如何实现SQLite到应用程序？

我从一个教程制作了一个简单的SQLite应用程序，现在正试图将它与我制作的游戏拼接在一起。我想要使用SQLite更新和保存高分，但我遇到了一些问题。现在游戏起作用了，但是当我输了，并且调用了cancel（）方法时，我得到了一个“尝试调用虚方法void'对空对象引用”的错误。我不太清楚SQL是如何工作的，所以任何反馈都是值得赞赏的。如何实现SQLite到应用程序？

主要活动

import android.app.Activity; 
import android.content.Intent; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.widget.Button; 
import android.view.View; 
import android.widget.EditText; 
import android.widget.TextView; 



public class MainActivity extends Activity { 

    //sqlite 
    //EditText input; 
    //first high score 
    TextView productText; 
    MyDBHandler dbHandler; 

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

     //sqlite 
     //input = (EditText)findViewById(R.id.editText_ID); 
     productText = (TextView)findViewById(R.id.textView1); 
     //GameScreen gameScreen = new GameScreen(); 
     dbHandler = new MyDBHandler(this, null, null, 1); 
     updateDatabase(); 

    } 
    public void sendMessage(View view) 
    { 
     Intent intent = new Intent(MainActivity.this, GameScreen.class); 
     startActivity(intent); 
    } 

    //sqlite 
    //add a product to the database 
    public void addButtonClicked(){ 
     GameScreen gameScreen = new GameScreen(); 
     Products product = new Products(gameScreen.countPassString); 
     dbHandler.addProduct(product); 
     updateDatabase(); 
    } 

    //sqlite 
    public void updateDatabase(){ 
     String dbString = dbHandler.databaseToString(); 
     productText.setText(dbString); 
    } 
}

GameScreen类

import android.app.Activity; 
import android.os.CountDownTimer; 
import android.os.Handler; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.view.View; 
import android.view.animation.Animation; 
import android.view.animation.TranslateAnimation; 
import android.widget.Button; 
import android.widget.ImageButton; 
import android.widget.TextView; 
import android.widget.Toast; 



public class GameScreen extends Activity { 

    private TextView time; 
    private ImageButton start; 
    private ImageButton gameButton; 
    private ImageButton button2; 
    private CountDownTimer countDownTimer; 
    public static int count = 0; 
    public static int countPass = 0; 
    public String countPassString = ""; 

    final Handler handler = new Handler(); 
    final Runnable r = new Runnable() { 
     public void run() { 
      cancel(); 
     } 
    }; 




    private View.OnClickListener btnClickListener = new View.OnClickListener(){ 

     @Override 
     public void onClick(View v) { 

      switch(v.getId()){ 
       case R.id.start_ID : 
        start(); 
        break; 
       case R.id.gameButton_ID : 
        gameButton(); 
        break; 
       case R.id.button2_ID : 
        button2(); 
        break; 
      } 

     } 


    }; 


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


     start = (ImageButton) findViewById(R.id.start_ID); 
     start.setOnClickListener(btnClickListener); 
     time = (TextView) findViewById(R.id.time); 
     gameButton = (ImageButton) findViewById(R.id.gameButton_ID); 
     gameButton.setOnClickListener(btnClickListener); 
     button2 = (ImageButton) findViewById(R.id.button2_ID); 
     button2.setOnClickListener(btnClickListener); 

     button2.setVisibility(View.GONE); 

    } 

    public void start(){ 

     count = 0; 
     start.setVisibility(View.GONE); 

     time.setText("100"); 
     //this doesnt work and makes app crash when you hit start button 

     countDownTimer = new CountDownTimer(100 * 1000, 1000) { 
      @Override 
      public void onTick(long millsUntilFinished){ 
       time.setText("" + millsUntilFinished/1000); 

       //turns textview string to int 
       int foo = Integer.parseInt(time.getText().toString()); 

       if(foo == 93) { 
        time.setVisibility(View.GONE); 
        button2.setVisibility(View.VISIBLE); 
       } 
       else if(foo == 97) { 


gameButton.animate().translationX(200).setDuration(5000).start(); // move away 
       } 
       else if(foo == 90){ 
        gameButton.animate().translationY(200).setDuration(5000).start(); // move away 
       } 

       if(foo % 2 == 0){ 

        handler.postDelayed(r, 1000); 

       } 
      } 

      public void onFinish() { 
       start.setVisibility(View.VISIBLE); 
       time.setVisibility(View.VISIBLE); 
       time.setText("Done !"); 

       //need to check if score will reset if you win the game 

       Toast.makeText(getApplicationContext(), "You scored " + count, Toast.LENGTH_LONG).show(); 
       gameButton.clearAnimation(); 
       gameButton.animate().translationX(0).setDuration(500).start(); //move back 
       gameButton.animate().translationY(0).setDuration(500).start(); //move back 
       button2.setVisibility(View.GONE); 
      } 
     }; 
     countDownTimer.start(); 
    } 

    private void cancel(){ 
     if(countDownTimer != null){ 
      countDownTimer.cancel(); 
      countDownTimer = null; 
      start.setVisibility(View.VISIBLE); 
      time.setVisibility(View.VISIBLE); 
      gameButton.clearAnimation(); 
      gameButton.animate().translationX(0).setDuration(500).start(); //move back 
      gameButton.animate().translationY(0).setDuration(500).start(); //move back 
      button2.setVisibility(View.GONE); 

      //holds score at end of game 
      countPass = count; 
      //reset score for new game 
      count = 0; 
      Toast.makeText(getApplicationContext(), "You scored " + countPass, Toast.LENGTH_LONG).show(); 
      countPassString = Integer.toString(countPass); 
      MainActivity mainActivity = new MainActivity(); 
      mainActivity.addButtonClicked(); 
     } 
    } 

    private void gameButton(){ 


     int foo = Integer.parseInt(time.getText().toString()); 

     if(foo % 2 == 0) { 
      final Toast toast = Toast.makeText(getApplicationContext(), "+1", Toast.LENGTH_SHORT); 
      toast.show(); 

      // makes +1 toast half a second 
      Handler gameButtonToastHandler = new Handler(); 
      gameButtonToastHandler.postDelayed(new Runnable() { 
       @Override 
       public void run() { 
        toast.cancel(); 
       } 
      }, 500); 

      handler.removeCallbacks(r); 
      ++count; 
     } 
     else{ 
      cancel(); 
     } 
    } 

    private void button2(){ 
     cancel(); 
    } 

}

MyDBHandler类

import android.content.ContentValues; 
import android.content.Context; 
import android.database.Cursor; 
import android.database.sqlite.SQLiteDatabase; 
import android.database.sqlite.SQLiteOpenHelper; 

public class MyDBHandler extends SQLiteOpenHelper { 

private static final int DATABASE_VERSION = 1; 
private static final String DATABASE_NAME = "products.db"; 
public static final String TABLE_PRODUCTS = "products"; 
public static final String COLUMN_ID = "_id"; 
public static final String COLUMN_PRODUCTNAME = "productname"; 

public MyDBHandler(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) { 
    super(context, DATABASE_NAME, factory, DATABASE_VERSION); 
} 


@Override 
public void onCreate(SQLiteDatabase db) { 
    String query = "CREATE TABLE " + TABLE_PRODUCTS + "(" + 
      COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " + 
      COLUMN_PRODUCTNAME + " TEXT " + 
      ");"; 
    db.execSQL(query); 
} 

@Override 
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_PRODUCTS); 
    onCreate(db); 
} 

//add a new row to the database 
public void addProduct(Products product){ 
    ContentValues values = new ContentValues(); 
    values.put(COLUMN_PRODUCTNAME, product.get_productname()); 
    SQLiteDatabase db = getWritableDatabase(); 
    db.insert(TABLE_PRODUCTS, null, values); 
    db.close(); 
} 

/*//delete a product from the database 
public void deleteProduct(String productName){ 
    SQLiteDatabase db = getWritableDatabase(); 
    db.execSQL("DELETE FROM " + TABLE_PRODUCTS + " WHERE " + COLUMN_PRODUCTNAME + "=\"" + productName + "\";"); 
} 
*/ 

//print out the database as a string 
public String databaseToString(){ 
    String dbString = ""; 
    SQLiteDatabase db = getWritableDatabase(); 
    String query = "SELECT * FROM " + TABLE_PRODUCTS + " WHERE 1"; 


    //Cursor point to location in your results 
    /* 
    Cursor c = db.rawQuery(query, null); 
    //Move to the first row in your results 
    c.moveToFirst(); 

    while(!c.isAfterLast()){ 
     if(c.getString(c.getColumnIndex("productname")) != null){ 
      dbString += c.getString(c.getColumnIndex("productname")); 
      dbString += "\n"; 
     } 
     c.moveToNext(); 
    } 
    */ 

    db.close(); 
    return dbString; 

} 

}

产品类

public class Products { 
private int _id; 
private String _productname; 

public Products(){ 

} 

public Products(String productname){ 
    this._productname = productname; 
} 

public void set_id(int _id) { 
    this._id = _id; 
} 

public void set_productname(String _productname) { 
    this._productname = _productname; 
} 

public int get_id() { 
    return _id; 
} 

public String get_productname() { 
    return _productname; 
} 
}

用于MainActivity的XML也是我试图让SQL更改textViews的地方。

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:tools="http://schemas.android.com/tools" 
android:id="@+id/activity_main" 
android:background="@drawable/clock" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:paddingBottom="@dimen/activity_vertical_margin" 
android:paddingLeft="@dimen/activity_horizontal_margin" 
android:paddingRight="@dimen/activity_horizontal_margin" 
android:paddingTop="@dimen/activity_vertical_margin" 
tools:context="com.example.emilythacker.myapplication.MainActivity"> 

<TextView 
    android:text="HIGH SCORES" 
    android:textSize="35dp" 
    android:textColor="#4cfd00" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_alignParentTop="true" 
    android:layout_centerHorizontal="true" 
    android:layout_marginTop="45dp" 
    android:id="@+id/textView" /> 

<TextView 
    android:text="1:" 
    android:textSize="35dp" 
    android:textColor="#4cfd00" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_marginTop="17dp" 
    android:id="@+id/textView1" 
    android:layout_below="@+id/textView" 
    android:layout_alignStart="@+id/textView" /> 

<TextView 
    android:text="2:" 
    android:textSize="35dp" 
    android:textColor="#4cfd00" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_alignStart="@+id/textView1" 
    android:id="@+id/textView2" 
    android:layout_below="@+id/textView1" /> 

<TextView 
    android:text="3:" 
    android:textSize="35dp" 
    android:textColor="#4cfd00" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:id="@+id/textView3" 
    android:layout_below="@+id/textView2" 
    android:layout_alignStart="@+id/textView2" /> 

<TextView 
    android:text="4:" 
    android:textSize="35dp" 
    android:textColor="#4cfd00" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:id="@+id/textView4" 
    android:layout_below="@+id/textView3" 
    android:layout_alignStart="@+id/textView3" /> 

<TextView 
    android:text="5:" 
    android:textSize="35dp" 
    android:textColor="#4cfd00" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:id="@+id/textView5" 
    android:layout_below="@+id/textView4" 
    android:layout_alignStart="@+id/textView4" /> 

<Button 
    android:text="PLAY" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_below="@+id/textView5" 
    android:layout_centerHorizontal="true" 
    android:layout_marginTop="53dp" 
    android:id="@+id/playbutton_ID" 
    android:onClick="sendMessage"/> 

</RelativeLayout>

，这里是控制台消息

D/AndroidRuntime: Shutting down VM 
E/AndroidRuntime: FATAL EXCEPTION: main 
        Process: com.example.emilythacker.myapplication, PID: 31291 
        java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.emilythacker.myapplication.MyDBHandler.addProduct(com.example.emilythacker.myapplication.Products)' on a null object reference 
         at com.example.emilythacker.myapplication.MainActivity.addButtonClicked(MainActivity.java:46) 
         at com.example.emilythacker.myapplication.GameScreen.cancel(GameScreen.java:150) 
         at com.example.emilythacker.myapplication.GameScreen.access$000(GameScreen.java:19) 
         at com.example.emilythacker.myapplication.GameScreen$1.run(GameScreen.java:33) 
         at android.os.Handler.handleCallback(Handler.java:751) 
         at android.os.Handler.dispatchMessage(Handler.java:95) 
         at android.os.Looper.loop(Looper.java:154) 
         at android.app.ActivityThread.main(ActivityThread.java:6688) 
         at java.lang.reflect.Method.invoke(Native Method) 
         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468) 
         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358) 
Disconnected from the target VM, address: 'localhost:8603', transport: 'socket'

来源

2017-02-27 Ryder Thacker

之前，请参阅。 [什么是NullPointerException，以及如何解决它？]（http://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it） –

您应该添加

SQLiteDatabase DB = this.getWritableDatabase（）;

Contentvalues

来源

2017-02-27 03:24:38

如何实现SQLite到应用程序？

回答

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