初学者开发人员:未知错误:java.lang.nullPointerException

Question

我是开发Android应用程序的新手,我希望我能在正确的位置发布这个.我正在尝试学习如何让手机蓝牙连接到我的蓝牙模块.我正在尝试使用我在参考书中找到的代码,但它给了我一个错误.我之前用java编写过,但我很难理解Android应用程序的结构.无论如何,我得到的错误是未知错误:java.lang.nullPointerException.我可能忘记导入我需要的库,或者在创建项目时是否犯了包装错误？这是代码:

package android.app.bluetooth;
//import java.io.InputStream;
//import java.io.OutputStream;
import java.util.ArrayList;
import java.util.UUID;
import java.io.IOException;
import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
//import android.os.Handler;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothServerSocket;
import android.bluetooth.BluetoothSocket;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
//import android.view.View.OnKeyListener;
import android.widget.ArrayAdapter;
import android.widget.AdapterView;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;    
import android.widget.TextView;
import android.widget.AdapterView.OnItemClickListener;

public class bluetooth extends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    //Get the bluetooth adapter (phone)
    configureBluetooth();

    //Setup ListView of discovered devices
    setupListView();

  //Setup search button
    setupSearchButton();

    //Setup listen button
    setupListenButton();
}

private BluetoothAdapter bluetooth;
private BluetoothSocket socket;
private UUID uuid = UUID.fromString("985c75a3-66ae-4b5b-9fac-894659d6f6ee");
private void configureBluetooth(){
     BluetoothAdapter bluetooth = BluetoothAdapter.getDefaultAdapter();
     }

//Create switchUI method that will be called once a connection is 
//established to enable views for reading and writing messages  
private ListView list;
private void switchUI(){
    final TextView messageText = (TextView)findViewById(R.id.text_messages);
    final EditText textEntry = (EditText)findViewById(R.id.text_message);

    messageText.setVisibility(View.VISIBLE);
    list.setVisibility(View.GONE);
    textEntry.setEnabled(true);
    }

//Create server listener.  Listen button will prompt user to enable discovery
//When discovery window returns, open bluetooth socket to listen for connection       requests for discovery duration
//Once a connection has been made, make a call to switchUI 

private static int DISCOVERY_REQUEST = 1;
private void setupListenButton(){
 Button listenButton = (Button)findViewById(R.id.button_listen);
 listenButton.setOnClickListener(new OnClickListener(){
  public void onClick(View view){
   Intent disc;
   disc = new Intent(BluetoothAdapter.ACTION_REQUEST_DISCOVERABLE);
   startActivityForResult(disc, DISCOVERY_REQUEST);
      }
     });
    }

//Find out if user has accepted or rejected the request

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data){
    if(requestCode == DISCOVERY_REQUEST){
     boolean isDiscoverable = resultCode > 0;
     if (isDiscoverable){
      String name = "bluetoothserver";
      try{
       final BluetoothServerSocket btserver =    bluetooth.listenUsingRfcommWithServiceRecord(name, uuid);
       AsyncTask<Integer, Void, BluetoothSocket> acceptThread = new AsyncTask<Integer, Void, BluetoothSocket>(){
        @Override
        protected BluetoothSocket doInBackground(Integer ... params){
         try{
          socket = btserver.accept(params[0]*1000);
          return socket;
          } catch (IOException e){
           Log.d("BLUETOOTH", e.getMessage());
          }
          return null;
         }
         @Override
         protected void onPostExecute(BluetoothSocket result){
          if (result != null)
           switchUI();
         }
        };
        acceptThread.execute(resultCode);
       } catch (IOException e){
        Log.d("BLUETOOTH", e.getMessage());
        }
      }
      //int discoverableDuration = resultCode;
     }
    }

//Provide a means for client device to search for listening server
private ArrayAdapter<BluetoothDevice> aa;
private ArrayList<BluetoothDevice> foundDevices;
private void setupListView(){
    aa = new ArrayAdapter<BluetoothDevice>(this, android.R.layout.simple_list_item_1, foundDevices);
    list = (ListView)findViewById(R.id.list_discovered);
    list.setAdapter(aa);

    //Include onItemClickListener that will attempt to asynchronously initiate a client-side connection
    //with the selected remote Bluetooth Device
    //If successful, keep a reference to the socket it creates and make a call to switchUI
    list.setOnItemClickListener(new OnItemClickListener() {
     public void onItemClick(AdapterView<?> arg0, View view, int index, long arg3){
      AsyncTask<Integer, Void, Void> connectTask = new AsyncTask<Integer, Void, Void>(){
       @Override
       protected Void doInBackground(Integer ... params){
        try{
         BluetoothDevice device = foundDevices.get(params[0]);
         socket = device.createRfcommSocketToServiceRecord(uuid);
         socket.connect();
        } catch(IOException e){
         Log.d("BLUETOOTH_CLIENT", e.getMessage());
         }
         return null;
        }

        @Override
        protected void onPostExecute(Void result){
         switchUI();
        }
       };
       connectTask.execute(index);
      }
     });
    }

//Create a broadcast receiver that listens for Bluetooth Device discovery broadcasts,
//adds each discovered device to the array of found devices and notifies the Array  Adapter
//Discover remote bluetooth devices
BroadcastReceiver discoveryResult = new BroadcastReceiver(){
@Override
public void onReceive(Context context, Intent intent){
 //String remoteDeviceName = intent.getStringName = intent.getStringExtra(BluetoothDevice.EXTRA_NAME);
 BluetoothDevice remoteDevice; //remote bluetooth device
 remoteDevice = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);
 if(bluetooth.getBondedDevices().contains(remoteDevice)){
  foundDevices.add(remoteDevice);
  aa.notifyDataSetChanged();
  }
 }
};

    //Register Broadcast Receiver and initiate discovery session
    private void setupSearchButton(){
     Button searchButton = (Button)findViewById(R.id.button_search);

     searchButton.setOnClickListener(new OnClickListener(){
      public void onClick(View view){
       registerReceiver(discoveryResult, new IntentFilter(BluetoothDevice.ACTION_FOUND));

       if (!bluetooth.isDiscovering()){
        foundDevices.clear();
        bluetooth.startDiscovery();
       }
      }
     });
    }
}

这是布局文件:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<EditText
 android:id="@+id/text_message"
 android:layout_width="fill_parent"
 android:layout_height="wrap_content"
 android:layout_alignParentBottom="true"
 android:enabled="false"
/>
<Button
 android:id="@+id/button_search"
 android:text="Search for listener"
 android:layout_width="fill_parent"
 android:layout_height="wrap_content"
 android:layout_above="@id/text_message"
/>
<Button
 android:id="@+id/button_listen"
 android:text="Listen for connection"
 android:layout_width="fill_parent"
 android:layout_height="wrap_content"
 android:layout_above="@id/button_search"
/> 
<ListView
 android:id="@+id/list_discovered"
 android:layout_width="fill_parent"
 android:layout_height="fill_parent"
 android:layout_above="@id/button_listen"
 android:layout_alignParentTop="true"
/>
<TextView 
android:id="@+id/text_messages" 
android:layout_width="fill_parent" 
android:layout_height="fill_parent"
android:layout_above="@id/button_listen"
android:layout_alignParentTop="true" 
android:visibility="gone"
/>
</RelativeLayout>

这是清单文件:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
  android:versionCode="1"
  android:versionName="1.0" package="android.app.bluetooth">
<application android:icon="@drawable/icon" android:label="@string/app_name">
    <activity android:name=".bluetooth"
              android:label="@string/app_name">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

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

就像我说我对此非常新,所以如果这没有多大意义我会道歉,但任何帮助都会非常感激.

Answer 1

尝试一步步调试。这样您就可以找到 NullPointerException 的原因。某些访问的字段可能为空。一旦知道哪个字段，您可以通过为该字段指定默认值来防止 NullPointerException。