登录和签名应用程序Android Studio

Question

我是Android编程的初学者，并尝试创建登录和注册活动。但是当我尝试登录时，应用程序崩溃。它在3级显示3错误。 有何评论？
代码如下：

错误消息

FATAL EXCEPTION: main
              Process: com.example.demir.carsharing, PID: 2314
              android.database.sqlite.SQLiteException: no such table: users (code 1): , while compiling: select * from  users where email = 'demir' and password = 'demir'
                  at android.database.sqlite.SQLiteConnection.nativePrepareStatement(Native Method)
                  at android.database.sqlite.SQLiteConnection.acquirePreparedStatement(SQLiteConnection.java:889)
                  at android.database.sqlite.SQLiteConnection.prepare(SQLiteConnection.java:500)
                  at android.database.sqlite.SQLiteSession.prepare(SQLiteSession.java:588)
                  at android.database.sqlite.SQLiteProgram.<init>(SQLiteProgram.java:58)
                  at android.database.sqlite.SQLiteQuery.<init>(SQLiteQuery.java:37)
                  at android.database.sqlite.SQLiteDirectCursorDriver.query(SQLiteDirectCursorDriver.java:44)
                  at android.database.sqlite.SQLiteDatabase.rawQueryWithFactory(SQLiteDatabase.java:1318)
                  at android.database.sqlite.SQLiteDatabase.rawQuery(SQLiteDatabase.java:1257)
                  at com.example.demir.carsharing.DbHelper.getUser(DbHelper.java:66)
                  at com.example.demir.carsharing.LoginActivity.login(LoginActivity.java:58)
                  at com.example.demir.carsharing.LoginActivity.onClick(LoginActivity.java:43)
                  at android.view.View.performClick(View.java:5610)
                  at android.view.View$PerformClick.run(View.java:22265)
                  at android.os.Handler.handleCallback(Handler.java:751)
                  at android.os.Handler.dispatchMessage(Handler.java:95)
                  at android.os.Looper.loop(Looper.java:154)
                  at android.app.ActivityThread.main(ActivityThread.java:6077)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

申请已终止。

Dbhelper (error is in line 66 ----->Cursor cursor = db.rawQuery(selectQuery, null);)

package com.example.demir.carsharing;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;


public class DbHelper extends SQLiteOpenHelper {
    public static final String TAG = DbHelper.class.getSimpleName();
    public static final String DB_NAME="carsharing.db";
    public static final int DB_VERSION =1;

    public static final String USER_TABLE = "users";
    public static final String COLUMN_ID = "_id";
    public static final String COLUMN_EMAIL = "email";
    public static final String COLUMN_PASS = "password";

    /*
 create table users(
     id integer primary key autoincrement,
     email text,
     password text);
  */
    public static final String CREATE_TABLE_USERS = "CREATE TABLE " + USER_TABLE + "("
            + COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT,"
            + COLUMN_EMAIL + " TEXT,"
            + COLUMN_PASS + " TEXT);";

    public DbHelper(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(CREATE_TABLE_USERS);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXIST"+USER_TABLE);
        onCreate(db);
    }

    public void addUser(String email, String password) {
        SQLiteDatabase db = this.getWritableDatabase();

        ContentValues values = new ContentValues();
        values.put(COLUMN_EMAIL, email);
        values.put(COLUMN_PASS, password);

        long id = db.insert(USER_TABLE, null, values);
        db.close();

        Log.d(TAG, "user inserted" + id);
    }

    public boolean getUser(String email, String pass){
        //HashMap<String, String> user = new HashMap<String, String>();
        String selectQuery = "select * from  " + USER_TABLE + " where " +
                COLUMN_EMAIL + " = " + "'"+email+"'" + " and " + COLUMN_PASS + " = " + "'"+pass+"'";

        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);
        // Move to first row
        cursor.moveToFirst();
        if (cursor.getCount() > 0) {

            return true;
        }
        cursor.close();
        db.close();

        return false;
    }
}

LoginActivity.java ----＆gt; error: if(db.getUser(email,pass)){..

@Override
        public void onClick(View v) {
            switch (v.getId()){
                case  R.id.btnLogin:
                    login(); ------> here
                    break;




package com.example.demir.carsharing;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class LoginActivity extends AppCompatActivity implements View.OnClickListener {
    private Button login,register;
    private TextView etEmail,etPass;
    private DbHelper db;
    private Session session;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        db = new DbHelper(this);
        session = new Session(this);
        login =(Button)findViewById(R.id.btnLogin);
        register=(Button)findViewById(R.id.btnReg);
        etEmail=(EditText)findViewById(R.id.etEmail);
        etPass=(EditText)findViewById(R.id.etPass);

        login.setOnClickListener(this);
        register.setOnClickListener(this);

        if(session.loggedin()){
            startActivity(new Intent(LoginActivity.this, MainActivity.class));
            finish();
        }
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()){
            case  R.id.btnLogin:
                login();
                break;
            case R.id.btnReg:
                startActivity(new Intent(LoginActivity.this, RegisterActivity.class));
                break;
            default:

        }

    }

    private void login(){
        String email = etEmail.getText().toString();
        String pass = etPass.getText().toString();

        if(db.getUser(email,pass)){
            session.setLoggedin(true);
            startActivity(new Intent(LoginActivity.this,MainActivity.class));
            finish();
        }else{
            Toast.makeText(getApplicationContext(), "Wrong email/password",Toast.LENGTH_SHORT).show();
        }
    }
}