android - 在Android app_project中的SQLite数据库上读写

我在LogCat上遇到java.lang.NullPointer错误，但我不知道为什么。似乎错误在于第二个OnClick过程。有人可以帮助我吗？

package com.example.cinemaodeon;


 import com.example.cinemaodeon.Main;



public class Main extends Activity {

private DatabaseHelper DatabaseHelper;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);


    setContentView(R.layout.main_layout);

    final Toast toast = Toast.makeText(this,
            "Il nome utente non è presente!!! registra un nuovo nome utente!",
            Toast.LENGTH_LONG);

    final Toast toast2 = Toast.makeText(this,
            "Il nome utente è già presente!!!Puoi accedere con questo nome utente!",
            Toast.LENGTH_LONG);


    Button btnGO = (Button) findViewById(R.id.main_btnentra);
     btnGO.setOnClickListener(new OnClickListener(){
        @Override
        public void onClick(View arg0) {

            SQLiteDatabase db = DatabaseHelper.getReadableDatabase();
            String[] columns = { "_id_user" };
            Cursor cursor = db.query("users", columns, null, null, null, null, null);
            String[] result = new String[]{};
            int count = cursor.getCount();
            while (cursor.moveToNext()) {
                result[count]=cursor.getString(0);
                count=count+1;
                }

            EditText editusername =(EditText) findViewById(R.id.main_editusername);
            final String username = editusername.getText().toString();

            for (int i = 0; i < count; i++) {
            if (username==result[count]){

                Intent openProgrammazione = new Intent(Main.this,Programmazione.class);
                startActivity(openProgrammazione);

                }
            }
            toast.show();
            }
        });


     Button btnReg = (Button) findViewById(R.id.main_btnregistra);
     btnReg.setOnClickListener(new OnClickListener(){

似乎该错误是通过OnClick程序覆盖的，可能与databaseHelper的定义有关

        @Override
        public void onClick(View arg0) {

            SQLiteDatabase db = DatabaseHelper.getReadableDatabase();
            String[] columns = { "_id_user" };
            Cursor cursor = db.query("users", columns, null, null, null, null, null);
            String[] result = new String[]{};
            int count = cursor.getCount();
            while (cursor.moveToNext()) {
                result[count]=cursor.getString(0);
                count=count+1;
                }


            EditText editusername =(EditText) findViewById(R.id.main_editusername_registra);
            final String username = editusername.getText().toString();


            for (int i = 0; i < count; i++) {
            if (username==result[count]){

                toast2.show();
                }
            }
                SQLiteDatabase dbW = DatabaseHelper.getWritableDatabase();
                ContentValues values = new ContentValues();
                values.put("_id_user", username);
                @SuppressWarnings("unused")
                long id = dbW.insert("users", null, values);

            Intent openProgrammazione = new Intent(Main.this,Programmazione.class);
            startActivity(openProgrammazione);
            }

     });


}

最佳答案

您尚未初始化DatabaseHelper DatabaseHelper。

尝试初始化

eg:DatabaseHelper DatabaseHelper=new DatabaseHelper();