Weird ByteBuffer Outputs

Question

I was working with ByteBuffers and IntBuffers when I came across this weird problem. Here's the code -

public static void main(String[] args) {
    File file = null;
    FileOutputStream fos = null;
    FileChannel fc = null;
    ByteBuffer bb = ByteBuffer.allocate(1024);
    IntBuffer ib = bb.asIntBuffer();

    ib.put(458215);
    // System.out.println(bb.getInt());  //uncomment this for the program to work flawlessly

    try {
        // Initialize with object references
        file = new File("E:/demjavafiles/text.txt");
        fos = new FileOutputStream(file);
        fc = fos.getChannel();
        System.out.println("Object refrence variables file, fos, fc initialized. Now writing to file....");
        System.out.println("before flip positon : " + bb.position()+ "\n limit: " + bb.limit());
        bb.flip();
        System.out.println("after flip positon : " + bb.position()+ "\n limit: " + bb.limit());

        fc.write(bb);// write buffer
        fc.close();
        System.out.println("buffer written successfully!!");

    } catch (Exception e) {
        System.outprintln("oh oh an error occured D: heres some message 'bout it.");
        e.getMessage();
        e.printStackTrace(System.out);
    } finally {

        System.out.println("End of program....");

    }

}

Now the program as you can see creates a ByteBuffer and an IntBuffer and using the put() method of the IntBuffer class it adds an int to the ByteBuffer as 4 bytes. When I ran the program with the System.out.println(bb.getInt()); before the try block statement commented this was my output-

Object reference variables file, fos, fc initialized. Now writing to file....
before flip position : 0
limit: 1024
after flip posiiton : 0
limit: 0
buffer written successfully!!
End of program....

Now when I ran it again with the System.out.println(bb.getInt()); statement uncommented this was my output-

458215
Object reference variables file, fos, fc initialized. Now writing to file....
before flip position : 4
limit: 1024
after flip position : 0
limit: 4
buffer written successfully!!
End of program....

So can anyone please tell me why is this happening?

@schmop, of course, this was just "by-the-way" comment, `slim` is right — Dmitry Ginzburg, May 07 '14 at 09:52
IntBuffer and ByteBuffer have different positions to the same underlying data. I would just not use an IntBuffer here, you can use plain ByteBuffer and it would be simpler. — Peter Lawrey, May 07 '14 at 09:54

schmop · Answer 1 · 2014-05-07T09:52:57.877

1

bb.getInt() advances the position in your buffer by four bytes (all java.nio buffers have an internal position for relative puts and gets). Use getInt(someIndex) - the absolute position variant - if you wish to keep the position.

edited May 07 '14 at 09:52

answered May 07 '14 at 09:45

schmop

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score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

From JavaDoc

public final Buffer flip()

Flips this buffer. The limit is set to the current position and then the position is set to zero. If the mark is defined then it is discarded.

Before you read anything from the buffer, position == 0, limit ==0. So after the flip, position == 0, limit == 0.

When you getInt(), position increments by 4. So position == 4, limit == 0. Then flip() does what the JavaDoc says: position == 0, limit == 4.

score 0 · Answer 3 · answered May 07 '14 at 09:59

You are maintaining two different Buffers over the same data and they don't have the same positions so when you write to one, it doesn't change the position of the other.

I suggest you step through your code in your debugger to see what is happening.

This is how I would write it.

public static void main(String... ignored) throws IOException {
    try (FileChannel fc = new FileOutputStream("binary.data").getChannel()) {
        ByteBuffer bb = ByteBuffer.allocate(1024);
        bb.putInt(458215);
        bb.flip();
        fc.write(bb);
        System.out.println("File has " + fc.size() + " bytes in it");
    }
}

prints

File has 4 bytes in it

Weird ByteBuffer Outputs

3 Answers3