Output

The simplest way a program can communicate with its user is to print data on the screen.

The computer screen is called the standard output device ("standard out").
```
  System.out.println("You were " + age + " in 2010.");
```
Error messages are usually sent to the standard error device. This is also your computer screen, but could be set to a text file.
```
  System.err.println("Your age cannot be negative!");
```