java CharsetDetector: How to detect ASCII

Question

i am using CharsetDetector to detect the charset of a text file.

This is the code to detect the charset of the given file:

private String getCharset(File file) {
        String charset = "";
        try {
            InputStream is = new FileInputStream(file);
            BufferedInputStream bis = new BufferedInputStream(is);
            CharsetDetector cd = new CharsetDetector();
            cd.setText(bis);
            CharsetMatch cm = cd.detect();
            if (cm != null) {
               Reader reader = cm.getReader();
               charset = cm.getName();
            }
            bis.close();
            is.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return charset;
    } 

For a ASCII text file it returns UTF-8. ASCII is a subset of UTF-8 but i like to detect ASCII if it is ascii only and UTF-8 if there is a sign which is not in ASCII.

But how can i check it?

Answer 1

First, I reviewed your code and want to put out some hints:

1. Your method first reads through the entire file to detect the character set using CharsetDetector, and then it read through the file again to check if all the characters are ASCII. For large files, this can be inefficient since you are essentially doubling the I/O operations required.
2. In your code, if the CharsetMatch was null (i.e., the CharsetDetector couldn't determine the charset), you returned an empty string. You may want to return a sensible default charset or handle this case explicitly.

My suggestion would be to improve your method like this:

import java.io.*;
import java.nio.charset.Charset;

public class CharsetDetection {
    public static void main(String[] args) {
        File file = new File("path_to_your_file.txt");
        String charset = getCharset(file);
        System.out.println("Detected Charset: " + charset);
    }

    private static String getCharset(File file) {
        String charset = "UTF-8"; // Default to UTF-8
        boolean isPureAscii = true;

        try (InputStream is = new FileInputStream(file);
             BufferedInputStream bis = new BufferedInputStream(is)) {

            int b;
            while ((b = bis.read()) != -1) {
                if (b > 127) { // Non-ASCII character found
                    isPureAscii = false;
                    break; // No need to continue checking, it's not ASCII
                }
            }

            // If all characters are ASCII, set charset to US-ASCII
            if (isPureAscii) {
                charset = "US-ASCII";
            }

        } catch (IOException e) {
            e.printStackTrace();
        }

        return charset;
    }
}

This method is more efficient because it only requires a single pass through the file and stops as soon as it finds a non-ASCII character. However, it doesn't use the CharsetDetector class. If you need to detect character sets other than ASCII and UTF-8, you might need a more complex solution.

Good Luck!