java錯誤：無法啓動活動ComponentInfo：java.lang.NullPointerException

Question

我一直面臨這種類型的錯誤約java.lng.NullPointerExeption。任何人都知道如何解決它？

main.java package com.example.shoppingassistantnew;

import greendroid.app.GDActivity; 
import greendroid.widget.ActionBarItem; 
import greendroid.widget.ActionBarItem.Type; 


import android.os.Bundle; 
import android.app.Activity; 
import android.content.Intent; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.support.v4.app.NavUtils; 

public class main extends GDActivity { 


    private static final int EXPAND = 0; 

    private static final int ADD = 1; 


    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setActionBarContentView(R.layout.shoppinglist); 

     initActionBar(); 
    } 
    private void initActionBar(){ 

     addActionBarItem(Type.List,EXPAND); 
     addActionBarItem(Type.Add,ADD); 

    } 


    @Override 
     public boolean onHandleActionBarItemClick(ActionBarItem item, int position) { 
      // TODO Auto-generated method stub 
     switch(item.getItemId()){ 
     case ADD: 
      Intent intent=new Intent(main.this,AddShoppingList.class); 
      startActivity(intent); 

     } 
      return super.onHandleActionBarItemClick(item, position); 
     } 

    static final class ProductData{ 

      String barcode; 
      String format; 
     } 
} 

AddShoppingList.java

package com.example.shoppingassistantnew; 

import com.cyrilmottier.android.greendroid.R; 
import com.example.shoppingassistantnew.main.ProductData; 


import android.app.ActionBar.LayoutParams; 
import android.app.Activity; 
import android.app.AlertDialog; 
import android.app.Dialog; 
import android.content.Context; 
import android.content.DialogInterface; 
import android.os.Bundle; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.view.WindowManager; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.ListView; 
import greendroid.app.GDActivity; 

public class AddShoppingList extends Activity implements OnClickListener { 
    private static final ProductData mProductData = new ProductData(); 
    private Button mSaveBttn; 
    EditText mBarcodeEdit; 
    EditText mFormatEdit; 
    ProductDatabase mProductDb; 


    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     // TODO Auto-generated method stub 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.addshoppinglist); 
     mBarcodeEdit = (EditText) findViewById(R.id.shoppingListNameEdit); 
     mFormatEdit = (EditText) findViewById(R.id.shoppingDateEdit); 

     mSaveBttn.setOnClickListener(this); 


    } 



    private void showInfoDialog(Context context, String title, 
      String information) { 
     new AlertDialog.Builder(context).setMessage(information) 
       .setTitle(title) 
       .setPositiveButton("OK", new DialogInterface.OnClickListener() { 

        public void onClick(DialogInterface dialog, int which) { 
         dialog.dismiss(); 

        } 
       }).show(); 
    } 



    public void onClick(View v) { 
     // TODO Auto-generated method stub 

    } 

} 

ProductDatabase.java

package com.example.shoppingassistantnew; 
import com.example.shoppingassistantnew.main.ProductData; 
import android.content.ContentValues; 
import android.content.Context; 
import android.database.sqlite.SQLiteDatabase; 
import android.database.sqlite.SQLiteDatabase.CursorFactory; 
import android.database.sqlite.SQLiteOpenHelper; 
import android.util.Log; 

public class ProductDatabase { 
private static final String PRODUCT_TABLE="products"; 
private static final String DATABASE_NAME="spot_pay.db"; 
private static final int DATABASE_VERSION=1; 
private SQLiteDatabase db; 


private static class ProductDatabaseHelper extends SQLiteOpenHelper { 

    private static final String TAG = null; 

    public ProductDatabaseHelper(Context context) { 
     super(context, DATABASE_NAME, null, DATABASE_VERSION); 
     // TODO Auto-generated constructor stub 
    } 

    @Override 
    public void onCreate(SQLiteDatabase db) { 
     // TODO Auto-generated method stub 
     StringBuilder sql=new StringBuilder(); 

     sql.append("create table ").append(PRODUCT_TABLE) 
      .append("( ") 
      .append(" _id integer primary key,") 
      .append(" barcode text,") 
      .append(" format text,") 
      .append(") "); 


     db.execSQL(sql.toString()); 
      Log.d(TAG, PRODUCT_TABLE + "table created");  
    } 

    @Override 
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 
     // TODO Auto-generated method stub 
     db.execSQL("drop table if exists " + PRODUCT_TABLE);      
      onCreate(db); 
    } 



} 



public ProductDatabase(Context context){ 
    ProductDatabaseHelper helper = new ProductDatabaseHelper(context); 
    db = helper.getWritableDatabase(); 

} 
public boolean insert(ProductData product){ 
    ContentValues vals = new ContentValues(); 
     vals.put("barcode", product.barcode); 
     vals.put("format", product.format); 
    return db.insert(PRODUCT_TABLE, null, vals) != -1; 
} 
} 

的AndroidManifest.xml

<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
    package="com.example.shoppingassistantnew" 
    android:versionCode="1" 
    android:versionName="1.0" > 

    <uses-sdk 
     android:minSdkVersion="7" 
     android:targetSdkVersion="15" /> 

    <application 
     android:icon="@drawable/ic_launcher" 
     android:label="@string/app_name" 
     android:theme="@style/Theme.GDActionBarExample" 
      android:name=".GDIntroApp"> 
     <activity 
      android:name=".main" 
      android:label="@string/title_activity_main" > 
      <intent-filter> 
       <action android:name="android.intent.action.MAIN" /> 

       <category android:name="android.intent.category.LAUNCHER" /> 
      </intent-filter> 
     </activity> 

     <activity android:name=".AddShoppingList" android:theme="@style/myDialog"></activity> 
    </application> 

</manifest> 

logcat的

10-27 10:18:15.545: D/AndroidRuntime(1633): Shutting down VM 
10-27 10:18:15.545: W/dalvikvm(1633): threadid=1: thread exiting with uncaught exception (group=0x4001d800) 
10-27 10:18:15.555: E/AndroidRuntime(1633): FATAL EXCEPTION: main 
10-27 10:18:15.555: E/AndroidRuntime(1633): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.shoppingassistantnew/com.example.shoppingassistantnew.AddShoppingList}: java.lang.NullPointerException 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at android.app.ActivityThread.access$2300(ActivityThread.java:125) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at android.os.Handler.dispatchMessage(Handler.java:99) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at android.os.Looper.loop(Looper.java:123) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at android.app.ActivityThread.main(ActivityThread.java:4627) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at java.lang.reflect.Method.invokeNative(Native Method) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at java.lang.reflect.Method.invoke(Method.java:521) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at dalvik.system.NativeStart.main(Native Method) 
10-27 10:18:15.555: E/AndroidRuntime(1633): Caused by: java.lang.NullPointerException 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at com.example.shoppingassistantnew.AddShoppingList.onCreate(AddShoppingList.java:39) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627) 
10-27 10:18:15.555: E/AndroidRuntime(1633):  ... 11 more 
10-27 10:18:17.825: I/Process(1633): Sending signal. PID: 1633 SIG: 9 

Answer 1

在 AddShoppingList.java , mSaveBttn is null

給refrence首先從XML，然後用它

mFormatEdit = (EditText) findViewById(R.id.shoppingDateEdit); 

    mSaveBttn =(Button)findViewById(R.id.mSaveBttn);<<<< Give Refrence from xml 
    mSaveBttn.setOnClickListener(this); 
    ^^^^^^^^^