R.java file is not there

Question

The below given is the code for MyActivity.java file.

package com.technomentis.ledonoff;


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


import android.app.Activity;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothSocket;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;
import android.R;

public class MainActivity extends Activity {
    private static final String TAG = "LEDOnOff";

    Button btnOn, btnOff;

    private static final int REQUEST_ENABLE_BT = 1;
    private BluetoothAdapter btAdapter = null;
    private BluetoothSocket btSocket = null;
    private OutputStream outStream = null;

    // Well known SPP UUID
    private static final UUID MY_UUID =
            UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");

    // Insert your server's MAC address
    private static String address = "00:00:00:00:00:00";

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Log.d(TAG, "In onCreate()");

        setContentView(R.layout.main);

        btnOn = (Button) findViewById(R.id.btnOn);
        btnOff = (Button) findViewById(R.id.btnOff);

        btAdapter = BluetoothAdapter.getDefaultAdapter();
        checkBTState();

        btnOn.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
                sendData("1");
                Toast msg = Toast.makeText(getBaseContext(),
                        "You have clicked On", Toast.LENGTH_SHORT);
                msg.show();
            }
        });

        btnOff.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
                sendData("0");
                Toast msg = Toast.makeText(getBaseContext(),
                        "You have clicked Off", Toast.LENGTH_SHORT);
                msg.show();
            }
        });
    }

    @Override
    public void onResume() {
        super.onResume();

        Log.d(TAG, "...In onResume - Attempting client connect...");

        // Set up a pointer to the remote node using it's address.
        BluetoothDevice device = btAdapter.getRemoteDevice(address);

        // Two things are needed to make a connection:
        //   A MAC address, which we got above.
        //   A Service ID or UUID.  In this case we are using the
        //     UUID for SPP.
        try {
            btSocket = device.createRfcommSocketToServiceRecord(MY_UUID);
        } catch (IOException e) {
            errorExit("Fatal Error", "In onResume() and socket create failed: " + e.getMessage() + ".");
        }

        // Discovery is resource intensive.  Make sure it isn't going on
        // when you attempt to connect and pass your message.
        btAdapter.cancelDiscovery();

        // Establish the connection.  This will block until it connects.
        Log.d(TAG, "...Connecting to Remote...");
        try {
            btSocket.connect();
            Log.d(TAG, "...Connection established and data link opened...");
        } catch (IOException e) {
            try {
                btSocket.close();
            } catch (IOException e2) {
                errorExit("Fatal Error", "In onResume() and unable to close socket during connection failure" + e2.getMessage() + ".");
            }
        }

        // Create a data stream so we can talk to server.
        Log.d(TAG, "...Creating Socket...");

        try {
            outStream = btSocket.getOutputStream();
        } catch (IOException e) {
            errorExit("Fatal Error", "In onResume() and output stream creation failed:" + e.getMessage() + ".");
        }
    }

    @Override
    public void onPause() {
        super.onPause();

        Log.d(TAG, "...In onPause()...");

        if (outStream != null) {
            try {
                outStream.flush();
            } catch (IOException e) {
                errorExit("Fatal Error", "In onPause() and failed to flush output stream: " + e.getMessage() + ".");
            }
        }

        try     {
            btSocket.close();
        } catch (IOException e2) {
            errorExit("Fatal Error", "In onPause() and failed to close socket." + e2.getMessage() + ".");
        }
    }

    private void checkBTState() {
        // Check for Bluetooth support and then check to make sure it is turned on

        // Emulator doesn't support Bluetooth and will return null
        if(btAdapter==null) {
            errorExit("Fatal Error", "Bluetooth Not supported. Aborting.");
        } else {
            if (btAdapter.isEnabled()) {
                Log.d(TAG, "...Bluetooth is enabled...");
            } else {
                //Prompt user to turn on Bluetooth
                Intent enableBtIntent = new Intent(btAdapter.ACTION_REQUEST_ENABLE);
                startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
            }
        }
    }

    private void errorExit(String title, String message){
        Toast msg = Toast.makeText(getBaseContext(),
                title + " - " + message, Toast.LENGTH_SHORT);
        msg.show();
        finish();
    }

    private void sendData(String message) {
        byte[] msgBuffer = message.getBytes();

        Log.d(TAG, "...Sending data: " + message + "...");

        try {
            outStream.write(msgBuffer);
        } catch (IOException e) {
            String msg = "In onResume() and an exception occurred during write: " + e.getMessage();
            if (address.equals("00:00:00:00:00:00"))
                msg = msg + ".\n\nUpdate your server address from 00:00:00:00:00:00 to the correct address on line 37 in the java code";
            msg = msg +  ".\n\nCheck that the SPP UUID: " + MY_UUID.toString() + " exists on server.\n\n";

            errorExit("Fatal Error", msg);
        }
    }
}

I am getting following errors,

Error:(46, 32) error: cannot find symbol variable main
Error:(49, 43) error: cannot find symbol variable btnOn
Error:(50, 44) error: cannot find symbol variable btnOff

Btw I am using Android Studio. And new to android development.

Answer 1

Remove this line import android.R; Go through this steps:-

first :- go cursor on "R" text, it shows hint that many types of packages,then select your package name ,your problem solved.

second:- if your first step is not working then you have implement project again.

I hope it is useful for you. :)

Answer 2

Remove this line..

import android.R;

and press CTRL+SHIFT+O to Fix imports.

Answer 3

这意味着您缺少 xml 文件 main.xml 单击布局 -> 新建 aml 文件，您必须在该 xml 文件上插入 btnOn 和 btnoff 按钮。