Understanding Delimiters with Scanner.useDelimiter in Java

The Scanner class in Java provides the useDelimiter method, allowing you to specify a delimiter (a character or pattern) that separates tokens. However, using delimiters can be confusing for beginners. Let's break it down in simpler terms.

Consider the snippet:

sc = new Scanner(new File(dataFile));
sc.useDelimiter(",|\r\n");

In this example, the delimiter is defined as a combination of a comma (",") and a carriage return followed by a newline ("\r\n"). The scanner will use this delimiter to separate tokens in the data file.

Understanding Regular Expressions

The delimiter you specify in useDelimiter is a regular expression. Regular expressions are patterns used to match characters or sequences. The delimiter ",|\r\n" specifies that a token either ends with a comma or a carriage return followed by a newline.

An Example

Let's take an example:

String input = "1 fish 2 fish red fish blue fish";
Scanner s = new Scanner(input).useDelimiter("\\s*fish\\s*");
System.out.println(s.nextInt());   // prints: 1
System.out.println(s.nextInt());   // prints: 2
System.out.println(s.next());      // prints: red
System.out.println(s.next());      // prints: blue
s.close();

In this example, the delimiter is "\sfish\s." The \\s* indicates that there can be any number of whitespaces (including zero) before or after the word "fish." This means the scanner will match and extract tokens like "1 fish," "2 fish," and so on.

Conclusion

Remember that when using useDelimiter in Java, it's important to understand regular expressions. You can find detailed tutorials online to enhance your knowledge of regex patterns. With a proper understanding of delimiters and regular expressions, you can efficiently parse and extract data from text files using the Scanner class.