Question

I would like to have a method that would return a list of BufferedReader objects (for example for all files in a directory):

private List<BufferedReader> getInputReaders(List<String> filenames) {
    List<BufferedReader> result = new ArrayList<BufferedReader>();
    for(String filename : filenames)
        result.add(new BufferedReader(new InputStreamReader(new FileInputStream(filename), "UTF-8")));
    }
    return result;
}

Will this be a major waste of resources?

Will all those streams be opened at the moment of creation and remain so therefore holding system resources?

If yes, can I create those readers in “passive” mode without actually opening streams, or is there any other workaround (so I can build a List with thousands of readers safely)?

Answer 1

Yes, the constructor for FileInputStream invokes open() in its constructor. open() is a native method, which will most likely reserve a file descriptor for the file.

Instead of immediately returning a list of BufferedReaders, why not return a list of something that will open the underlying stream as needed? You can create a class that holds onto a filename and simply open the resource when called.