我们，我正在Android上进行练习，点击按钮可以在区域中检索WIFI的所有名称。我需要保存一些关于数据库的信息，但是当我运行项目时，请使用NullPointerException停止，然后打开数据库
logcat的
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: FATAL EXCEPTION: main
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: java.lang.RuntimeException: Error receiving broadcast Intent { act=android.net.wifi.SCAN_RESULTS flg=0x8000010 } in com.example.davide.prova.DisplayMessageActivity$WifiScanReceiver@42465e28
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:765)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at android.os.Handler.handleCallback(Handler.java:615)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:92)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at android.os.Looper.loop(Looper.java:137)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:4867)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at java.lang.reflect.Method.invokeNative(Native Method)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at java.lang.reflect.Method.invoke(Method.java:511)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1007)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:774)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at dalvik.system.NativeStart.main(Native Method)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: Caused by: java.lang.NullPointerException
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at com.example.davide.prova.DisplayMessageActivity$WifiScanReceiver.onReceive(DisplayMessageActivity.java:89)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:755)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at android.os.Handler.handleCallback(Handler.java:615)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:92)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at android.os.Looper.loop(Looper.java:137)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:4867)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at java.lang.reflect.Method.invokeNative(Native Method)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at java.lang.reflect.Method.invoke(Method.java:511)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1007)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:774)
03-04 18:44:16.225 12670-12670/com.example.davide.prova E/AndroidRuntime: at dalvik.system.NativeStart.main(Native Method)
这是我声明DB的类：
public class DBAdapter {
private static final String LOG_TAG = "DatabaseUtil";
/*NOME DEL DATABASE*/
private static final String DB_NAME = "DB_WIFIdiscover";
private static final int DB_VERSION = 1;
/*NOMI TABELLE*/
private static final String DB_TABLE_WIFI_INFO = "Table_stato_wifi";
private static final String DB_TABLE_LOCATION = "Table_dove_sei";
/*DATABASE INFORMAZIONI RELATIVE AL WIFI TROVATO*/
/*DATABASE COORDINATE*/
public static final String COLUMN_LOCATION_ID = "LocationID";
public static final String COLUMN_LATITUDINE = "Longitudine";
public static final String COLUMN_LONGITUDINE = "Latitudine";
private static final String SQL_CREATE_ENTRIES = "create table " //
+ DB_TABLE_LOCATION + " (" //
+ COLUMN_LOCATION_ID + " integer primary key autoincrement, " //
+ COLUMN_LATITUDINE + " DOUBLE NOT NULL, " //
+ COLUMN_LONGITUDINE + " DOUBLE NOT NULL);";
private final Context mCtx;
private FROpenHelper dbHelper;
private SQLiteDatabase mDb;
private String TAG = this.getClass().getCanonicalName();
//private SQLiteDatabase db; //si riferisce al database
//private FROpenHelper dbHelper; //si riferisce all'OpenHelper
private static class FROpenHelper extends SQLiteOpenHelper {
public FROpenHelper(Context context) {
super(context, DB_NAME, null, DB_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) throws SQLException {
Log.w(LOG_TAG, "Creating database.");
try {
db.execSQL(SQL_CREATE_ENTRIES);
} catch (SQLException e) {
Log.e(LOG_TAG, e.getMessage());
throw e;
}
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
Log.w(LOG_TAG, "Upgrading from version " + oldVersion + " to " + newVersion
+ ", which will destroy all old data");
// Create a new one.
onCreate(db);
}
}
public DBAdapter(Context context) {
this.mCtx = context;
}
public DBAdapter open() throws SQLException {
try {
dbHelper = new FROpenHelper(mCtx);
mDb = dbHelper.getWritableDatabase();
} catch (SQLiteException e) {
Log.e(TAG, e.getMessage());
throw e;
}
return this;
}
/**
* Close the DB.
*/
public void close() {
dbHelper.close();
}
这是我打开方法的类：
public class DisplayMessageActivity extends AppCompatActivity {
ListView lv;
WifiManager wifi;
String wifis[];
WifiScanReceiver wifiReciever;
public DBAdapter dbAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_display_message);
//
lv = (ListView)findViewById(R.id.listView);
wifi = (WifiManager)getSystemService(Context.WIFI_SERVICE);
wifiReciever = new WifiScanReceiver();
wifi.startScan();
//layout dell'activity
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
}
protected void onPause() {
unregisterReceiver(wifiReciever);
super.onPause();
}
protected void onResume() {
registerReceiver(wifiReciever, new IntentFilter(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));
super.onResume();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
private class WifiScanReceiver extends BroadcastReceiver {
@TargetApi(Build.VERSION_CODES.M)
public void onReceive(Context c, Intent intent) {
List<ScanResult> wifiScanList = wifi.getScanResults();//lista degli hotspot trovati
wifis = new String[wifiScanList.size()];//array di stringhe della lunghezza del numeri di hotspot trovati
//for(int i = 0; i < wifiScanList.size(); i++){
//if(wifiScanList.get(i).is80211mcResponder() == true){//controlla se è un canale 802.11, se lo è lo salva nel database
// wifis[i] = ((wifiScanList.get(i).SSID).toString());
//Inserisco nel DB i valori della rete wireless
//long idpos = dbAdapter.insertDB();
//if(idpos == -1){
//}
dbAdapter.open();
dbAdapter.insertDB();//passa valori della rete
dbAdapter.close();
//}
//SSID nome della rete, BSSID modalità di funzionamento wifi, frequency, level(dbm)
//}
lv.setAdapter(new ArrayAdapter<String>(getApplicationContext(),android.R.layout.simple_list_item_1,wifis));//reference to an built-in XML layout document that is part of the Android OS
}
}
}
请帮助我，因为我不知道问题出在哪里！
答案 0 :(得分：1)
You need to learn how to read a Java stacktrace
Nullpointers are the most easy error to fix usually.
Caused by: java.lang.NullPointerException
at com.example.davide.prova.DisplayMessageActivity$WifiScanReceiver.onReceive(DisplayMessageActivity.java:89)
You always want to look for the first cause of you error (Caused By) This has nothing to do with Android or the database.
Look at line 89 of your file. Something in that line is null and you call a method on it. Understand why that is null / when can it be will lead to fixing the exception.