Java Android NullPointerInserting数据中的异常

我明白这一点

它工作得很好，但是当我修改插入方法时，我在这一行中得到了NullpointerException：

return DB.insert(tableName, null, initialValues);

tableName和initialvalue也被分配。我不知道我为什么会有NullpointerException

我的代码：

public class MainActivity extends ListActivity {

    private ArrayList<String> results = new ArrayList<String>();
    private String tableName = DBHelper.tableName;
    private SQLiteDatabase newDB;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.activity_main);

        InsertTheData();
        openAndQueryDatabase(); 
        displayResultList();
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    private void displayResultList() {
        TextView tView = new TextView(this);
        tView.setText("This data is retrieved from the database and only 4 " +
                "of the results are displayed");
        getListView().addHeaderView(tView);

        setListAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, results));
        getListView().setTextFilterEnabled(true);

    }

    private void InsertTheData()
    {
        try {
            DBHelper dbHelper = new DBHelper(this.getApplicationContext()); 
            newDB = dbHelper.getWritableDatabase();
            dbHelper.insertSomeItmes();
        } catch (Exception e) {
            // TODO Auto-generated catch block
            Log.e(getClass().getSimpleName(), "Could not Insert data");
        }
         finally {
            if (newDB != null) 
                newDB.execSQL("DELETE FROM " + tableName);
                newDB.close();
         }
    }


    private void openAndQueryDatabase() {
        try {
            DBHelper dbHelper = new DBHelper(this.getApplicationContext());
            newDB = dbHelper.getWritableDatabase();
            Cursor c = newDB.rawQuery("SELECT FirstName, Age FROM " +
                    tableName +
                    " where Age > 10 LIMIT 4", null);

            if (c != null ) {
                if  (c.moveToFirst()) {
                    do {
                        String firstName = c.getString(c.getColumnIndex("FirstName"));
                        int age = c.getInt(c.getColumnIndex("Age"));
                        results.add("Name: " + firstName + ",Age: " + age);
                    }while (c.moveToNext());
                } 
            }           
        } catch (SQLiteException se ) {
            Log.e(getClass().getSimpleName(), "Could not create or Open the database");
        } finally {
            if (newDB != null) 
                newDB.execSQL("DELETE FROM " + tableName);
                newDB.close();
        }

    }
}

---

我只是猜测，但是当您到达

DBHelper#onCreate

并且如果

DBHelper#checkDbExists

返回true，则可能数据库字段仍然未分配

---

您应该已经提供了堆栈跟踪。

我已经在某个地方读到了，主键是必需的

您应该在插入之前打开一个数据库……我如何打开一个数据库？此代码：DBHelper DBHelper=newdbhelper（this.getApplicationContext（））；newDB=dbHelper.getWritableDatabase（）；不打开数据库？移动此

DB=currentContext.openOrCreateDatabase（DBName，0，null）

public class DBHelper extends SQLiteOpenHelper {

    public SQLiteDatabase DB;
    public String DBPath;
    public static String DBName = "sample";
    public static final int version = '1';
    public static Context currentContext;
    public static String tableName = "Resource";

    public static final String KEY_LastName = "LastName";
    public static final String KEY_FirstName = "FirstName";
    public static final String KEY_Country = "Country";
    public static final String KEY_Age = "Age"; 

    private static final String TAG = "Create_The_DB";

    private static final String TABLE_CREATE = "CREATE TABLE IF NOT EXISTS " +
            tableName + " ( "+ KEY_LastName + " VARCHAR, "+ KEY_FirstName +"  VARCHAR," + KEY_Country + "  VARCHAR,"+ KEY_Age +"  INT(3))";

    public DBHelper(Context context) {
        //super(context, name, factory, version);
        // TODO Auto-generated constructor stub

        super(context, DBName, null, version);
        currentContext = context;
        DBPath = "/data/data/" + context.getPackageName() + "/databases";
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
      // TODO Auto-generated method stub
      Log.w(TAG, TABLE_CREATE);
      boolean dbExists = checkDbExists();       
      if (dbExists) {
        // do nothing
      } else {
        DB = currentContext.openOrCreateDatabase(DBName, 0, null);
        DB.execSQL(TABLE_CREATE);
    }

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub
         Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
                 + newVersion + ", which will destroy all old data");
               db.execSQL("DROP TABLE IF EXISTS " + TABLE_CREATE);
               onCreate(db);

    }

    private boolean checkDbExists() {
        SQLiteDatabase checkDB = null;

        try {
            String myPath = DBPath + DBName;
            checkDB = SQLiteDatabase.openDatabase(myPath, null,
                    SQLiteDatabase.OPEN_READONLY);

        } catch (SQLiteException e) {
            // database does't exist yet.
        }

        if (checkDB != null) {
            checkDB.close();
        }

        return checkDB != null ? true : false;
    }

    public long createItmes(String LeLastName, String LeFirstName, String LeCountry, int LeAge) {

              ContentValues initialValues = new ContentValues();
              initialValues.put(KEY_LastName, LeLastName);
              initialValues.put(KEY_FirstName, LeFirstName);
              initialValues.put(KEY_Country, LeCountry);
              initialValues.put(KEY_Age, LeAge);

              return DB.insert(tableName, null, initialValues);
             }

    public void insertSomeItmes() {

        createItmes("AFG","Afghanistan","Asia",5);
        createItmes("ALB","Albania","Europe",9);
        createItmes("DZA","Algeria","Africa",52);   
        createItmes("AND","Andorra","Europe",55);
        createItmes("AGO","Angola","Africa",63);
        createItmes("AIA","Anguilla","North America",75);

         }  
}