joined on.