Android app crashes on Bluetooth connect?

Question

I'm trying to connect to other Android devices using Bluetooth via my app. The app works fine while discovering nearby Bluetooth devices. However, upon connecting, the app crashes.

I have two JAVA files other than MainActivity.java that are responsible for discovering & connecting to other Bluetooth devices. Their codes are posted below:

SearchBTDevice.java (for discovering nearby devices)

package vertex2016.mvjce.edu.bluealert;

import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothGattDescriptor;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.content.pm.ActivityInfo;
import android.os.Bundle;
import android.bluetooth.BluetoothAdapter;
import android.provider.Settings;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.AttributeSet;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

import java.util.Set;

public class SearchBTDevice extends AppCompatActivity {

    public BluetoothAdapter BlueAdapter = BluetoothAdapter.getDefaultAdapter();
    public ArrayAdapter PairedArrayAdapter;
    public ArrayAdapter BTArrayAdapter;
    BluetoothDevice btd;

    public ListView devicesFound;


    private final BroadcastReceiver BTReceiver= new BroadcastReceiver(){

       public void onReceive(Context context, Intent intent)
       {
           String action = intent.getAction();

           if (BluetoothDevice.ACTION_FOUND.equals(action)) {
                   btd = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);

                   BTArrayAdapter.add(btd.getName() + "\t" + btd.getAddress() + "\n");

               }
           }

    };

    IntentFilter filter1 = new IntentFilter(BluetoothDevice.ACTION_FOUND);




    @Override
    protected void onResume() {
        super.onResume();
        this.registerReceiver(BTReceiver,filter1);



    }

    @Override
    protected void onPause() {
        super.onPause();
        BlueAdapter.cancelDiscovery();
        this.unregisterReceiver(BTReceiver);
        Toast.makeText(SearchBTDevice.this, "Discovery Stopped!!", Toast.LENGTH_SHORT).show();
    }


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_search_btdevice);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LOCKED);




       searchBTDevices();


    }


    public void searchBTDevices()
    {
        if(!BlueAdapter.startDiscovery())
            Toast.makeText(SearchBTDevice.this, "Failed to Start Discovery", Toast.LENGTH_SHORT).show();
        else
            Toast.makeText(SearchBTDevice.this, "Discovery Startred", Toast.LENGTH_SHORT).show();
        BTArrayAdapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1);
        devicesFound = (ListView)findViewById(R.id.searchpagelistView);
        devicesFound.setAdapter(BTArrayAdapter);
        devicesFound.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Intent connectedBTintent = new Intent(SearchBTDevice.this, ConnectedBTDevice.class);
                connectedBTintent.putExtra("BluetoothDevice", btd);
                startActivity(connectedBTintent);

            }
        });

    }


}

This is updated ConnectedBTDevice.java, responsible for connecting devices

package vertex2016.mvjce.edu.bluealert;

import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothSocket;
import android.content.pm.ActivityInfo;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import java.io.IOException;
import java.util.UUID;

public class ConnectedBTDevice extends AppCompatActivity {

    public BluetoothDevice btd;
    public BluetoothSocket btSocket, tempSocket;
    private UUID myUUID;
    ArrayAdapter arr;
    ListView lv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_connected_btdevice);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LOCKED);


        arr  = new ArrayAdapter(this, android.R.layout.simple_list_item_2);

        btd = getIntent().getParcelableExtra("BluetoothDevice");

        connectBT();
        displayStuff();

    }

    public void connectBT() {
        Thread myThread = new Thread() {

            public void run() {
                tempSocket = null;

                try {
                    tempSocket = btd.createRfcommSocketToServiceRecord(myUUID);
                } catch (IOException e) {
                    e.printStackTrace();
                }

                BluetoothAdapter.getDefaultAdapter().cancelDiscovery();

                try {
                    tempSocket.connect();
                    arr.add("CONNECTED TO-->" + btd.getName());
                } catch (IOException e) {
                    e.printStackTrace();
                    try {
                        tempSocket.close();
                    } catch (IOException e1) {
                        e1.printStackTrace();
                    }
                }


            }

        };
        myThread.start();
    }




    public void displayStuff()
    {
        lv = (ListView)findViewById(R.id.connectedBTlistView);
        lv.setAdapter(arr);
    }

}

This is activity_connected_btdevice.xml for ConnectedBTDevice.java activity

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context="vertex2016.mvjce.edu.bluealert.SearchBTDevice">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.NoActionBar.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.NoActionBar.PopupOverlay" />

    </android.support.design.widget.AppBarLayout>

    <include layout="@layout/content_connected_btdevice" />

</android.support.design.widget.CoordinatorLayout>

This is content_connected_btdevice.xml for ConnectedBTDevice.java

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"

    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="vertex2016.mvjce.edu.bluealert.ConnectedBTDevice"
    tools:showIn="@layout/activity_connected_btdevice">

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/connectedBTimageView"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true"
        android:src="@drawable/bluealert_bg"
        android:scaleType="centerCrop"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Connected Bluetooth Device"
        android:id="@+id/connectedBTtextextView"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="105dp"
        android:textSize="25dp"
        android:textAlignment="center"/>

    <ListView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/connectedBTlistView"
        android:layout_below="@+id/connectedBTtextextView"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="65dp" />


</RelativeLayout>

This is AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="vertex2016.mvjce.edu.bluealert">

    <uses-permission android:name="android.permission.BLUETOOTH" />
    <uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/bluealerticon"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme.NoActionBar">
        <activity
            android:name=".SplashScreen"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".MainActivity" />
        <activity
            android:name=".SearchBTDevice"
            android:label="@string/title_activity_search_btdevice"
            android:parentActivityName=".MainActivity"
            android:theme="@style/AppTheme.NoActionBar">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="vertex2016.mvjce.edu.bluealert.MainActivity" />
        </activity>
        <activity
            android:name=".ConnectedBTDevice"
            android:label="@string/title_activity_connected_btdevice"
            android:parentActivityName=".SearchBTDevice"
            android:theme="@style/AppTheme.NoActionBar">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="vertex2016.mvjce.edu.bluealert.SearchBTDevice" />
        </activity>
    </application>

</manifest>

Here's the exception that my updated logcat shows

03-24 00:19:40.541 7205-9703/vertex2016.mvjce.edu.bluealert E/AndroidRuntime: FATAL EXCEPTION: Thread-35890
                                                                              Process: vertex2016.mvjce.edu.bluealert, PID: 7205
                                                                              java.lang.NullPointerException: Attempt to invoke virtual method 'long java.util.UUID.getMostSignificantBits()' on a null object reference
                                                                                  at android.os.ParcelUuid.writeToParcel(ParcelUuid.java:129)
                                                                                  at android.bluetooth.IBluetooth$Stub$Proxy.connectSocket(IBluetooth.java:1767)
                                                                                  at android.bluetooth.BluetoothSocket.connect(BluetoothSocket.java:309)
                                                                                  at vertex2016.mvjce.edu.bluealert.ConnectedBTDevice$1.run(ConnectedBTDevice.java:63)

I don't understand what the problem is. I have tried several online tutorials, but nothing seemed to work. I know the problem is in my ConnectedBTDevice.java, but can't figure out the point at which it's throwing the exception.

Thank you for your time.

Answer 1

You are assigning BluetoothSocket to tempSocket, and then you try to invoke connect() method on btSocket which is null.