不太了解InputStream.read（byte []，int，int）的工作方式

Question

I know this is trivial, but it doesn't make sense to me. Java can't pass pointers/references as parameters, yet the read() function is passed a buffer into which the data is read , and only returns an int as the total number of bytes read into the buffer .

I am expecting to read five separate bytes from this device, but when I pass the function a buffer, and try to access it afterwards it continues to be null . If I print out the return value from the function it gives me the int 5 , which is expected. But how can I access the data which was actually put into the buffer?

Here is a link to the JavaDocs ....

EDIT:

This is the original call to the read function.

public boolean onOptionsItemSelected( MenuItem item ) {
    switch( item.getItemId() ) {
    case R.id.connect:
        startActivityForResult( new Intent( this, DeviceList.class ), 1 );
        return true;
    case R.id.readTest:
        Log.i(TAG,  "Before write." );
        byte[] b = {'$'};
        for( int i = 0 ; i < 3 ; i++ ) {
            mService.write( b );
        }
        Log.i(TAG, "After write." );
        return true;

    case R.id.readData:
        byte[] c = mService.read( 5 );

        Toast.makeText(this, Integer.toString( mService.bytes ), Toast.LENGTH_LONG).show();
    default:
        return super.onContextItemSelected( item );
    }
}

Note, this read function is a function declared in my class called BluetoothService. This class contains another class called ConnectedThread, which calls the InputStream read...

Here is MY read function....

public byte[] read( int length ) {
    Log.i( TAG, "Inside read." );
    ConnectedThread r;
    buffer = null;
    synchronized( this ) {
        if( mState != STATE_CONNECTED ) return null;
        r = mConnectedThread;
    }
    Log.i(TAG,  "Before run." );
    r.run( length );
    Log.i( TAG, "After run." );
    Log.i( TAG, Integer.toString( bytes ) );
    return buffer;

}

And here is the ConnectedThread class, which calls read itself....

/**
 * This thread runs during a connection with a remote device.
 * It handles all incoming and outgoing transmissions.
 */
private class ConnectedThread extends Thread {
    private final BluetoothSocket mmSocket;
    private final InputStream mmInStream;
    private final OutputStream mmOutStream;

    public ConnectedThread(BluetoothSocket socket, String socketType) {
        Log.d(TAG, "create ConnectedThread: " + socketType);
        mmSocket = socket;
        InputStream tmpIn = null;
        OutputStream tmpOut = null;

        // Get the BluetoothSocket input and output streams
        try {
            tmpIn = socket.getInputStream();
            tmpOut = socket.getOutputStream();
        } catch (IOException e) {
            Log.e(TAG, "temp sockets not created", e);
        }

        mmInStream = tmpIn;
        mmOutStream = tmpOut;
    }

    public void run(int length) {
        Log.i(TAG, "BEGIN mConnectedThread");
        byte[] buffer = new byte[1024];


        // Keep listening to the InputStream while connected
        //while (true) {
            try {
                // Read from the InputStream
                bytes = mmInStream.read(buffer, 0, length);

                // Send the obtained bytes to the UI Activity
                mHandler.obtainMessage(MainMenu.MESSAGE_READ, bytes, -1, buffer)
                        .sendToTarget();
            } catch (IOException e) {
                Log.e(TAG, "disconnected", e);
                connectionLost();
                // Start the service over to restart listening mode
                BluetoothService.this.start();
                //break;
            }
       // }
            Log.i(TAG, "MADE IT HERE" );
    }

    /**
     * Write to the connected OutStream.
     * @param buffer  The bytes to write
     */
    public void write(byte[] buffer) {
        try {
            mmOutStream.write(buffer);

            // Share the sent message back to the UI Activity
            mHandler.obtainMessage(MainMenu.MESSAGE_WRITE, -1, -1, buffer)
                    .sendToTarget();
        } catch (IOException e) {
            Log.e(TAG, "Exception during write", e);
        }
    }

    public void cancel() {
        try {
            mmSocket.close();
        } catch (IOException e) {
            Log.e(TAG, "close() of connect socket failed", e);
        }
    }
}

Answer 1

Java can't pass pointers/references as parameters

Yes it can.

A fundamental misconception like this would leave you very confused ...

The way that read(...) is supposed to be used is like this:

byte[] buffer = new byte[42];
int nosRead = is.read(buffer, 0, buffer.length);

You allocate a buffer object and pass it as a parameter to the read method. Bytes are read and copied into the buffer starting at (in this case) offset zero. When the method call returns, it returns the number of bytes actually read ... and the bytes are in the buffer ready for the caller to process.

As you can see, this relies on passing a byte array reference as a parameter to the read method.

Answer 2

您必须初始化数组，否则它将继续保持为空。

Answer 3

Perhaps you have a C background that confused you on making the call to read() appropriate. Instead of returning an array or creating one using a pointer to a pointer, read(byte[]) and read(byte[] b, int offset, int len) expect an actual byte array (in the latter form, of size bigger or equal to offset+len) in the argument list. Assuming the value x is returned from the method, the array elements from offset to offset+x-1 will be defined in the array (unless -1 was returned, which indicates an EOF).