Java中的阅读器在字符缓冲区的情况下如何内部工作？

Question

public class ReadFileUsingFileReader {

public static void main(String[] args) {
        String path = "D:\\sticky_notes.txt";
        readFileUsingFileReader(path);
    }

    public static void readFileUsingFileReader(String file) {

        try {
            //read the file
            FileReader reader = new FileReader(file);
            char[] buffer = new char[1024];
            int noOfCharsRead = reader.read(buffer);
            while (noOfCharsRead != -1) {
                System.out.println(String.valueOf(buffer, 0, noOfCharsRead));
                noOfCharsRead = reader.read(buffer);
            }
            reader.close();
     } catch (FileNotFoundException ex) {
            ex.printStackTrace();
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }
}

Here the variable "reader" reads the first 1024 characters from the file and save it into the buffer. Now again in the while loop, it reads the next 1024 characters. My question is how does in the second time reader comes to know from which index it should start reading the next characters. Is there some kind of flag, if yes how to access it.

Answer 1

This is the purpose of classes that implements the concept of opened files . There is an underlying structure which contains the current position, just like a bookmark when you read a book. Java FileReader doesn't provide you access to it because Readers almost just let you read from the beginning to the end. So the current position is exactly the number of bytes you actually read.

But you can have access to it if you use a RandomAccessFile on which you can call long getFilePointer() and void seek(long position) methods. RandomAccessFile is the only file manipulation class that let you control where input/output will occur.

---- EDIT (thanks EJP) ----

This is true only for package java.io . There exists another advanced IO package java.nio for which things are slightly different.