How to extract starting of a String in Java

Question

I have a text file with more than 20,000 lines and i need to extract specific line from it. The output of this program is completely blank file.

There are 20,000 lines

Answer 1

We will assume that you use Java 7, since this is 2014.

Here is a method which will return a List<String> where each element is an ISDN:

private static final Pattern ISDN = Pattern.compile("ISDN=(.*)");

// ...

public List<String> getISDNsFromFile(final String fileName)
    throws IOException
{
    final Path path = Paths.get(fileName);
    final List<String> ret = new ArrayList<>();

    Matcher m;
    String line;

    try (
        final BufferedReader reader
            = Files.newBufferedReader(path, StandardCharsets.UTF_8);
    ) {
        while ((line = reader.readLine()) != null) {
            m = ISDN.matcher(line);
            if (m.matches())
                ret.add(m.group(1));
        }
    }

    return ret;
}