# Booleans

How do we write code that tells us whether a user is logged in to our program? Booleans are the solution to these questions.

## What are Booleans?

A boolean refers to a value that is true or false. Those are the only values of a boolean expression, and these are useful if we want to check if something is true or false.

## Meet George Boole

Let's meet the fellow behind the name, "booleans," George Boole. Mr. Boole was an English-born mathematician, philosopher, and logician. He introduced boolean algebra in *The Mathematical Analysis of Logic* (1847) and *Investigation of the Laws of Thought* (1854). Many consider him to be one of the founders of Computer Science.

![George Boole](https://3899603656-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-M4CE_DjWtTxVmIYccGI%2F-M4CEbFz20voXskDd956%2F-M4CF5oRvhwkCeOhHvzf%2Fgeorge_boole.jpg?generation=1586139040330488\&alt=media)

## Working with Booleans

How about an example? Let's create a variable and set it equal to true. Then, we'll print the variable.

We first want to declare the variable, `loggedIn`, and set it to true or false.

```
boolean loggedIn = false;
```

We can similarly set the variable to true instead:

```
boolean loggedIn = true;
```

You can imagine a boolean variable as a box that can hold only the values true or false. The box below shows the current value of `loggedIn`:

![CodeHS](https://3899603656-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-M4CE_DjWtTxVmIYccGI%2F-M4CEbFz20voXskDd956%2F-M4CF5oT_Selz7qwi0Fe%2Fboolean_loggedin.png?generation=1586139040196554\&alt=media)

Notice that we do not need to have quotations around `true` or `false`.

The full example looks like:

```
// In this program we declare a boolean
// value to represent whether the user
// is logged in, and then print it out.
public class LoggedIn extends ConsoleProgram
{
    public void run()
    {
        boolean loggedIn = false;
        System.out.println("User logged in?: " + loggedIn);
    }
}
```

Remember, use booleans if you want to set a variable to true or false!