Question

我正在尝试构建基于数据库的应用。但是当我按下搜索按钮时，应用程序意外停止。现在我能做些什么...

我的logcat消息是：

12-09 18:05:52.177    1552-1552/com.example.bank E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.NullPointerException
        at com.example.bank.search$1.onClick(search.java:44)
        at android.view.View.performClick(View.java:4204)
        at android.view.View$PerformClick.run(View.java:17355)
        at android.os.Handler.handleCallback(Handler.java:725)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:5041)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
        at dalvik.system.NativeStart.main(Native Method)

，search.java是：

public class search extends AppCompatActivity {
    DatabaseOperations myDB;
    Button search;



protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.search);

    search = (Button) findViewById(R.id.btn_search);
    all_data();
}

public void showMessage(String title, String message)
{
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setCancelable(true);
    builder.setTitle(title);
    builder.setMessage(message);
    builder.show();


}

public void all_data()
{
    search.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            Cursor res = myDB.getAllData();
            if (res.getCount() == 0)
            {
                //message
                showMessage("ERROR", "Nothig is Found");
                return;
            }

            else
            {
                StringBuffer buffer = new StringBuffer();
                while (res.moveToNext())
                {
                    buffer.append("EMAIL : "+res.getString(0)+"\n");
                    buffer.append("FULL_NAME : "+res.getString(1)+"\n\n");


                }

                //show all data
                showMessage("Data", buffer.toString());
            }

        }
    });

}




}

DataBaseOperations.java：

public class DatabaseOperations extends SQLiteOpenHelper{
public static final int database_version = 1;
public String CREATE_QUERY = "CREATE TABLE " + TableData.TableInfo.TABLE_NAME+"("+ TableData.TableInfo.EMAIL+" TEXT,"+ TableData.TableInfo.FULL_NAME+" TEXT,"+ TableData.TableInfo.USER_NAME+" TEXT,"+ TableData.TableInfo.USER_PASS+" TEXT,"+ TableData.TableInfo.DISTRICT+" TEXT,"+ TableData.TableInfo.CITY+" TEXT,"+ TableData.TableInfo.VARSITY+" TEXT,"+ TableData.TableInfo.BLOOD_GROUP+" TEXT,"+ TableData.TableInfo.LAST_DATE+" TEXT,"+ TableData.TableInfo.PHONE_NUM+" TEXT );" ;


public DatabaseOperations(Context context) {
    /*
    some change is made here
    check it for problem...
     */

    super(context, TableData.TableInfo.DATABASE_NAME, null, database_version);
    Log.d("Database operations", "Database Created");
}

@Override
public void onCreate(SQLiteDatabase sdb) {

    sdb.execSQL(CREATE_QUERY);
    Log.d("Database operations", "Table Created");
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

}

public void putInformation(DatabaseOperations dop, String email, String full_name, String user_name, String user_pass, String district, String city, String varsity, String blood_group, String last_date, String ph_num)
{
    SQLiteDatabase SQ = dop.getWritableDatabase();
    ContentValues cv = new ContentValues();
    cv.put(TableData.TableInfo.EMAIL, email);
    cv.put(TableData.TableInfo.FULL_NAME, full_name);
    cv.put(TableData.TableInfo.USER_NAME, user_name);
    cv.put(TableData.TableInfo.USER_PASS, user_pass);
    cv.put(TableData.TableInfo.DISTRICT, district);
    cv.put(TableData.TableInfo.CITY, city);
    cv.put(TableData.TableInfo.VARSITY, varsity);
    cv.put(TableData.TableInfo.BLOOD_GROUP, blood_group);
    cv.put(TableData.TableInfo.LAST_DATE, last_date);
    cv.put(TableData.TableInfo.PHONE_NUM, ph_num);


    long k = SQ.insert(TableData.TableInfo.TABLE_NAME, null, cv);


    Log.d("Database operations", "One Raw Inserted");
}


public Cursor getInformation(DatabaseOperations dop)
{
    SQLiteDatabase SQ = dop.getReadableDatabase();
    String[] columns = {TableData.TableInfo.USER_NAME, TableData.TableInfo.USER_PASS};
    Cursor CR = SQ.query(TableData.TableInfo.TABLE_NAME, columns, null, null, null, null, null);
    return CR;

}

public Cursor getAllData()
{
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor res = db.rawQuery("Select * from "+TableData.TableInfo.TABLE_NAME, null);
    return res;

}

}

帮我解决这个问题。提前谢谢

Answer 1

在您提供的代码中，myDB永远不会被初始化。

因此Cursor res = myDB.getAllData();会产生异常。

如何修复零点异常

1 个答案: