Question

How can I read 20 characters of a text file progressively, for example, If i have a function read_next, calling it the first time would return the first 20 characters in a string, calling it the second time would return the next 20 characters of the file. Please note, I don’t want to read the whole file into an array then break it up.

Answer 1

Basically, you want to use InputStream#read(byte[])

Reads some number of bytes from the input stream and stores them into
the buffer array b. The number of bytes actually read is returned as
an integer

public int read(InputStream is, byte[] bytes) throws IOException {
    return is.read(bytes);
}

Then you basically want to call this method…

byte[] bytes = new byte[20];
int bytesRead = -1;
while ((bytesRead = read(is, bytes)) != -1) {
    // Process the bytes..
    // Note, while bytes.length will always == 20
    // there will only ever be bytesRead worth of 
    // values in the array
}

UPDATED

After some nice feedback, you could also apply the same idea to UFT-8 encoded file using a Reader

public int read(Reader reader, char[] chars) throws IOException {
    return reader.read(chars);
}

And call the method as such…

Reader reader = new InputStreamReader(new FileInputStream("file"), "UTF-8");
char[] chars = new char[20];
int charsRead = -1;
while ((charsRead = read(reader, chars)) != -1) {
    // Process chars, the same caveats apply as above...
}