TopolearnYou need to read the skeleton of a Java program fluently before anything else makes sense.
All Java code lives inside a class. When you launch a program, the JVM looks for a method with this exact signature and starts there:
public static void main(String[] args)public— callable from outside the class (the JVM needs that).static— belongs to the class itself, so no object has to be created first.void— returns nothing.String[] args— the command-line arguments, e.g.java App.java alice 42givesargs[0] == "alice".
Inside the method you write statements, each ending with a semicolon. Blocks of statements are grouped with braces { }. Java is case-sensitive: System and system are different names.
To print, use System.out.println(x) (adds a newline) or System.out.print(x) (does not). System.err is the error stream, useful for diagnostics.
Comments come in three flavours: // to end of line, /* block */, and /** Javadoc */ placed above classes and methods to generate API documentation.
Naming conventions are not enforced by the compiler but every Java developer follows them: classes in PascalCase, methods and variables in camelCase, constants in UPPER_SNAKE_CASE.
You may read about simplified "instance main methods" (void main() with no class). That is a preview feature in Java 21, so it needs --enable-preview; stick with the classic form for code that has to compile everywhere.
Example
/** A tiny program that greets everyone named on the command line. */
public class Greeter {
public static void main(String[] args) {
if (args.length == 0) {
System.err.println("usage: java Greeter.java NAME...");
return; // leave main early
}
for (String name : args) {
System.out.println("Hello, " + name + "!");
}
}
}
// java Greeter.java Ana Bo
// Hello, Ana!
// Hello, Bo!Remember
- Execution starts at
public static void main(String[] args) - Statements end with
;, blocks use{ }, and names are case-sensitive - Classes are PascalCase, methods and variables camelCase
Watch out for
- Misspelling the main signature (e.g.
Mainorstring[]): it compiles but the launcher reports that no main method was found.