SQLiteDatabase对象

Question

我有一个Helper类，它扩展了SQLiteOpenHelper，因为我有四个方法

onCreate(SQLiteDatabase sqLiteDatabase)

onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1)

addquestions()

addingeachquestions(Questions question)// I also have one class named as Questions

我将SQLiteDatabase对象声明为＆＃34; db＆＃34;在我的Helperclass下 但是当我在我的onCreate＆amp;中做"db=sqLiteDatabase"时onUpgrade方法它工作正常，但是当我不在我的"db=sqLiteDatabase"方法中生成语句onCreate & onUpgrade时，logcat窗口在行处显示空指针异常

"db.insert(Helperclass.TABLE_NAME, null, values);"

这是我的代码

Helperclass.java

public class Helperclass extends SQLiteOpenHelper {
SQLiteDatabase db;
Context context;
Helperclass helperclass;
private static final String DATABASE_NAME = "DATABASE1";
private static final int DATABASE_VERSION = 52;
private static final String TABLE_NAME = "TABLE1";
private static final String UID = "_ID";
private static final String QUESTION = "QUESTION";
private static final String OPT1 = "OPT1";
private static final String OPT2 = "OPT2";
private static final String OPT3 = "OPT3";
private static final String ANSWER = "ANSWER";
private static final String CREATE_TABLE =
        "CREATE TABLE " + TABLE_NAME + " (" + UID + " INTEGER PRIMARY KEY, " + QUESTION + " VARCHAR(255), " + OPT1 + " VARCHAR(255), " + OPT2 + " VARCHAR(255), " + OPT3 + " VARCHAR(255), " + ANSWER + " VARCHAR(255));";
private static final String DROP_TABLE = "DROP TABLE IF EXISTS " + TABLE_NAME;      


public Helperclass(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
    this.context = context;
    Toast.makeText(context, "constructor called", Toast.LENGTH_SHORT).show();
}

@Override
public void onCreate(SQLiteDatabase sqLiteDatabase) {
    db = sqLiteDatabase;  //when i mentions this it works fine but when i don't it gives error. Why i have to mention this?         
    sqLiteDatabase.execSQL(CREATE_TABLE);
     addquestions();
}

@Override
public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {
    db = sqLiteDatabase;  //when i mentions this it works fine but when i don't it gives error.
   sqLiteDatabase.execSQL(DROP_TABLE);
    onCreate(sqLiteDatabase);
}

public void addquestions() {
    Questions q1 = new Questions("what is your hobby?", "singing", "dancing", "reading", "sining");
    this.addingeachquestions(q1);
    Questions q2 = new Questions("how are you?", "fine", "good", "sad", "good");
    this.addingeachquestions(q2);
    Questions q3 = new Questions("whats your name?", "sam", "jhon", "alice", "sam");
    this.addingeachquestions(q3);
    Toast.makeText(context, "addquestion method called", Toast.LENGTH_SHORT).show();
}

private void addingeachquestions(Questions question) {
    ContentValues values = new ContentValues();
    values.put(Helperclass.QUESTION, question.getQUESTION());
    values.put(Helperclass.OPT1, question.getOPTA());
    values.put(Helperclass.OPT2, question.getOPTB());
    values.put(Helperclass.OPT3, question.getOPTC());
    values.put(Helperclass.ANSWER, question.getANSWER());
    db.insert(Helperclass.TABLE_NAME, null, values);
}

Answer 1

无需在db = sqLiteDatabase;中添加onCreate行。在getWritableDatabase方法调用db中调用insert方法之前调用addingeachquestions方法初始化getWritableDatabase()对象，然后再使用它。

db=getWritableDatabase();
db.insert(Helperclass.TABLE_NAME, null, values);

Answer 2

你必须在“addingeachquestions”方法中得到Db的对象，如下所示：

SQLiteDatabase db = this.getWritableDatabase();

或

db = this.getWritableDatabase();

Answer 3

在调用助手类时初始化db： -

Helperclass helperClassObj = new Helperclass(context);
db = helperClassObj.getWritableDatabase();

Answer 4

这是因为SQLiteDatabase，db的实例为NULL。

您完全不需要在onCretae（SQLiteDatabase sqLiteDatabase）和db = sqLiteDatabase;中添加onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1)，而是使用以下代码。

if (db ==null){ 
    db = this.getWritableDatabase();
 }