Question

What is the point of the synchronization here?

Why not just use mConnectedThread.write(out)?

The code snippet is from the BluetoothChat sample for Android (found here)

    /**
 * Write to the ConnectedThread in an unsynchronized manner
 * @param out The bytes to write
 * @see ConnectedThread#write(byte[])
 */
public void write(byte[] out) {
    // Create temporary object
    ConnectedThread r;
    // Synchronize a copy of the ConnectedThread
    synchronized (this) {
        if (mState != STATE_CONNECTED) return;
        r = mConnectedThread;
    }
    // Perform the write unsynchronized
    r.write(out);
}

Answer 1

Synchronization is needed to ensure that you don’t have inconsistent state.

Without the synchronization, the code would be:

public void write(byte[] out) {
    if (mState != STATE_CONNECTED) return;
    mConnectedThread.write(out);
}

Now, if the connection where to close between the if statement check and the method invocation, mConnectedThread might be assigned to null, before the method invocation execution. That would result into a NullPointerException.