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.

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, for example, bad user input or a bad sector in a disk file. A Java program may have statements that catch exceptions. When an exception occurs, the Java virtual machine creates an object of class Exception (big 'E') which holds information about the problem. The Exception object is used 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.