尝试使用蓝牙时出现Nullpointer异常

问题描述：

我正在尝试创建一个使用设备蓝牙的Android应用程序.但是，当我尝试模拟应用程序时，它给了我这个错误:不幸的是，应用程序已停止.

I'm trying to create an Android application that uses the Bluetooth of the device. However, when I try to emulate the application, it gives me this error: unfortunately Application has stopped.

我一直在寻找解决方案，但是却遇到了问题，并且执行了以下操作:

I've searched for a solution but for the problem, and I've done the following:

使用实际设备代替仿真器，因为无法仿真蓝牙功能
根据我在网上找到的一些建议修改代码
在支持Bluetooth的计算机上使用VM代替普通的仿真器.

以上所有尝试均被证明是徒劳的，无济于事.

All of the above attempts proved futile and were of no avail.

我的java文件:

package com.example.application;

import android.app.Activity;
import android.bluetooth.BluetoothAdapter;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener; 
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;


public class MainActivity extends Activity{
/** Called when the activity is first created. */
private BluetoothAdapter btAdapter;

public TextView statusUpdate;
public Button connect;
public Button disconnect;
public ImageView logo;

BroadcastReceiver bluetoothState = new BroadcastReceiver(){
    @Override
    public void onReceive(Context context, Intent intent){
        String prevStateExtra=BluetoothAdapter.EXTRA_PREVIOUS_STATE;
        String stateExtra=BluetoothAdapter.EXTRA_STATE;
        int state = intent.getIntExtra(prevStateExtra,  -1);
        int previousState = intent.getIntExtra(prevStateExtra, -1);
        String toastText="";
        switch(state){
        case(BluetoothAdapter.STATE_TURNING_ON) :
        {
            toastText="Bluetooth turning on";
            Toast.makeText(MainActivity.this, toastText,          Toast.LENGTH_SHORT).show();
            break;
        }   
        case(BluetoothAdapter.STATE_ON) :
        {
            toastText="Bluetooth on";
            Toast.makeText(MainActivity.this, toastText, Toast.LENGTH_SHORT).show();
            setupUI();
            break;
        }   
        case(BluetoothAdapter.STATE_TURNING_OFF) :
        {
            toastText="Bluetooth turning off";
            Toast.makeText(MainActivity.this, toastText, Toast.LENGTH_SHORT).show();
            break;
        }       
        case(BluetoothAdapter.STATE_OFF) :
        {
            toastText="Bluetooth off";
            Toast.makeText(MainActivity.this, toastText, Toast.LENGTH_SHORT).show();
            setupUI();
            break;
        }

        }

    }


};
@Override
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
setupUI();  
}//end onCreate



private void setupUI(){
//get references
//final TextView statusUpdate = (TextView) findViewById(R.id.result);
final Button connect = (Button)findViewById(R.id.connect);
final Button disconnect = (Button)findViewById(R.id.disconnect);
//final ImageView logo = (ImageView)findviewById(R.id.logo);
//set display view

disconnect.setVisibility(View.GONE);
logo.setVisibility(View.GONE);
btAdapter = BluetoothAdapter.getDefaultAdapter();
if(btAdapter.isEnabled()){
    String address = btAdapter.getAddress();
    String name = btAdapter.getName();
    String statusText = name + " : " + address;
    statusUpdate.setText(statusText);
}
else{
    connect.setVisibility(View.VISIBLE);
    statusUpdate.setText("Bluetooth is not on");
}

connect.setOnClickListener(new OnClickListener(){
    @Override
    public void onClick(View v){
        String actionStateChanged =     BluetoothAdapter.ACTION_CONNECTION_STATE_CHANGED;
        String actionRequestEnable = BluetoothAdapter.ACTION_REQUEST_ENABLE;
        IntentFilter filter = new IntentFilter(actionStateChanged);
        registerReceiver(bluetoothState, filter);
        startActivityForResult(new Intent(actionRequestEnable), 0);


    }

}); //end connect onClickListener

disconnect.setOnClickListener(new OnClickListener(){
    @Override
    public void onClick(View v){

    }

}); //end disconnect onClickListener

}
} //end setupUI

Main.xml:

<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"
android:padding="1dp" >

<TextView
    android:id="@+id/textView1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true"
    android:gravity="center"
    android:text="Sherline Android App"
    android:textColor="#0000ff"
    android:textSize="20dp"
    android:textStyle="bold" />

    <requestFocus />

    <Button
        android:id="@+id/connect"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/textView1"
        android:layout_below="@+id/textView1"
        android:layout_marginTop="36dp"
        android:text="Connect Bluetooth"
        android:textSize="12sp"
        android:textStyle="bold" />

    <EditText
        android:id="@+id/editText1"
        android:layout_width="200dp"
        android:layout_height="300dp"
        android:layout_alignTop="@+id/connect"
        android:layout_toRightOf="@+id/disconnect"
        android:background="#808080"
        android:ems="10"
        android:hint="G-code goes here..."
        android:inputType="textMultiLine"
        android:textColorHint="#FFFFFFFF"
        android:width="150dp" >                     

        <requestFocus />
    </EditText>

    <Button
        android:id="@+id/Button01"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/disconnect"
        android:layout_alignRight="@+id/disconnect"
        android:layout_below="@+id/disconnect"
        android:radius="3dp"
        android:text="Stop"
        android:textSize="12dp"
        android:textStyle="bold" />

    <Button
        android:id="@+id/Button02"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/Button01"
        android:layout_alignRight="@+id/Button01"
        android:layout_centerVertical="true"
        android:radius="3dp"
        android:text="Pause"
        android:textSize="12dp"
        android:textStyle="bold" />

    <Button
        android:id="@+id/Button03"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/Button02"
        android:layout_alignRight="@+id/Button02"
        android:layout_below="@+id/Button02"
        android:radius="3dp"
        android:text="Resume"
        android:textSize="12dp"
        android:textStyle="bold" />

    <Button
        android:id="@+id/disconnect"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignRight="@+id/connect"
        android:layout_below="@+id/connect"
        android:radius="3dp"
        android:text="Disconnect Bluetooth"
        android:textSize="12sp"
        android:textStyle="bold" />

</RelativeLayout>

logcat显示以下错误:

logcat shows the following error(s):

01-31 10:34:41.511: E/AndroidRuntime(978): FATAL EXCEPTION: main
01-31 10:34:41.511: E/AndroidRuntime(978): java.lang.RuntimeException: Unable to start          activity ComponentInfo{com.example.application/com.example.application.MainActivity}:     java.lang.NullPointerException
01-31 10:34:41.511: E/AndroidRuntime(978):  at         android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
01-31 10:34:41.511: E/AndroidRuntime(978):  at     android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
01-31 10:34:41.511: E/AndroidRuntime(978):  at android.app.ActivityThread.access$600(ActivityThread.java:141)
01-31 10:34:41.511: E/AndroidRuntime(978):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
01-31 10:34:41.511: E/AndroidRuntime(978):  at android.os.Handler.dispatchMessage(Handler.java:99)
01-31 10:34:41.511: E/AndroidRuntime(978):  at android.os.Looper.loop(Looper.java:137)
01-31 10:34:41.511: E/AndroidRuntime(978):  at android.app.ActivityThread.main(ActivityThread.java:5039)
01-31 10:34:41.511: E/AndroidRuntime(978):  at java.lang.reflect.Method.invokeNative(Native Method)
01-31 10:34:41.511: E/AndroidRuntime(978):  at java.lang.reflect.Method.invoke(Method.java:511)
01-31 10:34:41.511: E/AndroidRuntime(978):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
01-31 10:34:41.511: E/AndroidRuntime(978):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
01-31 10:34:41.511: E/AndroidRuntime(978):  at dalvik.system.NativeStart.main(Native     Method)
01-31 10:34:41.511: E/AndroidRuntime(978): Caused by: java.lang.NullPointerException
01-31 10:34:41.511: E/AndroidRuntime(978):  at     com.example.application.MainActivity.setupUI(MainActivity.java:87)
01-31 10:34:41.511: E/AndroidRuntime(978):  at com.example.application.MainActivity.onCreate(MainActivity.java:73)
01-31 10:34:41.511: E/AndroidRuntime(978):  at android.app.Activity.performCreate(Activity.java:5104)
01-31 10:34:41.511: E/AndroidRuntime(978):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
01-31 10:34:41.511: E/AndroidRuntime(978):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
01-31 10:34:41.511: E/AndroidRuntime(978):  ... 11 more
01-31 10:54:41.175: E/AndroidRuntime(1149): FATAL EXCEPTION: main
01-31 10:54:41.175: E/AndroidRuntime(1149): java.lang.RuntimeException: Unable to start     activity ComponentInfo{com.example.application/com.example.application.MainActivity}:     java.lang.NullPointerException
01-31 10:54:41.175: E/AndroidRuntime(1149):     at     android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at     android.app.ActivityThread.access$600(ActivityThread.java:141)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at     android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at android.os.Handler.dispatchMessage(Handler.java:99)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at android.os.Looper.loop(Looper.java:137)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at android.app.ActivityThread.main(ActivityThread.java:5039)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at java.lang.reflect.Method.invokeNative(Native Method)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at java.lang.reflect.Method.invoke(Method.java:511)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at dalvik.system.NativeStart.main(Native Method)
01-31 10:54:41.175: E/AndroidRuntime(1149): Caused by: java.lang.NullPointerException
01-31 10:54:41.175: E/AndroidRuntime(1149):     at com.example.application.MainActivity.setupUI(MainActivity.java:87)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at com.example.application.MainActivity.onCreate(MainActivity.java:73)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at android.app.Activity.performCreate(Activity.java:5104)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
01-31 10:54:41.175: E/AndroidRuntime(1149):     at     android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
01-31 10:54:41.175: E/AndroidRuntime(1149):     ... 11 more

请帮助！谢谢！

答

好吧，您得到NullPointerException，可能有2种情况引起关注.

Well, you are getting NullPointerException, and there may be 2 cases of interest.

statusUpdate和logo变量是NULL，但是已经有其他答案解释.

statusUpdate, and logo variables are NULL, but other answers already explaining that.

BluetoothAdapter.getDefaultAdapter()可能返回NULL 硬件不支持.因此，您也需要处理这种情况.

BluetoothAdapter.getDefaultAdapter() may return NULL if bluetooth is not supported on hardware. So you need to handle that case too..

btAdapter = BluetoothAdapter.getDefaultAdapter();
if(btAdapter != null && btAdapter.isEnabled()){  // see changes in this line..

尝试使用蓝牙时出现Nullpointer异常

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