Скачать или смотреть Fix Scanner with Regex: Reading Entire File in Java

Learn how to fix the common issue of Java's `Scanner` not reading the entire file due to regex mismatch, and discover an effective solution to read all words correctly in your input.
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This video is based on the question https://stackoverflow.com/q/63584722/ asked by the user 'C0DeX' ( https://stackoverflow.com/u/9576749/ ) and on the answer https://stackoverflow.com/a/63584926/ provided by the user 'jb.' ( https://stackoverflow.com/u/116286/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: Scanner and Regex Not Reading the Entire File

When working with file input in Java, many developers rely on the Scanner class for efficient parsing. However, issues can arise when using regex to tokenize words from a file. A common problem is that the Scanner will fail to read certain parts of the file, leading developers to believe they have reached the end of the text prematurely.

In this guide, we will explore why this happens and provide a detailed solution to ensure that your Scanner reads the entire file correctly.

The Problem Encountered

Consider the following scenario: you have a text file and a method that utilizes a regex to read words. However, upon execution, the Scanner only reads part of the file, leading to an incorrect count of words. Here’s the parsing method you might be using:

[[See Video to Reveal this Text or Code Snippet]]

Given an example input file, the output will only match some words, while ignoring others.

Why is This Happening?

The key issue here is how Scanner tokenizes text. By default, Scanner uses whitespace as a token separator. When combined with your specific regex, which also expects characters without trailing punctuation (like commas), it can lead to mismatches. If a token ends with a comma, like "bank,", it does not match the regex, causing the scanner to stop reading further.

The Solution: Simplifying the Regex Approach

To troubleshoot and resolve this issue, there’s an effective strategy you can employ:

Use Default Whitespace Tokens: Keep the default behavior of Scanner to use whitespace for token separation.

Match with Regex After Reading: Instead of coupling the regex with hasNext(), use it after you acquire each token.

Here’s the refactored code:

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Solution:

s.hasNext(): This continues to check for any tokens in the file, ignoring the regex for the separation.

Matcher m = wordPattern.matcher(s.next());: A Matcher is created for the token obtained from the Scanner.

if (m.find()): This checks if the token matches the defined regex for words.

Print the Matched Word: Finally, if a match is found, it prints the word.

Conclusion

In conclusion, the combination of Scanner and regex can lead to frustrating situations if not configured correctly. By understanding how tokens are read and applying regex after the fact, you can efficiently parse your files without missing critical content.

Make sure to adjust your code as suggested, and you should see a significant improvement in your file reading process. Happy coding!