D:\cs151\chap80>java  Square 
Enter an integer:
Rats
Exception in thread "main" java.lang.NumberFormatException: Rats
        at java.lang.Integer.parseInt(Integer.java:409)
        at java.lang.Integer.parseInt(Integer.java:458)
        at Square .main(NFException.java:18)

D:\cs151\chap80>

Exceptions and Errors

Nothing is wrong with the program. The problem is that parseInt cannot convert "Rats" into an int. When parseInt found the problem it threw a NumberFormatException. The Java run-time system caught the exception, halted the program, and printed the error messages.

An exception is a problem that occurs when a program is running. Often the problem is caused by circustances outside the control of the program, such as bad user input. When an exception occurs, the Java virtual machine creates an object of class Exception which holds information about the problem. A Java program itself may catch an exception. It can then use the Exception object to recover from the problem.

An error, also, is a problem that occurs when a program is running. An error is represented by an object of class Error. But an error is too severe for a program to handle. The program must stop running.