How do I engage a Bluetooth connection with another device?

Question

I'm working on a app that will connect to a OBD2 device through Bluetooth, but I haven't yet figured out how to connect to a Bluetooth device.

I followed a video tutorial, and am currently following this tutorial with hopefully better luck, but I don't really understand how to implement the code since I can't see the full code (I'm a beginner).

Any help is appreciated!

Image:

(Here's where I want to press on a device and connect to it.)

BluetoothActivity:

package com.example.asabanov.powersupplytool;

import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothSocket;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.view.Window;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.Set;
import java.util.UUID;

public class BluetoothActivity extends AppCompatActivity implements OnItemClickListener {

    public static final UUID MY_UUID = UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");
    protected static final int SUCCESS_CONNECT = 0;
    protected static final int MESSAGE_READ = 1;
    ArrayAdapter<String> listAdapter;
    ListView listView;
    BluetoothAdapter btAdapter;
    Set<BluetoothDevice> devicesArray;
    ArrayList<String> pairedDevices;
    ArrayList<BluetoothDevice> devices;
    IntentFilter filter;
    BroadcastReceiver receiver;
    //String tag = "debugging";

    Handler mHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            switch (msg.what) {
                case SUCCESS_CONNECT:
                    //do something
                    ConnectedThread connectedThread = new ConnectedThread((BluetoothSocket) msg.obj);
                    Toast.makeText(getApplicationContext(), "Connect", Toast.LENGTH_SHORT).show();
                    String s = "Successfully connected";
                    connectedThread.write(s.getBytes());
                    break;
                case MESSAGE_READ:
                    byte[] readBuf = (byte[]) msg.obj;
                    String string = new String(readBuf);
                    Toast.makeText(getApplicationContext(), "Connect", Toast.LENGTH_SHORT).show();
                    break;
            }
        }
    };

    @Override
    public void onCreate(Bundle savedInstanceState) {
        this.requestWindowFeature(Window.FEATURE_NO_TITLE);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_bluetooth);

        init(); //initiate Bluetooth
        if (btAdapter == null) {
            Toast.makeText(getApplicationContext(), "No bluetooth detected on this device", Toast.LENGTH_SHORT).show();
            finish();
        } else {
            if (!btAdapter.isEnabled()) {
                turnOnBT();
            }

            pairedDevices=getPairedDevices();
            startDiscovery();
        }


    }

    private void startDiscovery() {
        btAdapter.cancelDiscovery();
        btAdapter.startDiscovery();

    }

    private void turnOnBT() {
        Intent intent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
        startActivityForResult(intent, 1);
    }

    private ArrayList getPairedDevices() {
        devicesArray = btAdapter.getBondedDevices();
        if (devicesArray.size() > 0) {
            //add devices in array to list array
            for (BluetoothDevice device : devicesArray) {
                pairedDevices.add(device.getName() + "\n" + device.getAddress());
            }
        }
        return pairedDevices;
    }

    private void init() {
        listView = (ListView) findViewById(R.id.listView);
        listView.setOnItemClickListener(this);
        listAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, 0);
        listView.setAdapter(listAdapter);
        btAdapter = BluetoothAdapter.getDefaultAdapter();
        pairedDevices = new ArrayList<String>();
        filter = new IntentFilter(BluetoothDevice.ACTION_FOUND);
        devices = new ArrayList<BluetoothDevice>();
        receiver = new BroadcastReceiver() {
            @Override
            public void onReceive(Context context, Intent intent) {
                String action = intent.getAction();
                if (BluetoothDevice.ACTION_FOUND.equals(action)) {
                    BluetoothDevice device = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);

                    devices.add(device);
                    String s = "";

                    for (int a = 0; a < pairedDevices.size(); a++) {
                        if (device.getName().equals(pairedDevices.get(a))) {
                            s = "Paired";
                            break;
                        }
                    }
                    // matt-hp (paired)
                    listAdapter.add(device.getName() + " " + s + " " + "\n" + device.getAddress());
                } else if (BluetoothAdapter.ACTION_DISCOVERY_STARTED.equals(action)) {
                    //run some code


                } else if (BluetoothAdapter.ACTION_DISCOVERY_FINISHED.equals(action)) {
                    //run some code

                } else if (BluetoothAdapter.ACTION_STATE_CHANGED.equals(action)) {
                    if (btAdapter.getState() == btAdapter.STATE_OFF) {
                        turnOnBT();
                    }
                }
            }
        };
        registerReceiver(receiver, filter);
        IntentFilter filter = new IntentFilter(BluetoothAdapter.ACTION_DISCOVERY_STARTED);
        registerReceiver(receiver, filter);
        filter = new IntentFilter(BluetoothAdapter.ACTION_DISCOVERY_FINISHED);
        registerReceiver(receiver, filter);
        filter = new IntentFilter(BluetoothAdapter.ACTION_STATE_CHANGED);
        registerReceiver(receiver, filter);
    }

    @Override
    protected void onResume() {
        super.onResume();
        registerReceiver(receiver, filter);

    }

    @Override
    protected void onPause() {
        super.onPause();
        unregisterReceiver(receiver);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (resultCode == RESULT_CANCELED) {
            Toast.makeText(getApplicationContext(), "Bluetooth must be enabled to continue", Toast.LENGTH_SHORT).show();
            finish();
        }
    }

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        if (btAdapter.isDiscovering()) {
            btAdapter.cancelDiscovery();
        }

        if (listAdapter.getItem(position).contains("Paired")) {

            BluetoothDevice selectedDevice = devices.get(position);
            ConnectThread connect = new ConnectThread(selectedDevice);
            connect.start();
            Toast.makeText(getApplicationContext(), "Device is paired", Toast.LENGTH_SHORT).show();
        } else {
            Toast.makeText(getApplicationContext(), "Device is not paired", Toast.LENGTH_SHORT).show();
        }
    }

    private class ConnectThread extends Thread {
        private final BluetoothSocket mmSocket;
        private final BluetoothDevice mmDevice;

        public ConnectThread(BluetoothDevice device) {
            // Use a temporary object that is later assigned to mmSocket,
            // because mmSocket is final
            BluetoothSocket tmp = null;
            mmDevice = device;

            // Get a BluetoothSocket to connect with the given BluetoothDevice
            try {
                // MY_UUID is the app's UUID string, also used by the server code
                tmp = device.createRfcommSocketToServiceRecord(MY_UUID);
            } catch (IOException e) {
            }
            mmSocket = tmp;
        }

        public void run() {
            // Cancel discovery because it will slow down the connection
            btAdapter.cancelDiscovery();

            try {
                // Connect the device through the socket. This will block
                // until it succeeds or throws an exception
                mmSocket.connect();
            } catch (IOException connectException) {
                // Unable to connect; close the socket and get out
                try {
                    mmSocket.close();
                } catch (IOException closeException) {
                }
                return;
            }

            // Do work to manage the connection (in a separate thread)
            mHandler.obtainMessage(SUCCESS_CONNECT, mmSocket).sendToTarget();
        }


        /**
         * Will cancel an in-progress connection, and close the socket
         */
        public void cancel() {
            try {
                mmSocket.close();
            } catch (IOException e) {
            }
        }
    }

    private class ConnectedThread extends Thread {
        private final BluetoothSocket mmSocket;
        private final InputStream mmInStream;
        private final OutputStream mmOutStream;

        public ConnectedThread(BluetoothSocket socket) {
            mmSocket = socket;
            InputStream tmpIn = null;
            OutputStream tmpOut = null;

            // Get the input and output streams, using temp objects because
            // member streams are final
            try {
                tmpIn = socket.getInputStream();
                tmpOut = socket.getOutputStream();
            } catch (IOException e) {
            }

            mmInStream = tmpIn;
            mmOutStream = tmpOut;
        }

        public void run() {
            byte[] buffer;   // buffer store for the stream
            int bytes; // bytes returned from read()

            // Keep listening to the InputStream until an exception occurs
            while (true) {
                try {
                    // Read from the InputStream
                    buffer = new byte[1024];
                    bytes = mmInStream.read(buffer);
                    // Send the obtained bytes to the UI activity
                    mHandler.obtainMessage(MESSAGE_READ, bytes, -1, buffer)
                            .sendToTarget();

                } catch (IOException e) {
                    break;
                }
            }
        }

        /* Call this from the main activity to send data to the remote device */
        public void write(byte[] bytes) {
            try {
                mmOutStream.write(bytes);
            } catch (IOException e) {
            }
        }

        /* Call this from the main activity to shutdown the connection */
        public void cancel() {
            try {
                mmSocket.close();
            } catch (IOException e) {
            }
        }
    }
}