java.lang.NullPointerException無法解決

Question

我的仿真程序顯示應用程序意外停止的消息。我無法找到我的代碼中的錯誤。

這是我的主類名爲Database2Activity：

package data.base; 

import com.example.phone_no.DBhelper; 

import android.app.Activity; 
import android.os.Bundle; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.widget.Button; 
import android.widget.EditText; 

public class Database2Activity extends Activity { 
EditText editText; 
DBhelper DB; 
/** Called when the activity is first created. */ 
@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.main); 
    Button button = (Button)findViewById(R.id.button); 
    DB = new DBhelper(Database2Activity.this); 
    button.setOnClickListener(new OnClickListener() { 

     @Override 
     public void onClick(View arg0) { 
      // TODO Auto-generated method stub 
      String rr = editText.getText().toString(); 

      DB.open(); 
      DB.adddata("A", rr, "122"); 
       DB.getAlldata(); 
       DB.close(); 

     } 
    }); 


} 
} 

而且這是我的第二個Java類名爲DBhelper：

package com.example.phone_no; 



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

public class DBhelper { 
public static final String KEY_ID = "id"; 
public static final String KEY_NAME = "names"; 

public static final String KEY_PHONE = "phoneno"; 

private final Context ourContext; 
private static final String DATABASE_TABLE = "Contactinfo"; 
private static final int DATABASE_VERSION = 27; 
private static final String DATABASE_NAME = "contactdata.db"; 
private DbHelper ourHelper; 
private SQLiteDatabase ourDatabase; 
// end for location 
private static class DbHelper extends SQLiteOpenHelper { 
    public DbHelper(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 

     /*db.execSQL("CREATE TABLE " + DATABASE_TABLE + " (" + KEY_ID 
       + " INTEGER PRIMARY KEY, " + KEY_NAME + " TEXT NULL , " 
       + KEY_PHONE + " INTEGER NULL);");*/ 
     db.execSQL("CREATE TABLE " + DATABASE_TABLE + " (" + KEY_ID 
       + " INTEGER , " + KEY_NAME + " TEXT NULL , " 
       + KEY_PHONE + " INTEGER NULL);"); 

     // string value 
     String y = "CREATE TABLE " + DATABASE_TABLE + " (" + KEY_ID 
       + " INTEGER PRIMARY KEY , " + KEY_NAME + " TEXT NULL , " 
       + KEY_PHONE + " INTEGER NULL);"; 

     System.out.println("query" + y); 
     Log.d("query", y); 






    } 

    @Override 
    public void onUpgrade(SQLiteDatabase db, int arg1, int arg2) { 
     // TODO Auto-generated method stub 
     // TODO Auto-generated method stub 
     db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE); 
     onCreate(db); 

    } 

} 

public DBhelper(Context c) { 
    ourContext = c; 
} 
public DBhelper open() throws SQLException 
{ 
    ourHelper = new DbHelper(ourContext); 
    ourDatabase = ourHelper.getWritableDatabase(); 
    return this; 
} 

public void close() 
{ 
    ourHelper.close(); 
} 
public long adddata(String id,String name,String number) 
{ 
    // TODO Auto-generated method stub 
    // add the custom Image Gallery Image Path to Data Base 
    ContentValues cv = new ContentValues(); 
    cv.put(KEY_ID, id); 
    cv.put(KEY_NAME, name); 
    cv.put(KEY_PHONE, number); 
    return ourDatabase.insert(DATABASE_TABLE, null, cv); 
} 

public void getAlldata() 
{ 
    Cursor details = null; 
    if (ourDatabase.isOpen() == false) 

     ourDatabase = ourHelper.getWritableDatabase(); 
    if (ourDatabase.isOpen()) 
    { 
     details = ourDatabase.query(DATABASE_TABLE, null, null, null, null, null, null); 
     for(details.moveToFirst();!details.isAfterLast();details.moveToNext()) 
     { 
      String a=details.getString(0); 
      String b=details.getString(1); 
      String c=details.getString(2); 
      System.out.println("id--"+a+"name"+b+"phoneno"+c); 
     } 



    } 


} 
public long delete_image(String id) 
    { 
    if (ourDatabase.isOpen() == false) 
     ourDatabase = ourHelper.getWritableDatabase(); 
     if (ourDatabase.isOpen()) 
     { 
      return ourDatabase.delete(DATABASE_TABLE, KEY_ID + "=" + id, null); 
     } 
     return 0; 
    } 



} 

這是logcat的窗口中的錯誤消息：

03-11 23:20:32.677: E/AndroidRuntime(444): FATAL EXCEPTION: main 
03-11 23:20:32.677: E/AndroidRuntime(444): java.lang.NullPointerException 
03-11 23:20:32.677: E/AndroidRuntime(444): at data.base.Database2Activity$1.onClick(Database2Activity.java:27) 
03-11 23:20:32.677: E/AndroidRuntime(444): at android.view.View.performClick(View.java:2408) 
03-11 23:20:32.677: E/AndroidRuntime(444): at android.view.View$PerformClick.run(View.java:8816) 
03-11 23:20:32.677: E/AndroidRuntime(444): at android.os.Handler.handleCallback(Handler.java:587) 
03-11 23:20:32.677: E/AndroidRuntime(444): at android.os.Handler.dispatchMessage(Handler.java:92) 
03-11 23:20:32.677: E/AndroidRuntime(444): at android.os.Looper.loop(Looper.java:123) 
03-11 23:20:32.677: E/AndroidRuntime(444): at android.app.ActivityThread.main(ActivityThread.java:4627) 
03-11 23:20:32.677: E/AndroidRuntime(444): at java.lang.reflect.Method.invokeNative(Native Method) 
03-11 23:20:32.677: E/AndroidRuntime(444): at java.lang.reflect.Method.invoke(Method.java:521) 
03-11 23:20:32.677: E/AndroidRuntime(444): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868) 
03-11 23:20:32.677: E/AndroidRuntime(444): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626) 
03-11 23:20:32.677: E/AndroidRuntime(444): at dalvik.system.NativeStart.main(Native Method) 

Answer 1

永遠不要致電findViewById()查詢editText所以它保持null。您需要在嘗試使用它之前這樣做：

editText = (EditText) findViewById(R.id.your_edittext_id);//replace with your id 
button.setOnClickListener(new OnClickListener() { 
    //rest of code 

Answer 2

必須使用findViewById它返回視圖中給ID獲取視圖。

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.main); 
    editText = (EditText)findViewById(R.id.editText); 

    ..... 
}