Java Program Structure

1A class is the box; main is the door

A Java program you run lives in a class, and the machine starts at a method named main. Think of the class as the named box and main as the door.

Until later units you will put almost every line inside that door. The class name and the file name should match: TicketPrinter lives in TicketPrinter.java.

Figure. The outer box is the class. The inner box is main — the only place the machine starts. Lines you want to run on launch go inside that inner box.

How it works

1. A class is the box; main is the doorThe class is the named box. main is the door the machine starts at.

Smallest runnable ticket printer

public class TicketPrinter {
    public static void main(String[] args) {
        System.out.println("Seat A12");
    }
}

Coding lab. Print Meera's seat runs in the app, with checks on your output.

2The compiler reads tokens, not a story

The compiler does not read the file as a story. It splits the text into tokens: words, numbers, symbols, and quoted strings.

class, TicketPrinter, and { are separate tokens. A misspelled token is not almost right — it is a different token, or an error.

No diagram — the idea is carried by the prose or the Java listing.

3A statement is one instruction

A statement is one instruction the machine carries out. In Java most statements end with a semicolon.

A missing semicolon is not a style issue. The compiler cannot see where the instruction ended, so it stops.

Figure. A statement is one instruction. These two print lines are two statements, each ended by a semicolon. A missing semicolon is not style — the compiler cannot see where the instruction ended.

How it works

1. A statement is one instructionOne instruction, then a semicolon. The next instruction is a new statement.

Two statements, two semicolons

System.out.println("Show: 6pm");
System.out.println("Seat A12");

4Comments do not run; names are for people

A // comments out the rest of that line. A /* ... */ block can span lines. Comments do not run.

Style is for humans: class names like TicketPrinter, names that say what they hold. The compiler accepts a and b; a reader cannot tell which is the seat.

Figure. A // comment does not run. The println of Seat A12 does. Names are for the person reading the file; the compiler accepts a and b, a reader cannot tell which is the seat.

How it works

1. Comments do not run; names are for peopleComments do not run. Names are for the person reading the file.

Comment versus a statement that runs

// This line does not run.
System.out.println("Seat A12");
/* A block comment
   can cover several lines. */

5A backslash changes the next character

Inside a string, a backslash starts an escape. The backslash is not printed; it changes the next character.

\n is a new line, \t a tab, \" a quote that can sit inside quotes. A lone backslash at the end of a string is an error — it has nothing to change.

No diagram — the idea is carried by the prose or the Java listing.

One string, two lines of output

System.out.println("Seat A12\nPrice 200");

6Words before start, words after start

Command-line arguments arrive in main's args array — words typed after the program name, before it starts. args[0] is the first of those words, if any were given.

User input is different: the program is already running and waits. Scanner on System.in reads a line the person types. Do not treat args as a prompt; the person never saw a question.

Figure. args is ready at launch — words typed after the program name, before it starts. Scanner on System.in waits after launch. Do not treat args as a prompt; the person never saw a question.

How it works

1. Words before start, words after startargs is ready at launch. Scanner waits for a person after launch.

args[0] then a typed seat

import java.util.Scanner;

public class TicketPrinter {
    public static void main(String[] args) {
        System.out.println(args[0]);
        Scanner in = new Scanner(System.in);
        String seat = in.nextLine();
        System.out.println(seat);
    }
}