Write a class Scanner with the following interface:
- Scanner(String filename) -- creates a scanner object that reads from
- String next() -- returns the next token in the file, null if there
is no next token.
- boolean hasNext() -- returns true if there is another token
available (i.e., the file is open and it's not EOF), false otherwise.
The Scanner constructor opens the given file for reading text, and
nextToken() returns the next token in the file. For our purposes
a token is one of two things:
- A string of contiguous alphabetic and numeric
characters terminated by any non-aphabetic/non-numeric character (including
whitespace) or EOF.
- A single non-alphabetic, non-numeric character.
So the text below contains the 12 tokens shown:
public class Test5
Things to keep in mind:
- Whitespace may include spaces, tabs, and end-of-lines and
may be encountered at any place, and in any combination, in the file. In particular,
the file may start or end with whitespace.
- Most Java IO methods throw exceptions, so you will have to deal
with these using try and catch. However, your Scanner methods should not throw
any IO exceptions. If an IO exception is thrown when you
try to open the file (in the constructor), catch it, print an appropriate
warning message, and
initialize the scanner to indicate that there is no
next token (as if it were EOF). If an IO exception
is thrown while you are reading a token, catch it, print an appropriate
warning message, and return null.
- Since you are reading text from a file, you'll want to use a FileReader.
(You may want to refer to the online Java documentation at
http://java.sun.com/j2se/1.4.2/docs/api to review the FileReader API.)
Remember that FileReader has a int read() method that returns the
character read, or -1 if it's EOF. Of course, to use the returned value
as a character you'll have to cast it to char.
- Since you detect EOF based on the value of the last character read, you
may find it useful to store that character in an instance variable. This
variable will retain its value between calls to next(), and can be
used to determine what hasNext will return.
- You may find it useful to write a little method to skip whitespace, leaving
the first non-whitespace character in an instance variable.
- You may also find it useful to write a little method to read an alphanumeric
token once the first character of one is encountered.