Your first program

Problem

The computer does exactly what it says

The computer does not guess what you want. It runs commands, one after another, exactly as they are written. A sequence of such commands is a program.

Here is a whole program. It prints two lines on the screen:

Main.java

public class Main {
    public static void main(String[] args) {
        System.out.println("Welcome!");
        System.out.println("This is your first program.");
    }
}

Output

Welcome!
This is your first program.

It looks like a lot of code for two lines of text. Most of it is a wrapper that you write in every program. Your commands are only the two lines in the middle.

The idea

What is in the program

• public static void main(String[] args) is the place where the program starts. We call it main. The computer runs the lines in it from top to bottom.
• System.out.println(...) prints the text in the parentheses and moves to a new line.
• The text is in double quotes: "Welcome!". The quotes are not printed.
• Every command ends with ;.
• The curly braces { and } group the lines that belong together.

public class Main is the wrapper of every program. You will learn what it means in the "Classes and objects" module. For now you copy it and write your code inside main. Later in the lessons we show only the lines from main.

More precisely, if you are curious

• Uppercase and lowercase letters matter. system.out.println is not the same as System.out.println, and the program will not compile.

• The file is named after the class: public class Main is in a file called Main.java.
• From Java 25 on, a small program can also be written in a shorter way, without the class. Here we write the full form, because it works with every version and because that is how you will see it in other people's code.

The idea

Printing with and without a new line

Sometimes you want several pieces of text on one line. There is a second command for that: System.out.print. It prints the text and stays on the same line.

System.out.print("Order ");
System.out.print("#1024");
System.out.println(" is confirmed.");
System.out.println("Delivery: tomorrow");

Output

Order #1024 is confirmed.
Delivery: tomorrow

The first two lines of the code use print, so "Order", "#1024" and "is confirmed." come out together. After println the next text starts on a new line.

Pay attention to the spaces. They are inside the quotes, because the computer does not add a space by itself.

Follow the cursor: the text always comes out where it is.

System.out.print("Order ");
System.out.print("#1024");
System.out.println(" is confirmed.");
System.out.println("Delivery: tomorrow");

Step by step, in words

1. print prints "Order " and the cursor stays on the same line.
2. print prints "#1024" and the cursor stays on the same line.
3. println prints " is confirmed." and the cursor moves to a new line.
4. println prints "Delivery: tomorrow" and the cursor moves to a new line.
5. The spaces are inside the quotes: the computer does not add a space by itself.

Try it

On how many lines?

What will the program print?

System.out.print("Train ");
System.out.println("8:15");
System.out.print("Platform ");
System.out.print("3");
System.out.println();

Answer

Train 8:15
Platform 3

Train 8:15 Platform 3

Train
8:15
Platform
3

Check

The idea

Comments: notes for people

Sometimes you want to leave a note in the code: why it is like this, or what comes next. You write it as a comment. The computer skips comments.

• // starts a comment that runs to the end of the line.
• /* and */ surround a comment over several lines.

/* The coffee shop receipt.
   Prices are in euros. */
System.out.println("Coffee    €2.50");
// System.out.println("Croissant €1.80");
System.out.println("Total     €2.50");  // out of croissants

Output

Coffee    €2.50
Total     €2.50

The croissant line is code, but it is behind //, so it does not run. This is how you switch off a line for a while without deleting it.

A good comment says why, not what. The code itself shows what it does.

Try it

What will come out?

What will the program print?

System.out.println("9:00  Math");
// System.out.println("10:00 Physics");
System.out.println("11:00 History");  // lunch is next

Answer

9:00  Math
11:00 History

9:00  Math
10:00 Physics
11:00 History

9:00  Math
11:00 History  // lunch is next

Check

Problem

The program does not start

You wrote the program, you run it, and it does not even begin. Instead of output you get a message:

Main.java

public class Main {
    public static void main(String[] args) {
        System.out.println("Welcome to the course!")
        System.out.println("Here we go.");
    }
}

Message from the compiler

Main.java:3: error: ';' expected
        System.out.println("Welcome to the course!")
                                                    ^
1 error

Before it runs, the program goes through the compiler. It translates the code for the computer and checks that it is written by the rules. If it is not, the compiler stops and does not run anything. This is called a compile error.

The idea

How to read the message

The message tells you three things: the file, the line and what is wrong.

Main.java:3 means the file Main.java, line 3. ';' expected means "I expected a ;". The ^ under the code points to the place where the compiler noticed the problem.

The most common errors at the start:

• a ; is missing at the end of a command;
• a quote or a parenthesis is opened but not closed;
• a lowercase letter instead of an uppercase one: system.out.println.

Fix the first error in the list first. One missing quote can confuse the compiler and show several more errors further down, which go away along with it.

More precisely, if you are curious

• The compiler tells you where it noticed the problem. Sometimes the error is a little higher up: if you forgot to close a }, it finds out only at the end of the file.
• There are also errors that nobody catches: the program starts, but it does something other than what you want. We will hunt those in almost every lesson.

Try it

Find the bug

The program should print the exam schedule. It does not compile. Which line is the bug on?

Pick the line with the bug.

1System.out.println("Time: 10:30");2System.out.println("Room: 204);3System.out.println("Programming exam");

Check

Exercise

Your first programs

Solve the tasks in your editor. Each one is a separate program. Compare your output with the sample, character by character.

1. Hello World. The traditional first program. Print:

Output

Hello, World!

2. Business card. Print a business card of three lines: name, job and email. You can use your own.

Sample output

Maria Ivanova
web developer
maria@example.com

3. Frame. Print today's cafeteria menu in a frame of asterisks. Watch the spaces, so the right side comes out straight.

Output

***************
* Today: soup *
* €5.40       *
***************

More tasks

• One sentence in three parts. With three print commands and one println, print on one line "The shop is open from 9:00 to 19:00."
• A schedule with a comment. Print a schedule of four classes. Then switch off the third line with //, without deleting it, and add a comment saying why.

• An error on purpose. Remove the ; from a line in task 1, then remove a quote as well. Read what the compiler says each time.

Test

Check yourself

1. Where does the program start running?

At mainAt the last lineAt the first line of the file

2. What is the difference between print and println?

println moves to a new line after the text, print does notThere is no differenceprint prints faster

3. What does the computer do with a comment?

It runs it, if it is codeIt skips itIt prints it

4. The compiler says that ; is missing on line 3. What does that mean?

The program runs, but line 3 is skippedThe program does not run at all until you fix the errorThe compiler adds the ; itself

5. Which one will compile?

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

Check

Start the lesson over