无法弄清楚为什么我有null对象引用

-1

dbString在printDatabase（）中获取空对象引用。我很难搞清楚为什么它是一个空对象，当我刚刚从我以前工作的应用程序中移动此代码时，但现在我得到此错误。以下是错误日志无法弄清楚为什么我有null对象引用

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference 
                     at binarysoup.mommy_info.ActivitiesAndFragments.KickCounter.printDatabase(KickCounter.java:178) 
                     at binarysoup.mommy_info.ActivitiesAndFragments.KickCounter.onCreate(KickCounter.java:75)

KickCounter.java frgment这是存在的。

public class KickCounter extends Fragment { 
// TODO: Rename parameter arguments, choose names that match 
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER 
private static final String ARG_PARAM1 = "param1"; 
private static final String ARG_PARAM2 = "param2"; 

// TODO: Rename and change types of parameters 
private String mParam1; 
private String mParam2; 

private OnFragmentInteractionListener mListener; 

public KickCounter() { 
    // Required empty public constructor 
} 

/** 
* Use this factory method to create a new instance of 
* this fragment using the provided parameters. 
* 
* @param param1 Parameter 1. 
* @param param2 Parameter 2. 
* @return A new instance of fragment KickCounter. 
*/ 
// TODO: Rename and change types and number of parameters 
public static KickCounter newInstance(String param1, String param2) { 
    KickCounter fragment = new KickCounter(); 
    Bundle args = new Bundle(); 
    args.putString(ARG_PARAM1, param1); 
    args.putString(ARG_PARAM2, param2); 
    fragment.setArguments(args); 
    return fragment; 
} 

//SQLite 
MyDBHandler dbHandler; 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    if (getArguments() != null) { 
     mParam1 = getArguments().getString(ARG_PARAM1); 
     mParam2 = getArguments().getString(ARG_PARAM2); 
    } 
    //SQLite 
    dbHandler = new MyDBHandler(getContext(), null, null, 1); 
    printDatabase(); 
} 

//SQLite 
//Add a product to the database 
public void addButtonClicked(View view){ 
    Product product = new Product(kickString); 
    dbHandler.addProduct(product); 
    printDatabase(); 
} 

Button kickButton; 
Button removeKickButton; 
Button startButton; 
Chronometer chronometer; 
TextView kickCount; 
Integer kick = 0; 
String kickString = kick.toString(); 
//SQLite 
TextView outputText; 

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 

    View rootView = inflater.inflate(R.layout.fragment_kick_counter, container, false); 

    kickButton = (Button)rootView.findViewById(R.id.kick_button_ID); 
    kickButton.setVisibility(View.INVISIBLE); 
    removeKickButton = (Button)rootView.findViewById(R.id.remove_kick_button_ID); 
    removeKickButton.setVisibility(View.INVISIBLE); 
    startButton = (Button)rootView.findViewById(R.id.start_button_ID); 
    chronometer = (Chronometer)rootView.findViewById(R.id.chronometer_ID); 
    kickCount = (TextView)rootView.findViewById(R.id.kickCount_ID); 
    //SQLite 
    outputText = (TextView)rootView.findViewById(R.id.outputText_ID); 

    startButton.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      startButton.setVisibility(View.GONE); 
      kickButton.setVisibility(View.VISIBLE); 
      removeKickButton.setVisibility(View.VISIBLE); 

      chronometer.setBase(SystemClock.elapsedRealtime()); 
      chronometer.start(); 

      chronometer.setOnChronometerTickListener(new Chronometer.OnChronometerTickListener() { 

         @Override 
         public void onChronometerTick(Chronometer chronometer) { 
          if(chronometer.getText().toString().equalsIgnoreCase("00:05")) { 
           chronometer.stop(); 
           kickButton.setVisibility(View.GONE); 
           removeKickButton.setVisibility(View.GONE); 
           Snackbar.make(getView(),"Your text",Snackbar.LENGTH_LONG).show(); 

          } 
          if(kick == 10) { 
           chronometer.stop(); 
           kickButton.setVisibility(View.GONE); 
           removeKickButton.setVisibility(View.GONE); 
           Snackbar.make(getView(),"heo",Snackbar.LENGTH_INDEFINITE).show(); 
          } 
         } 
      }); 

     } 
    }); 

    kickButton.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      if(kick < 10){ 
       kick++; 
       setKickCountText(); 
      } 
      else{ 
       return; 
      } 
     } 
    }); 

    removeKickButton.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      if(kick != 0){ 
       kick--; 
       setKickCountText(); 
      } 
      else{ 
       return; 
      } 
     } 
    }); 

    // Inflate the layout for this fragment 
    return rootView; 
} 

//SQLite 
//Print the database 
public void printDatabase(){ 
    String dbString = dbHandler.databaseToString(); 
    outputText.setText(dbString); 
    //buckysInput.setText(""); 
} 

public void setKickCountText(){ 
    kickCount.setText(kick.toString()); 
} 

// TODO: Rename method, update argument and hook method into UI event 
public void onButtonPressed(Uri uri) { 
    if (mListener != null) { 
     mListener.onFragmentInteraction(uri); 
    } 
} 

@Override 
public void onAttach(Context context) { 
    super.onAttach(context); 
    if (context instanceof OnFragmentInteractionListener) { 
     mListener = (OnFragmentInteractionListener) context; 
    } else { 
     throw new RuntimeException(context.toString() 
       + " must implement OnFragmentInteractionListener"); 
    } 
} 

@Override 
public void onDetach() { 
    super.onDetach(); 
    mListener = null; 
} 

/** 
* This interface must be implemented by activities that contain this 
* fragment to allow an interaction in this fragment to be communicated 
* to the activity and potentially other fragments contained in that 
* activity. 
* <p> 
* See the Android Training lesson <a href= 
* "http://developer.android.com/training/basics/fragments/communicating.html" 
* >Communicating with Other Fragments</a> for more information. 
*/ 
public interface OnFragmentInteractionListener { 
    // TODO: Update argument type and name 
    void onFragmentInteraction(Uri uri);

MyDBHandler.java如果需要

public class MyDBHandler extends SQLiteOpenHelper { 

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

//We need to pass database information along to superclass 
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(Product product){ 
    SQLiteDatabase db = getWritableDatabase(); 
    ContentValues values = new ContentValues(); 
    values.put(COLUMN_PRODUCTNAME, product.get_productname()); 
    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 + "\";"); 
} 

public String databaseToString(){ 
    String dbString = ""; 
    SQLiteDatabase db = getWritableDatabase(); 
    String query = "SELECT * FROM " + TABLE_PRODUCTS + " WHERE 1"; 

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

    //Position after the last row means the end of the results 
    while (!c.isAfterLast()) { 
     if (c.getString(c.getColumnIndex("productname")) != null) { 
      dbString += c.getString(c.getColumnIndex("productname")); 
      dbString += "\n"; 
     } 
     c.moveToNext(); 
    } 
    db.close(); 
    return dbString; 
}

kickCounter.xml

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:app="http://schemas.android.com/apk/res-auto" 
xmlns:tools="http://schemas.android.com/tools" 
android:id="@+id/kick_counter_ID" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:background="@color/kickCounterFragmentBackground" 
tools:context="binarysoup.mommy_info.ActivitiesAndFragments.KickCounter"> 

<!-- TODO: Update blank fragment layout --> 

<Button 
    android:id="@+id/start_button_ID" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 

    android:text="Start" 
    android:layout_above="@+id/kick_button_ID" 
    android:layout_centerHorizontal="true" 
    android:layout_marginRight="8dp" 
    app:layout_constraintRight_toRightOf="parent" 
    android:layout_marginLeft="8dp" 
    app:layout_constraintLeft_toLeftOf="parent" 
    app:layout_constraintTop_toTopOf="parent" 
    android:layout_marginTop="8dp" 
    app:layout_constraintBottom_toBottomOf="parent" 
    android:layout_marginBottom="8dp" 
    app:layout_constraintVertical_bias="0.651" /> 

<Button 
    android:id="@+id/kick_button_ID" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:text="Kick!" 
    android:layout_alignParentBottom="true" 
    android:layout_alignParentLeft="true" 
    android:layout_alignParentStart="true" 
    android:layout_marginStart="71dp" 
    android:layout_marginLeft="8dp" 
    app:layout_constraintBottom_toBottomOf="parent" 
    android:layout_marginBottom="8dp" 
    app:layout_constraintTop_toTopOf="parent" 
    android:layout_marginTop="8dp" 
    app:layout_constraintVertical_bias="0.758" 
    app:layout_constraintLeft_toLeftOf="parent" 
    android:layout_marginRight="8dp" 
    app:layout_constraintRight_toRightOf="parent" 
    app:layout_constraintHorizontal_bias="0.728" /> 

<Button 
    android:id="@+id/remove_kick_button_ID" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:text="Remove Kick" 
    android:layout_alignBaseline="@+id/kick_button_ID" 
    android:layout_alignBottom="@+id/kick_button_ID" 
    android:layout_toRightOf="@+id/start_button_ID" 
    android:layout_toEndOf="@+id/start_button_ID" 
    app:layout_constraintBottom_toBottomOf="parent" 
    android:layout_marginBottom="8dp" 
    app:layout_constraintTop_toTopOf="parent" 
    android:layout_marginTop="8dp" 
    app:layout_constraintVertical_bias="0.758" 
    android:layout_marginLeft="8dp" 
    app:layout_constraintLeft_toLeftOf="parent" 
    android:layout_marginRight="8dp" 
    app:layout_constraintRight_toRightOf="parent" 
    app:layout_constraintHorizontal_bias="0.105" /> 

<Chronometer 
    android:id="@+id/chronometer_ID" 
    android:textSize="50dp" 
    android:layout_width="168dp" 
    android:layout_height="56dp" 
    android:layout_marginRight="8dp" 
    app:layout_constraintRight_toRightOf="parent" 
    android:layout_marginLeft="8dp" 
    app:layout_constraintLeft_toLeftOf="parent" 
    app:layout_constraintTop_toTopOf="parent" 
    android:layout_marginTop="8dp" 
    app:layout_constraintBottom_toBottomOf="parent" 
    android:layout_marginBottom="8dp" 
    app:layout_constraintVertical_bias="0.086" 
    app:layout_constraintHorizontal_bias="0.64" /> 

<TextView 
    android:id="@+id/kicksText_ID" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:textSize="25dp" 
    android:text="Kicks:" 
    android:layout_marginLeft="8dp" 
    app:layout_constraintLeft_toLeftOf="parent" 
    android:layout_marginRight="8dp" 
    app:layout_constraintRight_toRightOf="parent" 
    app:layout_constraintTop_toTopOf="parent" 
    android:layout_marginTop="8dp" 
    app:layout_constraintBottom_toBottomOf="parent" 
    android:layout_marginBottom="8dp" 
    app:layout_constraintHorizontal_bias="0.386" 
    app:layout_constraintVertical_bias="0.299" /> 

<TextView 
    android:id="@+id/kickCount_ID" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:textSize="25dp" 
    android:text="00" 
    android:layout_marginLeft="8dp" 
    android:layout_marginRight="136dp" 
    app:layout_constraintRight_toRightOf="parent" 
    app:layout_constraintHorizontal_bias="0.0" 
    app:layout_constraintTop_toTopOf="parent" 
    android:layout_marginTop="8dp" 
    app:layout_constraintBottom_toBottomOf="parent" 
    android:layout_marginBottom="8dp" 
    app:layout_constraintVertical_bias="0.298" 
    app:layout_constraintLeft_toRightOf="@+id/kicksText_ID" /> 

<TextView 
    android:id="@+id/outputText_ID" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_marginBottom="8dp" 
    android:layout_marginLeft="8dp" 
    android:layout_marginRight="8dp" 
    android:layout_marginTop="8dp" 
    android:text="TextView" 
    app:layout_constraintBottom_toBottomOf="parent" 
    app:layout_constraintHorizontal_bias="0.025" 
    app:layout_constraintLeft_toLeftOf="parent" 
    app:layout_constraintRight_toRightOf="parent" 
    app:layout_constraintTop_toTopOf="parent" 
    app:layout_constraintVertical_bias="0.016" />

来源

2017-05-27 Ryder Thacker

它会看起来好像是'outputText '为空。调试以找出原因。 – Carcigenicate

它会在'buckysInput.setText（“”）'行崩溃吗？如果是这样，问题在于你没有在你的'onCreateView（）'方法中初始化文本视图。 – Gaket

上传xml布局文件！ –

问题在于你对片段生命周期的解释。在片段生命周期onCreate()之前调用onCreateView()，所以TextView引用在printDatabase中为null。尝试在onCreateView()中添加onCreate的代码。

来源

2017-05-27 18:16:32

＃。而不是调用方法printDatabase()从onCreate()，从onCreateView()称它。您可以在onCreate()方法中执行任何non-graphical初始化。在onCreateView()中，您应该指定并声明以后要使用的任何View变量。

＃。在你的方法databaseToString()使用：

String query = "SELECT * FROM " + TABLE_PRODUCTS;

相反的：

String query = "SELECT * FROM " + TABLE_PRODUCTS + " WHERE 1";

更新您的databaseToString()方法如下：

public String databaseToString() { 

    String dbString = ""; 
    SQLiteDatabase db = getWritableDatabase(); 

    String query = "SELECT * FROM " + TABLE_PRODUCTS; 

    //Cursor points to a location in your results 
    Cursor c = db.rawQuery(query, null); 

    if(c.moveToFirst()) 
    { 
     do 
     { 
      if (c.getString(c.getColumnIndex(COLUMN_PRODUCTNAME)) != null) { 
       dbString += c.getString(c.getColumnIndex(COLUMN_PRODUCTNAME)); 
       dbString += "\n"; 
      } 

     }while(c.moveToNext()); 
    } 

    // Close 
    c.close(); 

    return dbString; 
}

希望这将有助于〜

来源

2017-05-27 18:22:12 FAT

无法弄清楚为什么我有null对象引用

回答

相关问题