I am getting ArrayOutOfBoundException on a byte[] array? And I don't know why

Question

I am trying to convert a file to byte array and send through socket to my server written in python. But when I try convert the file to byte I get this error -

java.lang.ArrayIndexOutOfBoundsException: length=1048576; regionStart=1048576; regionLength=1048576 

Code that runs when I press the upload button -

Button upload = binding.button2;
        upload.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //I have stored the path of the file which is a string in a arraylist 
                //"files" is an obj of that ArrayList
                System.out.println("Inside OnClick" + files.size()); 
                try {
                    for (int i = 0; i < files.size(); i++) {
                        Path path = Paths.get(files.get(i));
                        DataInputStream in = new DataInputStream(new FileInputStream(files.get(i)));
                        int size = (int)Files.size(path);
                        int iter = size/(1024 * 1024) + 1;
                        System.out.println(size + "  " + iter);
                        Thread send = new Thread(new Runnable() {
                                @Override
                                public void run() {
                                    try {
                                        ByteBuffer data = ByteBuffer.allocate(1048576); // 1048576 = 1MB
                                        DataOutputStream out = new DataOutputStream(SocketHandler.getSocket().getOutputStream());
                                        out.write("upl sample.mkv".getBytes(StandardCharsets.UTF_8));
                                        Thread.sleep(200);
                                        out.write(String.valueOf(size).getBytes(StandardCharsets.UTF_8));
                                        Thread.sleep(200);
                                        int off = 0;
                                        for(int j = 0;j < iter;j++){
                                            if(j == iter - 1){
                                                off += in.read(data.array(),off,size - off);
                                            }
                                            else {
                                                // This where I get the error 
                                                //And the error is always on the second iteration
                                                off += in.read(data.array(), off, 1048576); 
                                            }
                                            data.clear();
                                        }
                                        System.out.println(off);

                                    }catch(IOException | InterruptedException e){
                                        e.printStackTrace();
                                    }
                                }
                            });
                        send.start();
                        send.join();
                  }
                }catch(IOException | InterruptedException e){
                    e.printStackTrace();
                }
            }
        });

This is what was printed in logs -

D/ViewRootImpl@1d9e105[MainActivity2]: ViewPostIme pointer 1
I/System.out: Inside OnClick1
I/System.out: 259728246  248
E/AndroidRuntime: FATAL EXCEPTION: Thread-7
    Process: com.iot.ftpserver, PID: 30412
    java.lang.ArrayIndexOutOfBoundsException: length=1048576; regionStart=1048576; regionLength=1048576
        at java.util.Arrays.checkOffsetAndCount(Arrays.java:135)
        at libcore.io.IoBridge.read(IoBridge.java:496)
        at java.io.FileInputStream.read(FileInputStream.java:307)
        at java.io.DataInputStream.read(DataInputStream.java:152)
        at com.iot.ftpserver.ui.gallery.GalleryFragment$1$1.run(GalleryFragment.java:104)
        at java.lang.Thread.run(Thread.java:764)
I/Process: Sending signal. PID: 30412 SIG: 9

Can someone please help me? Thanks in advance