NullPointerException 与片段中的蓝牙数据传输[重复]

Question

所以我基本上有这个问题。我试图通过蓝牙与 arduino 通信，一切正常，将数据从主要活动发送到 arduino，直到我尝试将数据从我的片段传输到 arduino，我在 logcat 中抛出一个错误：

**java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.administradora.prueba.MainActivity$ConnectedThread.write(java.lang.String)' on a null object reference**

这里是用于通信的 ConnectedThread 类......

public class ConnectedThread extends Thread {

    private final InputStream mmInStream;
    private final OutputStream mmOutStream;

    //creation of the connect thread
    public ConnectedThread(BluetoothSocket socket) {
        InputStream tmpIn = null;
        OutputStream tmpOut = null;

        try {
            //Create I/O streams for connection
            tmpIn = socket.getInputStream();
            tmpOut = socket.getOutputStream();
        } catch (IOException e) { }

        mmInStream = tmpIn;
        mmOutStream = tmpOut;
    }


    public void run() {
        byte[] buffer = new byte[256];
        int bytes;

        // Keep looping to listen for received messages
        while (true) {
            try {
                bytes = mmInStream.read(buffer);            //read bytes from input buffer
                String readMessage = new String(buffer, 0, bytes);
                // Send the obtained bytes to the UI Activity via handler
                bluetoothIn.obtainMessage(handlerState, bytes, -1, readMessage).sendToTarget();
            } catch (IOException e) {
                break;
            }
        }
    }
    //write method
    public void write(String input) {
        byte[] msgBuffer = input.getBytes();           //converts entered String into bytes
        try {
            mmOutStream.write(msgBuffer);                //write bytes over BT connection via outstream
        } catch (IOException e) {
            //if you cannot write, close the application
            Toast.makeText(getBaseContext(), "La Conexión fallo", Toast.LENGTH_LONG).show();
            finish();

        }
    }
}`

这是我尝试发送数据的片段 java 类：

public class exterior extends Fragment {

public MainActivity.ConnectedThread mConnectedThread;
public exterior() {

    // Required empty public constructor
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View v=inflater.inflate(R.layout.fragment_exterior,container,false);

    ToggleButton garage,lucext;
    garage=(ToggleButton)v.findViewById(R.id.garage);


    garage.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
        @Override
        public void onCheckedChanged(CompoundButton compoundButton, boolean isChecked) {

            if(isChecked){

                mConnectedThread.write("7");

                Toast.makeText(getActivity(),"On",Toast.LENGTH_SHORT).show();
            }
            else{

                Toast.makeText(getActivity(),"Off",Toast.LENGTH_SHORT).show();
            }
        }
    });

return v;
 }
}

（下面是我添加的试图修复错误的行，但似乎没有多大作用）***

public MainActivity.ConnectedThread mConnectedThread;

希望有人知道如何解决，谢谢大家。

Answer 1

首先你的类名应该以大写开头，所以将 external 改为 Exterior

由于您说您已经在主活动中初始化了一个 ConnectedThread 对象，因此您需要在片段中创建一个 setter 方法，该方法将在新创建的片段上从主 Activity 设置 mConnectedThread。

在您启动片段事务的 MainActivity 中

Exterior fragment = new Exterior(); 
            getSupportFragmentManager().beginTransaction().add(android.R.id.content, fragment).commit();

fragment.setConnectedThread(mConnectedThread);

然后在你的片段中添加这个方法

public void setConnectedThread(MainActivity.ConnectedThread 
mConnectedThread){
    this.mConnectedThread = mConnectedThread;
}