Question

我正在开发一个应用程序，其中蓝牙控制Arduino汽车。我试图获得一个开关来打开和关闭电机（现在是一个LED），但是当我使用以下代码运行应用程序时，它崩溃了。

if (on_off_switch.isChecked()) {
    command = "1";
    try {
        outputStream.write(command.getBytes()); //transmits the value of command to the bluetooth module
    } catch (IOException e) {
        e.printStackTrace();
    }
} else {
    command = "10";
    try {
        outputStream.write(command.getBytes());
    } catch (IOException e) {
        e.printStackTrace();
    }
}

但是当我运行没有该代码部分的应用程序时，它运行得很好。但是，当我使用代码运行它时，该应用程序无法启动，并且Logcat会说：

---------崩溃开始 2018-11-10 14：22：36.570 3311-3311 / com.example.btcar2 E / AndroidRuntime：FATAL EXCEPTION：main 进程：com.example.btcar2，PID：3311 java.lang.RuntimeException：无法启动活动ComponentInfo {com.example.btcar2 / com.example.btcar2.MainActivity}：java.lang.NullPointerException：尝试调用虚拟方法'void java.io.OutputStream.write（byte [ ]）'在空对象引用上在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2830）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2905）在android.app.ActivityThread.-wrap11（未知来源：0）在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1606）在android.os.Handler.dispatchMessage（Handler.java:105）在android.os.Looper.loop（Looper.java:169）在android.app.ActivityThread.main（ActivityThread.java:6595）在java.lang.reflect.Method.invoke（本机方法）在com.android.internal.os.Zygote $ MethodAndArgsCaller.run（Zygote.java:240）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:767）原因：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法'void java.io.OutputStream.write（byte []）' 在com.example.btcar2.MainActivity.onCreate（MainActivity.java:51）在android.app.Activity.performCreate（Activity.java:7016）在android.app.Instrumentation.callActivityOnCreate（Instrumentation.java:1214）在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2783）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2905）在android.app.ActivityThread.-wrap11（未知来源：0）在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1606）在android.os.Handler.dispatchMessage（Handler.java:105）在android.os.Looper.loop（Looper.java:169）在android.app.ActivityThread.main（ActivityThread.java:6595）在java.lang.reflect.Method.invoke（本机方法）在com.android.internal.os.Zygote $ MethodAndArgsCaller.run（Zygote.java:240） com.android.internal.os.ZygoteInit.main（ZygoteInit.java:767）

我不知道如何解决它。如果您有任何问题可以帮助我解决问题，请写信，谢谢。

这是我其余的代码

public class MainActivity extends AppCompatActivity {
    final String DEVICE_ADDRESS = "00:12:12:24:06:48"; //MAC Address of Bluetooth Module
    private final UUID PORT_UUID = UUID.fromString("00001101-0000-1000-8000-00805f9b34fb");

    private BluetoothDevice device;
    private BluetoothSocket socket;
    private OutputStream outputStream;

    Button bluetooth_connect_btn;

    String command; //string variable that will store value to be transmitted to the bluetooth module

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Switch on_off_switch = (Switch) findViewById(R.id.on_off_switch);
        on_off_switch.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {

            @Override
            public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
                Log.v("Switch State=", "" + isChecked);
            }

        });

        if (on_off_switch.isChecked()) {
            command = "1";
            try {
                outputStream.write(command.getBytes()); //transmits the value of command to the bluetooth module
            } catch (IOException e) {
                e.printStackTrace();
            }

        } else {
            command = "10";
            try {
                outputStream.write(command.getBytes());
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        bluetooth_connect_btn = (Button) findViewById(R.id.bluetooth_connect_btn);

        //Button that connects the device to the bluetooth module when pressed
        bluetooth_connect_btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                if (BTint()) {
                    BTconnect();
                }

            }
        });

    }

    //Initializes bluetooth module
    public boolean BTint() {
        boolean found = false;

        BluetoothAdapter bluetoothAdapter = BluetoothAdapter.getDefaultAdapter();

        if (bluetoothAdapter == null) //Checks if the device supports bluetooth
            Toast.makeText(getApplicationContext(), "Device doesn't support bluetooth", Toast.LENGTH_SHORT).show();

        if (!bluetoothAdapter.isEnabled()) //Checks if bluetooth is enabled. If not, the program will ask permission from the user to enable it
        {
            Intent enableAdapter = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(enableAdapter, 0);

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        Set<BluetoothDevice> bondedDevices = bluetoothAdapter.getBondedDevices();

        if (bondedDevices.isEmpty()) //Checks for paired bluetooth devices
            Toast.makeText(getApplicationContext(), "Please pair the device first", Toast.LENGTH_SHORT).show();
        else {
            for (BluetoothDevice iterator : bondedDevices) {
                if (iterator.getAddress().equals(DEVICE_ADDRESS)) {
                    device = iterator;
                    found = true;
                    break;
                }
            }
        }

        return found;
    }

    public boolean BTconnect() {
        boolean connected = true;

        try {
            socket = device.createRfcommSocketToServiceRecord(PORT_UUID); //Creates a socket to handle the outgoing connection
            socket.connect();

            Toast.makeText(getApplicationContext(),
                    "Connection to bluetooth device successful", Toast.LENGTH_LONG).show();
        } catch (IOException e) {
            e.printStackTrace();
            connected = false;
        }

        if (connected) {
            try {
                outputStream = socket.getOutputStream(); //gets the output stream of the socket
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        return connected;
    }

    @Override
    protected void onStart() {
        super.onStart();
    }

}

Answer 1

outputStream为null，您必须为其创建一个新对象，就像您在BTconnect() outputStream = socket.getOutputStream();

中所做的一样

Answer 2

正如Amjad Alwareh所说，outputStream对象为空。您可以从Logcat的此语句中看到这一点：

java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法'void java.io.OutputStream.write（byte []）'

通过输入此代码

if (on_off_switch.isChecked()) {
    command = "1";
    try {
        outputStream.write(command.getBytes()); //transmits the value of command to the bluetooth module
    } catch (IOException e) {
        e.printStackTrace();
    }
} else {
    command = "10";
    try {
        outputStream.write(command.getBytes());
    } catch (IOException e) {
        e.printStackTrace();
    }
}

在onCreate中，您正在尝试通过蓝牙模块执行某些操作，而无需确保您已建立连接。

我不知道您的设计以及您要达到的目标。但我建议将上面的代码放在这样的单独方法中：

private void performAction() {
    if (on_off_switch.isChecked()) {
        command = "1";
        try {
            outputStream.write(command.getBytes()); //transmits the value of command to the bluetooth module
        } catch (IOException e) {
            e.printStackTrace();
        }
    } else {
        command = "10";
        try {
            outputStream.write(command.getBytes());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

如果要在建立连接后立即进行操作，请在BTconnect()中调用它：

    // code before
    if (connected) {
        try {
            outputStream = socket.getOutputStream(); //gets the output stream of the socket
            performAction();  // call it here
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

或在onCheckedChanged

中调用此方法

@Override
public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
    Log.v("Switch State=", "" + isChecked);
    performAction();  // call it here
}

“ if（switch.isChecked（））”语句使我的应用崩溃

2 个答案: