The err field represents a PrintStream object : System Class « Development « Java Tutorial

Its purpose is to display error messages that should get immediate attention of the user.

public class MainClass {

  public static void main(String[] args) {
    System.err.println("You have a runtime error.");

  }

}

You have a runtime error.

6.1.System Class
	6.1.1.	java.lang.System
	6.1.2.	PrintStream has many print method overloads that accept different types
	6.1.3.	println adds a line terminator at the end of the argument
	6.1.4.	The err field represents a PrintStream object
	6.1.5.	The 'in' field represents the standard input stream
	6.1.6.	Array copy method
	6.1.7.	Terminates the running program and the current JVM
	6.1.8.	Returns the computer time in milliseconds
	6.1.9.	public static long nanoTime(): provide more precision, i.e. in nanoseconds
	6.1.10.	Using System.getProperty(String key) to retrieve system properties
	6.1.11.	Get Name of User Currently Logged In to the Computer
	6.1.12.	Create a user-defined property or change the value of the current property
	6.1.13.	Returns a default value if the specified property does not exist
	6.1.14.	Returns all system properties
	6.1.15.	Exiting a Java program
	6.1.16.	Terminate virtual machine using System class
	6.1.17.	Redirect the output of System.out to a file.
	6.1.18.	Calculate process elapsed time