当我尝试打开列表时应用崩溃

Question

I've been progamming a bluettoth app and i've done a list por conect to the paired device, there are no compiler errors, but when i try to open it on my phone the app crashes I would be grateful if someone could help me, here follows the list code, the MainActivity code and the logcat error:

public class MainActivity extends AppCompatActivity {
    BluetoothAdapter BtAdapter = null;
    private final int REQUEST_ENABLE_BT = 1;
    private final int REQUEST_BT_CONECTION = 2;
    boolean conexao = false;
    public static String MAC = null;
    BluetoothDevice myDevice = null;
    BluetoothSocket mySocket = null;
    Button Frente, Tras, Esquerda, DIreita, Conectar;
    UUID myUUID = UUID.fromString("00001101-0000-1000-8000-00805f9b34fb");

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

    BtAdapter = BluetoothAdapter.getDefaultAdapter();

    Frente = (Button)findViewById(R.id.Frente);
    Tras = (Button)findViewById(R.id.Tras);
    Esquerda = (Button)findViewById(R.id.Esquerda);
    DIreita = (Button)findViewById(R.id.Direita);
    Conectar = (Button)findViewById(R.id.Conectar);

    if(BtAdapter == null){
        Toast.makeText(getApplicationContext(), "Seu dispositivo nao possui bluetooth", Toast.LENGTH_LONG).show();
    } else if(!BtAdapter.isEnabled()){
        Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
        startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
    }

    Conectar.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if(conexao){

            } else {
                Intent openList = new Intent(MainActivity.this, DevicesList.class);
                startActivityForResult(openList, REQUEST_BT_CONECTION);

                //code not finhished
            }
        }
    });





}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    //super.onActivityResult(requestCode, resultCode, data);
    switch(requestCode){
        case REQUEST_ENABLE_BT:
            if(resultCode == Activity.RESULT_OK){

                Toast.makeText(getApplicationContext(), "O bluetooth foi ativado com sucesso", Toast.LENGTH_LONG).show();


            } else {

                Toast.makeText(getApplicationContext(), "O bluetooth nao foi ativado com sucesso, o app sera encerado", Toast.LENGTH_LONG).show();
                finish();

            }
        case REQUEST_BT_CONECTION:
            if (resultCode == Activity.RESULT_OK){

                MAC = data.getDataString();
                myDevice = BtAdapter.getRemoteDevice(MAC);
                try {
                mySocket = myDevice.createRfcommSocketToServiceRecord(myUUID);
                } catch (IOException erro) {

                }

            } else {

                Toast.makeText(getApplicationContext(), "Falha ao obter o endereco MAc do dispositivo", Toast.LENGTH_LONG);

            }
    }

}
}

This is the deviceListActivity:

public class DevicesList extends ListActivity {
public BluetoothAdapter BtAdapter2 = null;
    public static String ENDERECO_MAC = null;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        ArrayAdapter<String> ArrayBluetooth = new ArrayAdapter<String>(DevicesList.this, android.R.layout.simple_list_item_1);

        Set<BluetoothDevice> dispositivosPareados = BtAdapter2.getBondedDevices();

        if(dispositivosPareados.size() > 0){
            for(BluetoothDevice device : dispositivosPareados){
                String nomeBt = device.getName();
                String macBt = device.getAddress();
                ArrayBluetooth.add(nomeBt + "/n" + macBt);
            }
        }
        setListAdapter(ArrayBluetooth);
    }


    @Override
    protected void onListItemClick(ListView l, View v, int position, long id) {
        super.onListItemClick(l, v, position, id);

        String infomacaoGeral = ((TextView) v).getText().toString();
         String enderecoMac = infomacaoGeral.substring(infomacaoGeral.length() - 17);

        Toast.makeText(getApplicationContext(), "Info:" + infomacaoGeral, Toast.LENGTH_LONG);

        Intent retornaMac = new Intent();
        retornaMac.putExtra(enderecoMac, ENDERECO_MAC);
        setResult(RESULT_OK);
        finish();

    }
}

This is the error stacktrace:

Process: com.example.lordz.bluetoothproject, PID: 25942 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.lordz.bluetoothproject/com.example.lordz.bluetoothproject.DevicesList}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.Set android.bluetooth.BluetoothAdapter.getBondedDevices()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2955)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3030)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6938)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
      **Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.Set android.bluetooth.BluetoothAdapter.getBondedDevices()' on a null object reference**
        at com.example.lordz.bluetoothproject.DevicesList.onCreate(DevicesList.java:29)
        at android.app.Activity.performCreate(Activity.java:7183)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1220)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2908)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3030) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696) 
        at android.os.Handler.dispatchMessage(Handler.java:105) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6938) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374) 

Answer 1

Your error is in the onCreate method, precisely in this line:

Set<BluetoothDevice> dispositivosPareados = BtAdapter2.getBondedDevices();

BtAdapter2 is null because you've declared in Activy class as null.

public BluetoothAdapter BtAdapter2 = null;

I think you may want to first declare the BluetoothAdapter in onCreate and then use it:

// inside onCreate
BtAdapter2 = BluetoothAdapter.getDefaultAdapter();
Set<BluetoothDevice> dispositivosPareados = BtAdapter2.getBondedDevices();

Your app has to check also bluetooth permissions and it may ask user for them. For more references visit this stackoverflow question about bt permissions and also visit the bluetooth overview in official documentation .