我想通过蓝牙发送和接收一些数据。我关注了这篇文章：Bluetooth Sending and Receiving Text Data
我的 MainActivity.java：
package com.example.proba2x8;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Set;
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.os.Handler;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
public class MainActivity extends Activity {
//@Override
//protected void onCreate(Bundle savedInstanceState) {
// super.onCreate(savedInstanceState);
// setContentView(R.layout.activity_main);
//}
//}
//public class MainActivity extends ActionBarActivity {
BluetoothAdapter mBluetoothAdapter;
BluetoothSocket mmSocket;
BluetoothDevice mmDevice;
OutputStream mmOutputStream;
InputStream mmInputStream;
Thread workerThread;
EditText myTextbox;
TextView myLabel;
byte[] readBuffer;
int readBufferPosition;
int counter;
volatile boolean stopWorker;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
try {
Button sendButton = (Button) findViewById(R.id.send);
Button openButton = (Button) findViewById(R.id.open);
myTextbox = (EditText) findViewById(R.id.editText);
myLabel = (TextView) findViewById(R.id.textView);
openButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
try {
findBT();
openBT();
} catch (IOException ex) {
}
}
});
// send data typed by the user to be printed
sendButton.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
try {
sendData();
} catch (IOException ex) {
}
}
});
} catch (NullPointerException e) {
e.printStackTrace();
} catch (Exception e) {
e.printStackTrace();
}
}
// This will find a bluetooth device
void findBT() {
try {
mBluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
if (mBluetoothAdapter == null) {
myLabel.setText("No bluetooth adapter available");
}
if (!mBluetoothAdapter.isEnabled()) {
Intent enableBluetooth = new Intent(
BluetoothAdapter.ACTION_REQUEST_ENABLE);
startActivityForResult(enableBluetooth, 0);
}
Set<BluetoothDevice> pairedDevices = mBluetoothAdapter
.getBondedDevices();
if (pairedDevices.size() > 0) {
for (BluetoothDevice device : pairedDevices) {
// Xperia mini pro is the name of the bluetooth device
if (device.getName().equals("Xperia mini pro")) {
mmDevice = device;
break;
}
}
}
myLabel.setText("Bluetooth Device Found");
} catch (NullPointerException e) {
e.printStackTrace();
} catch (Exception e) {
e.printStackTrace();
}
}
// Tries to open a connection to the bluetooth device
void openBT() throws IOException {
try {
// Standard SerialPortService ID
UUID uuid = UUID.fromString("00001101-0000-1000-8000-00805f9b34fb");
mmSocket = mmDevice.createRfcommSocketToServiceRecord(uuid);
mmSocket.connect();
mmOutputStream = mmSocket.getOutputStream();
mmInputStream = mmSocket.getInputStream();
beginListenForData();
myLabel.setText("Bluetooth Opened");
} catch (NullPointerException e) {
e.printStackTrace();
} catch (Exception e) {
e.printStackTrace();
}
}
// After opening a connection to bluetooth device,
// we have to listen and check if a data were sent to be printed.
void beginListenForData() {
try {
final Handler handler = new Handler();
// This is the ASCII code for a newline character
final byte delimiter = 10;
stopWorker = false;
readBufferPosition = 0;
readBuffer = new byte[1024];
workerThread = new Thread(new Runnable() {
public void run() {
while (!Thread.currentThread().isInterrupted()
&& !stopWorker) {
try {
int bytesAvailable = mmInputStream.available();
if (bytesAvailable > 0) {
byte[] packetBytes = new byte[bytesAvailable];
mmInputStream.read(packetBytes);
for (int i = 0; i < bytesAvailable; i++) {
byte b = packetBytes[i];
if (b == delimiter) {
byte[] encodedBytes = new byte[readBufferPosition];
System.arraycopy(readBuffer, 0,
encodedBytes, 0,
encodedBytes.length);
final String data = new String(
encodedBytes, "US-ASCII");
readBufferPosition = 0;
handler.post(new Runnable() {
public void run() {
myLabel.setText(data);
}
});
} else {
readBuffer[readBufferPosition++] = b;
}
}
}
} catch (IOException ex) {
stopWorker = true;
}
}
}
});
workerThread.start();
} catch (NullPointerException e) {
e.printStackTrace();
} catch (Exception e) {
e.printStackTrace();
}
}
/*
* This will send data to be printed by the bluetooth
*/
void sendData() throws IOException {
try {
// the text typed by the user
String msg = myTextbox.getText().toString();
msg += "\n";
mmOutputStream.write(msg.getBytes());
// tell the user data were sent
myLabel.setText("Data Sent");
} catch (NullPointerException e) {
e.printStackTrace();
} catch (Exception e) {
e.printStackTrace();
}
}
}
activity_main.xml中：
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="${relativePackage}.${activityClass}" >
<LinearLayout
android:id="@+id/linearLayout1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:orientation="vertical" >
<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Enter Text Here" />
<EditText
android:id="@+id/editText"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:ems="10" >
<requestFocus />
</EditText>
</LinearLayout>
<Button
android:id="@+id/open"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/send"
android:layout_alignBottom="@+id/send"
android:layout_alignRight="@+id/linearLayout1"
android:text="Open" />
<Button
android:id="@+id/send"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/linearLayout1"
android:layout_marginTop="15dp"
android:layout_toLeftOf="@+id/button1"
android:text="Send" />
<TextView
android:id="@+id/recievedText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/send"
android:layout_below="@+id/send"
android:layout_marginTop="20dp"
android:text="Received Text" />
<TextView
android:id="@+id/rtArea"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/recievedText"
android:layout_alignParentBottom="true"
android:layout_below="@+id/recievedText"
android:layout_marginTop="20dp"
android:text="Received Text Will be Displayed Here..." />
</RelativeLayout>
我的问题是，每次按下发送按钮时，LogCat中都会显示以下错误（NullPointerException）：
11-26 00:11:58.650: W/System.err(2236): java.lang.NullPointerException
11-26 00:11:58.650: W/System.err(2236): at com.example.proba2x8.MainActivity.sendData(MainActivity.java:215)
11-26 00:11:58.650: W/System.err(2236): at com.example.proba2x8.MainActivity$2.onClick(MainActivity.java:75)
11-26 00:11:58.654: W/System.err(2236): at android.view.View.performClick(View.java:2418)
11-26 00:11:58.654: W/System.err(2236): at android.view.View.onTouchEvent(View.java:4233)
11-26 00:11:58.654: W/System.err(2236): at android.widget.TextView.onTouchEvent(TextView.java:6645)
11-26 00:11:58.654: W/System.err(2236): at android.view.View.dispatchTouchEvent(View.java:3763)
11-26 00:11:58.654: W/System.err(2236): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:968)
11-26 00:11:58.654: W/System.err(2236): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:968)
11-26 00:11:58.654: W/System.err(2236): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:968)
11-26 00:11:58.654: W/System.err(2236): at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1659)
11-26 00:11:58.654: W/System.err(2236): at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1107)
11-26 00:11:58.654: W/System.err(2236): at android.app.Activity.dispatchTouchEvent(Activity.java:2064)
11-26 00:11:58.654: W/System.err(2236): at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1643)
11-26 00:11:58.654: W/System.err(2236): at android.view.ViewRoot.handleMessage(ViewRoot.java:1691)
11-26 00:11:58.654: W/System.err(2236): at android.os.Handler.dispatchMessage(Handler.java:99)
11-26 00:11:58.654: W/System.err(2236): at android.os.Looper.loop(Looper.java:123)
11-26 00:11:58.654: W/System.err(2236): at android.app.ActivityThread.main(ActivityThread.java:4370)
11-26 00:11:58.654: W/System.err(2236): at java.lang.reflect.Method.invokeNative(Native Method)
11-26 00:11:58.654: W/System.err(2236): at java.lang.reflect.Method.invoke(Method.java:521)
11-26 00:11:58.654: W/System.err(2236): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
11-26 00:11:58.654: W/System.err(2236): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
11-26 00:11:58.654: W/System.err(2236): at dalvik.system.NativeStart.main(Native Method)
为了更好地理解第215行：
mmOutputStream.write(msg.getBytes());
我不知道它是否重要，但第75行如下：
sendData();
感谢您的帮助。