Android的rawQuery出錯（）

Question

當我試圖用rawQuery從數據庫中提取一些數據時，我得到了nullPointerException。 這裏有一個錯誤：

03-11 18:09:01.522: E/AndroidRuntime(2057): Uncaught handler: thread main exiting due to uncaught exception 03-11 18:09:01.532: E/AndroidRuntime(2057): java.lang.NullPointerException 03-11 18:09:01.532: E/AndroidRuntime(2057): at com.math.scan.FormulaModel.setFormulaList(FormulaModel.java:27)

看我的代碼：

DbHelper

package com.math.scan; 

import java.io.IOException; 
import java.io.InputStream; 
import java.util.Scanner; 

import android.content.Context; 
import android.content.res.AssetManager; 
import android.database.sqlite.SQLiteDatabase; 
import android.database.sqlite.SQLiteOpenHelper; 
import android.util.Log; 

public class DbHelper extends SQLiteOpenHelper{ 

    public static final String TABLE_NAME = "formulas"; 

    public static final String COLUMN_ID = "_id"; 
    public static final String COLUMN_UNKNOWN = "unknown"; 
    public static final String COLUMN_FORMULA = "formula"; 
    public static final String COLUMN_CTG = "ctg"; 

    private static final String DB_NAME = "formulas.db"; 
    private static int DB_VERSION = 1; 

    private static final String DB_CREATE = "CREATE TABLE "+TABLE_NAME+ 
               "("+COLUMN_ID+" integer primary key autoincrement," 
               +COLUMN_UNKNOWN+" text not null," 
               +COLUMN_FORMULA+" text not null," 
               +COLUMN_CTG+" text not null);"; 

    public Context mCtx; 


    public DbHelper(Context context) { 
     super(context, DB_NAME, null, DB_VERSION); 
     this.mCtx = context; 
    } 

    @Override 
    public void onCreate(SQLiteDatabase database) { 

     database.execSQL(DB_CREATE); 

     // reading formulas 
     AssetManager asset = mCtx.getAssets(); 
     try { 
      InputStream input = asset.open("formulas"); 

      int size = input.available(); 

      byte[] buffer = new byte[size]; 
      input.read(buffer); 
      input.close(); 

      String content = new String(buffer); 

      Scanner scan = new Scanner(content); 

      String current; 
      String[] f = new String[2]; 
      String[] formula = new String[2]; 

      while(scan.hasNextLine()) { 
       current = scan.nextLine(); 
       // get category 
       f = current.split("!!"); 
       formula = f[1].split(" = "); 
       database.execSQL("INSERT INTO `formulas` VALUES (NULL, '"+formula[0]+"', '"+formula[1]+"', '"+f[0]+"');"); 
       Log.d("DB", "INSERT INTO `formulas` VALUES (NULL, '"+formula[0]+"', '"+formula[1]+"', '"+f[0]+"');"); 
      } 

     } catch (IOException e) { 
      e.printStackTrace(); 
     } 
    } 

    @Override 
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 
     Log.w(DbHelper.class.getName(), 
       "Upgrading database from version " + oldVersion + " to " 
         + newVersion + ", which will destroy all old data"); 
     db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME); 
     onCreate(db); 
    } 
} 

FormulaModel

package com.math.scan; 

import android.content.Context; 
import android.database.Cursor; 
import android.database.SQLException; 
import android.database.sqlite.SQLiteDatabase; 
import android.util.Log; 

public class FormulaModel { 
    private SQLiteDatabase database; 
    private DbHelper dbHelper; 

    public FormulaModel(Context context) { 
     dbHelper = new DbHelper(context); 
    } 

    public void open() throws SQLException { 
     dbHelper.getWritableDatabase(); 
    } 

    public void close() { 
     dbHelper.close(); 
    } 

    public void setFormulaList(String ctg) { 
     open(); 
     // app stops working here 
     Cursor cursor = database.rawQuery("SELECT unknown, formula FROM formulas WHERE ctg = '"+ctg+"'", null); 
     int results = cursor.getCount(); 
     cursor.moveToFirst(); 

     Global.FormuleResult = new String[results]; 
     Global.FormuleTable = new String[results]; 

     for(int i = 0; i < results; i++) { 
      Global.FormuleTable[i] = cursor.getString(1); 
      Global.FormuleResult[i] = cursor.getString(0); 
     } 
     close(); 
    } 
} 

這是活動的，在這裏我CA在FormulaModel類中設置setFormulaList（）方法。

package com.math.scan; 

import net.sourceforge.jeval.EvaluationException; 
import android.app.Activity; 
import android.os.Bundle; 
import android.view.View; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.TextView; 
import android.widget.Toast; 

public class ProblemActivity extends Activity { 

    private EditText prob; 
    private Button solve; 

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

     // text field 
     prob = (EditText) findViewById(R.id.problem); 

     // confirm btn 
     solve = (Button) findViewById(R.id.solve); 

     // check if confirm button was pressed 
     solve.setOnClickListener(new View.OnClickListener() { 

      @Override 
      public void onClick(View v) { 

       // get text from the field 
       String problem = prob.getText().toString(); 

       // check if expression is not empty 
       if(problem.length() == 0) { 
        // string is empty! 
        Toast.makeText(ProblemActivity.this, getString(R.string.empty_field), Toast.LENGTH_SHORT).show(); 
       } else { 
        FormulaModel f = new FormulaModel(ProblemActivity.this); 
        f.setFormulaList("mech"); 
        pears.doMagic(problem); 
        try { 
         String str = Global.eval.getVariableValue(Global.UNKNOWN); 
         TextView answ = (TextView) findViewById(R.id.answer); 
         answ.setText(getString(R.string.prob_answ) + str); 
        } catch (EvaluationException e) { 
         Toast.makeText(ProblemActivity.this, getString(R.string.prob_error), Toast.LENGTH_SHORT).show(); 
        } 
       } 
      } 
     }); 
    } 

} 

我不明白這裏有什麼問題。 任何想法？

Answer 1

你必須設置你的數據庫變量的open（）：

database = dbHelper.getWritableDatabase();