Hello World

In the first lesson here, we’ll learn the basics of Java.

The First Program

Below¹ is the famous hello world program for Java.

main.java

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

Output

Hello World

The code above, when run, will display “Hello World” in the terminal. Although not the most exciting result, the code to do it might look intimidating. Let’s discuss what is going on here.

Curly Braces

Curly braces {} define code blocks. There are 2 sets of them in this program. They are meant to encapsulate code within a program.

Classes

Every line of code needs to reside within a class. Classes are named the same as the file they reside in. In our case, that’s Main, for main.java, with a capital M because of the fact that classes must always start with a capital letter. It’s also important to note that java is case sensitive, Hello and hello are not the same thing.

Main method

The main method is where code starts running.

public static void main(String[] args) {
    // Code
}

For right now, don’t worry about public, static, void and all the extra fluff. What’s important right now is that you understand that code starts executing from inside the main method.

Statements

This is what you are probably thinking about when you think of code. It’s what actually gets executed, it’s the instructions that are run. In this program, we have exactly one:

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

What this line does it put what’s between the parentheses onto the screen. In this case "Hello World", which is what we see when we run the program.

At the end of every statement, we put a semicolon ; to tell the computer that our statement is complete.

More?

One hello is kinda boring, but don’t worry, you can add as many statements as you want to a code block. Like so,

System.out.println("Hello World");
System.out.println("Goodbye World");
System.out.println("Me when I forgor the semicolon :(");

As you would expect the output follows suit,

Hello World
Goodbye World
Me when I forgor the semicolon :(

Next, let’s make it a bit more complicated. Look at the next piece of code, before I tell you what it does, take a guess at what it do.

System.out.println(6 + 7);

What it puts on the screen is the number 13, which you probably guessed it would do. What you can learn from this is that you can do some math in java by using some basic symbols. Here’s some more examples:

System.out.println(5 * 3 + 1);         // Prints 16

System.out.println(6 / 2 * (1 + 2));   // Prints 9

Comments

I’ve also included comments the previous example, any text put after a // will not be run. It’s helpful for describing what code does inline without having to create a word doc or something. You’ll see them all the time in code snippets to tell you what going on but later on, we’ll start to not write as many because they will get redundant. In the robot’s code, we’ll normally write them to explain complicated logic or strange decisions. You can do multi line comments like so, if you would like.

/* The following line will do the following:
1. Add 6 and 4 to create 10
2. Multiply that by 9 to create 90
3. Subtract 9 to create 81
4. Display 81 to the screen
*/
System.out.println(9 * (6 + 4) - 9);

It’s /* to start a comment and */ to end the multi-line comment. Anything in-between will be a comment.