在RandomAccessFile中，为什么0是第一个位置，而8是第四个位置？

Question

Here is my program below,

public class RandomAccessDemo {
public static void main(String[] args) {
    double data[] = { 19.4, 10.1, 123.54, 33.0, 87.9, 74.25 };
    double d;
    // open and use a random access file
    try (RandomAccessFile raf = new RandomAccessFile("random", "rw")) {
        // write values to the file
        for (int i = 0; i < data.length; i++) {
            raf.writeDouble(data[i]);
        }
        // now read back specific values
        raf.seek(0);// seek to first double
        d = raf.readDouble();
        System.out.println("First Values is " + d);

        raf.seek(8);// seek to first double
        d = raf.readDouble();
        System.out.println("Second Values is " + d);

        raf.seek(8 * 3);// seek to first double
        d = raf.readDouble();
        System.out.println("Fourth Values is " + d);

        System.out.println();
        // Now read every other value
        System.out.println("Here is every other value:");
        for (int i = 0; i < data.length; i += 2) {
            raf.seek(8 * i);// seek to ith double
            d = raf.readDouble();
            System.out.println(d + " ");
        }

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

I was wondering why is 0 the first position, 8 the second value and 8 *3 the fourth value? What do these numbers correspond to? Also when it writes data to "random" does java create a file called random? I did not create the text file so where is this random file stored?

Answer 1

The positions are measured in bytes; 0 is the first byte, 1 is the second, etc. Doubles are 8 bytes each, so that's why you're getting the 8x behavior.

The word "random" in this context doesn't mean it's a random file, but that is a file you can access any part of at any time (as opposed to always having to start at the beginning and only going forward). From the system's perspective, you're able to access the file at random offsets. The file itself can be anywhere, and the arguments you pass to the constructor specify its path.

Answer 2

For your first question, Random access files permit nonsequential, or random, access to a file's contents. To access a file randomly, you open the file, seek a particular location, and read from or write to that file. Again, it is measured in bytes which is a 8 bit that's why it goes 0, 8, 8 * 2, and so on.

For your second question, "random" should be the location of your filename For eg. If you want to write your file at D: drive, you should create "D:\\random.txt" and it will generate random.txt file at the location D: drive. If you just create "D:\\random", yes it will create you a random file but it will be lacking what type of file it is.