想知道android为什么运用了数据库导致无法从别的界面跳到使用了数据库的界面

部分代码如下

public class MainActivity extends AppCompatActivity {
     EditText name,password;
     Button denglu,zhuce;
     String n=null;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        denglu=(Button) findViewById(R.id.denglu);
        zhuce=(Button) findViewById(R.id.zhuce);
        denglu.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String  usename=name.getText().toString().trim();
                String  usepassword=password.getText().toString().trim();
                SQLiteDatabase database=SQLiteDatabase.openDatabase("kk.db",null,SQLiteDatabase.OPEN_READONLY);
                Cursor cursor=database.rawQuery("select * from userTb where name =?",new String[]{usename});
                int success=0;
                while (cursor.moveToNext()){
                    @SuppressLint("Range") String String_upwd=cursor.getString(cursor.getColumnIndex("upwd"));
                    if (usepassword.equals(String_upwd)){
                        success=1;
                        Toast.makeText(MainActivity.this, usename+"用户登录成功", Toast.LENGTH_LONG).show();
                        Intent intent1=new Intent(MainActivity.this,MainActivity2.class);
                        startActivity(intent1);
                    }
                }
                if (success==0){
                    Toast.makeText(MainActivity.this, usename+"密码错误", Toast.LENGTH_LONG).show();
                }
            }
        });
         zhuce.setOnClickListener(new View.OnClickListener() {
           @Override
           public void onClick(View view) {
               Intent intent2=new Intent(MainActivity.this,MainActivity3.class);
               startActivity(intent2);
           }
       });
    }
}

public class MainActivity3 extends AppCompatActivity {
    Button queren,fanhui;
    EditText name2,password2,password3;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main3);
        queren=(Button) findViewById(R.id.denglu);
        fanhui=(Button) findViewById(R.id.zhuce);
        name2=(EditText) findViewById(R.id.name2);
        password2=(EditText) findViewById(R.id.password2);
        password3=(EditText) findViewById(R.id.password3);
        fanhui.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent3=new Intent(MainActivity3.this,MainActivity.class);
                startActivity(intent3);
            }
        });
       queren.setOnClickListener(new View.OnClickListener() {
           @Override
           public void onClick(View view) {
               SQLiteDatabase db= null;
               if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O_MR1) {
                   db = SQLiteDatabase.openDatabase(new File("kk.db"),null);
               }
               String usename=name2.getText().toString().trim();
               String usepassword=password2.getText().toString().trim();
               if (usename.equals("")||usepassword.equals("")){
                   Toast.makeText(MainActivity3.this,"请重新输入！",Toast.LENGTH_LONG).show();
               }else{
                   String sql="insert into userTb(name2,password2) values('"+usename+"','"+usepassword+"')";
                   db.execSQL(sql);
                   Toast.makeText(MainActivity3.this,"注册成功",Toast.LENGTH_LONG).show();
               }
           }
       });
    }
}

public class MainActivity2 extends AppCompatActivity {
      Button cx,gl;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        cx=(Button) findViewById(R.id.cx);
        gl=(Button) findViewById(R.id.gl);
        cx.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent4=new Intent(MainActivity2.this,chaxun.class);
                startActivity(intent4);
            }
        });
        gl.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent5=new Intent(MainActivity2.this,guanli.class);
                startActivity(intent5);
            }
        });
    }
}

就你这几行代码没看出有什么问题，往下你怎么写的。