Null pointer java.lang.NullPointerException, seek expert guidance

02-18 07:41:43.549: ERROR / AndroidRuntime (513): java.lang.NullPointerException

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase (SQLiteOpenHelper.java: 98)

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at com.nj.dao.UsersSerive.getUsersByLoginI (UsersSerive.java: 25)

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at com.nj.dao.UsersDaoImpl.getUsersByLoginId (UsersDaoImpl.java: 19)

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at com.nj.dao.One $ 1.onClick (One.java: 34)

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at android.view.View.performClick (View.java: 2408)

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at android.view.View $ PerformClick.run (View.java: 8816 )

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at android.os.Handler.handleCallback (Handler.java: 587)

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at android.os.Handler.dispatchMessage (Handler.java: 92)

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at android.os.Looper.loop (Looper.java: 123)

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at android.app.ActivityThread.main (ActivityThread.java: 4627)

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at java.lang.reflect.Method.invokeNative (Native Method)

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at java.lang.reflect.Method.invoke (Method.java: 521 )

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run (ZygoteInit . java: 868)

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at com.android.internal.os.ZygoteInit.main (ZygoteInit.java : 626)

02-18 07:41:43.549: ERROR / AndroidRuntime (513): at dalvik.system.NativeStart.main (Native Method)


This is in the running to SQLiteDatabase sqlite = dbhelper.getWritableDatabase (); This step out of the wrong seek guidance
Here is my code DBHelper

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

public class DBHelper extends SQLiteOpenHelper {
private static final String name = "mydata";
private static final int version = 1;

public DBHelper (Context context) {
super (context, name, null, version);
/ / TODO Auto-generated constructor stub
}

@ Override
public void onCreate (SQLiteDatabase db) {
/ / TODO Auto-generated method stub
db.execSQL ("create table if not exists users (id integer primary key autoincrement, loginId varchar (50), password varchar (50))");
db.execSQL ("insert into users (loginId, password) values ​​('123 ', '123')");
}

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

 
 / / Instantiate your dbhelper class 
 DBHelper dbhelper = new DBHelper (activity name. This); sqlite = dbhelper.getWritableDatabase ();  
 ------ Solution ------------------------------------ -------- 
 dbhelper is empty chanting, new DBHelper see if there is an exception when 
 best to initialize it private DBHelper dbhelper = null; 
 ------ Solution -------------------- ------------------------ 
 plus trace to see it. 
 
 UsersSerive.java: 25 
 This is NOT true? 
 ------ Solution ---------------------------------------- ---- 
 you have not been instantiated dbhelper 
 
 
 ------ For reference only ---------------------------------- ----- 
 null pointer error, double check your instance 
 ------ For reference only --------------------- ------------------ 
 
 
 / / Instantiate your dbhelper class 
 DBHelper dbhelper = new DBHelper (activity name. This); SQLiteDatabase sqlite = dbhelper.getWritableDatabase (); 
  
 ------ For reference only ---------------------------------- ----- 
 my serive code below 
 public class UsersSerive { 
 private DBHelper dbhelper; 
 public UsersSerive (Context context) { 
 dbhelper = new DBHelper (context); 
} 
 
 / / Save 
 public void save (Users user) { 
 SQLiteDatabase sqlite = dbhelper.getWritableDatabase (); 
 sqlite.execSQL ("insert into users (loginid, password) values ​​(?,?)", new Object [] {user.getLoginId (), user.getPassword ()}) ; 
} 
 
 / / Find the user name 
 public Users getUsersByLoginId (String loginId) { 
 Log.e ("userserive4", loginId); 
 SQLiteDatabase sqlite = dbhelper.getWritableDatabase (); 
 Log.e ("userserive", loginId); 
 Cursor cursor = sqlite.rawQuery ("select * from users where loginId =?", new String [] {loginId}); 
 Log.e ("userserive2", loginId); 
 if (cursor.moveToNext ()) { 
 Users user = new Users (); 
 Log.e ("userserive3", loginId); 
 user.setId (cursor.getInt (0)); 
 user.setLoginId (cursor.getString (1)); 
 user.setPassword (cursor.getString (2)); 
 return user; 
} 
 return null; 
} 
} 
 Chuan is a context where no activity 
 ------ For reference only ------------------------- -------------- 
 that there is an error in the above lines, the piece of code posted Come 
 ------ For reference only ----- ---------------------------------- 
 null pointer exception is the easiest exception, reported where empty take a look at where 
 ------ For reference only ---------------------------------- ----- 
 Thank you very much for replies, these days something could not Login Forum, tomorrow I will go to work tomorrow, the code is posted, ask you greatly enlighten 
 ------ For reference only --------------------------------------- 
 Below I posted up my DBHelper and UsersSerive and UsersDaoImpl code you greatly look oh 
 ------ For reference only -------------------------- ------------- 
 which instance is not initialized it 
 ------ For reference only ----------------- ---------------------- 
 DBHelper and UsersSerive code above the code given below UsersDaoImpl 
 package com.nj.dao; 
 
 import android.test.AndroidTestCase; 
 import android.util.Log; 
 
 import com.nj.entity.Users; 
 
 public class UsersDaoImpl extends AndroidTestCase { 
 public void save (Users user) { 
 UsersSerive userSerive = new UsersSerive (this.getContext ()); 
 userSerive.save (user); 
} 
 
 public Users getUsersByLoginId (String loginId) { 
 Log.e ("userDaoImpl", loginId); 
 UsersSerive userSerive = new UsersSerive (this.getContext ()); 
 Users user = new Users (); 
 Log.e ("userDaoImpl2", loginId); 
 user = userSerive.getUsersByLoginId (loginId); 
 return user; 
} 
} 
 ------ For reference only --------------------------------- ------ 
 above being given UsersSerive.java: 25 is the line: SQLiteDatabase sqlite = dbhelper.getWritableDatabase (); 
 UsersDaoImpl.java: 19 is: user = userSerive.getUsersByLoginId (loginId); 
 One.java: 34 yes: Users user = userDao.getUsersByLoginId (loginId.getText (). toString ()); 
 
 are ultimately dbhelper.getWritableDatabase wrong, the return should be empty seeking advice 
 
 
 ------ For reference only ---------------------------------- ----- 
 [code = Jav] dbhelper = new DBHelperUtil (context); 
 
 db = dbhelper.getWritableDatabase (); [/ code] 
 ------ For reference only ------------------- -------------------- 
 [code = Jav] 
 
 dbhelper = new DBHelperUtil (context); 
 
 db = dbhelper.getWritableDatabase (); 
 [/ code] 
 ------ For reference only ------------------------------ --------- 
 Upstairs positive solution! 
 ------ For reference only -------------------------------------- - 
 uh reconstruction method in my way when instantiated 
 private DBHelper dbhelper = null; 
 public UsersSerive (Context context) { 
 dbhelper = new DBHelper (context); 
} 
 ------ For reference only --------------------------------- ------ 
 not instantiate the object. . . 
 ------ For reference only -------------------------------------- - 
 Will not solved? I also encountered dbhelper not empty, but db = dbhelper.getWritableDatabase () when it reported this null pointer exception, db is null; 
 
 ------ For reference only ---------------------------------- ----- 
 I also encountered this problem, solve it, 
 ------ For reference only ------------------- -------------------- 
 I am also in the constructor, which has been declared a variable, in the end the problem lie? ? 
 ------ For reference only -------------------------------------- - 
 I have encountered this problem, how to solve, anxious, tangled!