试图在Sqlite中输入数据

Question

我试图在sqlite数据库表中插入一些数据，但是收到错误，并且我的另一个表上运行了一些查询和代码。我不明白为什么，错误显示（java.lang.NullPointerException）

Sqlite代码

    public boolean insertStudentData(String stName, String stFName, String contact)
        {


            SQLiteDatabase mydatabase = helper.getWritableDatabase();
            ContentValues contentValues = new ContentValues();
            contentValues.put(Helper.STUDENTNAME,stName);
            contentValues.put(Helper.FATHERNAME,stFName);
            contentValues.put(Helper.CONTACT, contact);
            long result = mydatabase.insert(Helper.STUDENT_TABLE,null,contentValues);
            if (result <= 0)
            {
                return false;
            }
            else
            {
                return true;
            }
        }
class Helper extends SQLiteOpenHelper
    {

        //database information
        private static final String DATABASE_NAME="mydatabase6";
        private static final int DTABASE_VERSION=25;

        //User_Registration_information
        private static final String TABLE_NAME="registration_table";
        private static final String NAME="name";
        private static final String USERNAME="username";
        private static final String PASSWORD="password";


        //Student_Registration_Information
        private static final String STUDENT_TABLE="student_table";
        private static final String STUDENTNAME="studentName";
        private static final String FATHERNAME="fatherName";
        private static final String CONTACT="contact";

        Context context;
        public Helper(Context context) {
            super(context,DATABASE_NAME, null, DTABASE_VERSION);
            this.context = context;
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
            //Create User Table
            String CREATE_TABLE=
                    "CREATE TABLE"+
                            " "+Helper.TABLE_NAME+" "+
                            "("
                            +Helper.NAME+" TEXT,"
                            +Helper.USERNAME+" TEXT,"
                            +Helper.PASSWORD+" TEXT);";
            db.execSQL(CREATE_TABLE);


            //Create Student Table
            String CREATE_STUDENT_TABLE=
                    "CREATE TABLE"+
                            " "+Helper.STUDENT_TABLE+" "+
                            "("
                            +Helper.STUDENTNAME+" TEXT,"
                            +Helper.FATHERNAME+" TEXT,"
                            +Helper.CONTACT+" TEXT);";
            db.execSQL(CREATE_STUDENT_TABLE);

            Toast.makeText(context, "Database is Created", Toast.LENGTH_LONG).show();
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            String DROP_TABLE = "DROP IF TABLE EXISTS"+Helper.TABLE_NAME;
            String DROP_TABLE_STUDENT = "DROP IF TABLE EXISTS"+Helper.STUDENT_TABLE;
            db.execSQL(DROP_TABLE);
            db.execSQL(DROP_TABLE_STUDENT);
            onCreate(db);
            Toast.makeText(context,"onUpgrrade Method called", Toast.LENGTH_LONG).show();
        }
    }

活动代码

boolean insertedStudent = database.insertStudentData( stName.getText().toString(), fName.getText().toString(), contact.getText().toString());
if (insertedStudent = true)
{
    Toast.makeText(Student_Insertion.this, "Student is Added", Toast.LENGTH_LONG).show();
    final android.os.Handler handler = new android.os.Handler();
    handler.postDelayed(new Runnable() {
        public void run() {
            // TODO: Your application init goes here.
            Intent mInHome = new Intent(Student_Insertion.this, MainActivity.class);
            Student_Insertion.this.startActivity(mInHome);
            Student_Insertion.this.finish();
        }
    }, 1000);
}
else
{
    Toast.makeText(Student_Insertion.this,"Student is not Added",Toast.LENGTH_LONG).show();
}

错误

02-10 23:11:32.684 21674-21674/com.example.ks.doit E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.ks.doit, PID: 21674 java.lang.NullPointerException: Attempt to invoke virtual method 'boolean com.example.ks.doit.sqlite_database.insertStudentData(java.lang.String, java.lang.String, java.lang.String)' on a null object reference
at com.example.ks.doit.Student_Insertion$1.onClick(Student_Insertion.java:43)
at android.view.View.performClick(View.java:5076)
at android.view.View$PerformClick.run(View.java:20279)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5910)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1405)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1200)