Want to get rid of an error in java

java.lang.StringIndexOutOfBoundsException: String index out of range: 0
at java.lang.String.charAt(String.java:687)
at rent.main(rent.java:24)

how to prevent this error?

It sounds like you have defined a list of strings(alpha numeric characters) and later in the sequence you are referring to a string in the list that is out of the defined range. For example if you define a list for 100 strings like 0-100 and you later refer to a string with an index of 101 or -1, they would be outside those boundaries.

To get rid of the error, you must either increase the boundaries or limit the range of indexes you're calling to numbers within the existing boundaries.

http://docs.oracle.com/javase/1.4.2/docs/api/java/lang/StringIndexOutOfBoundsException.html