Android SQLite数据库连接实现登录功能

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" >
<!--  布局的背景颜色-->
<!--  <solid android:color="#FFFFFF" />-->
<!--  边框线的粗细和颜色-->
  <stroke
      android:width="0.01dp"
      android:color="#000" />
  <padding
      android:bottom="5dp"
      android:left="5dp"
      android:right="5dp"
      android:top="5dp" />
<!--  圆角-->
  <corners android:radius="5dp" />
</shape>

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
<LinearLayout
        android:padding="5dp"
        android:background="@drawable/border"
        android:orientation="vertical"
        android:layout_gravity="center_horizontal"
        android:layout_width="360dp"
        android:layout_height="112dp">
      <LinearLayout
          android:orientation="horizontal"
          android:layout_gravity="center_horizontal"
          android:layout_width="match_parent"
          android:layout_height="50dp">
        <ImageView
            android:layout_marginRight="15dp"
            android:layout_gravity="center_vertical"
            android:layout_width="30dp"
            android:layout_height="30dp" app:srcCompat="@drawable/usn" android:id="@+id/usn"/>
        <!-- android:background="@null" 去掉下划线        -->
        <EditText
            android:singleLine="true"
            android:background="@null"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:inputType="text"
            android:hint="用户名"
            android:ems="10"
            android:id="@+id/username"/>
      </LinearLayout>
      <!-- 水平线-->
      <View
          android:layout_height="0.5dip"
          android:background="#686868"
          android:layout_width="match_parent"/>
      <LinearLayout
          android:orientation="horizontal"
          android:layout_gravity="center_horizontal"
          android:layout_width="match_parent"
          android:layout_height="50dp">
        <ImageView
            android:layout_marginRight="15dp"
            android:layout_gravity="center_vertical"
            android:layout_width="30dp"
            android:layout_height="30dp" app:srcCompat="@drawable/pwd" android:id="@+id/密码"/>
        <EditText
            android:singleLine="true"
            android:background="@null"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:inputType="textPassword"
            android:hint="密码"
            android:ems="10"
            android:id="@+id/password"/>
      </LinearLayout>
    </LinearLayout>
    <Button
        android:layout_gravity="center_horizontal"
        android:background="#EF8D89"
        android:layout_marginTop="20dp"
        android:text="登  录"
        android:onClick="userLogin"
        android:layout_width="360dp"
        android:layout_height="wrap_content" android:id="@+id/login"/>
</android.support.constraint.ConstraintLayout>

public class MainActivity extends AppCompatActivity implements View.OnClickListener{
  //访问数据库的类
  SQLiteDatabase db;
  //定义常量，作为消息的key
  public final static String MESSAGE_KEY="com.android2";
 @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    /**
     * (参数)1、context MainActivity
     * 2、name 数据库名
     * 3、
     * 4、版本号
     */
    final DatabaseHelper databaseHelper = new DatabaseHelper(this,"emis.db",null,2);
    //获得读取数据库权限
    db = databaseHelper.getReadableDatabase();
    setContentView(R.layout.activity_main);
  }
  /*响应*/
  private void userLogin() {
    EditText et1 = findViewById(R.id.username);
    String username = et1.getText().toString();
    EditText et2 = findViewById(R.id.password);
    String password = et2.getText().toString();
    //游标类Cursor 负责生成读写数据库的对象
    Cursor cursor = db.rawQuery("SELECT * FROM users WHERE username=? AND password=?",new String[]{username,password});
    //数据库中有此数据，登录成功
    if(cursor.getCount()>0){
      Intent intent = new Intent(this,ReceiveActivity.class);
      intent.putExtra(MESSAGE_KEY,username);
      startActivity(intent);
    }
    else{
      Toast.makeText(MainActivity.this,"用户名或密码错误!",Toast.LENGTH_SHORT).show();
    }
  }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".ReceiveActivity"
>
  <TextView
      android:textSize="24dp"
      android:layout_gravity="center_vertical"
      android:id="@+id/output"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      />
</LinearLayout>

package com.android02;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;
public class ReceiveActivity extends AppCompatActivity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_receive);
    //获取intent引用
    Intent intent = getIntent();
    //以MESSAGE_KEY获取获取编辑框文字
    String message = intent.getStringExtra(MainActivity.MESSAGE_KEY);
    //以id获取TextView
    TextView textView = findViewById(R.id.output);
    //显示message
    textView.setText("欢迎!"+message);
  }
}