java4.1

package com.example.test06;  import androidx.appcompat.app.AppCompatActivity;  import android.content.Intent; import android.content.SharedPreferences; import android.database.Cursor; import android.database.sqlite.SQLiteDatabase; import android.os.Bundle; import android.view.View; import android.widget.Button; import android.widget.EditText; import android.widget.Toast;  public class MainActivity extends AppCompatActivity {     EditText name,pwd;     Button btnlogin,btnreg;     Mysql mysql;     SQLiteDatabase db;     SharedPreferences sp1,sp2;     @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_main);         name = this.findViewById(R.id.name);            //用户名输入框         pwd = this.findViewById(R.id.pwd);              //密码输入框         btnlogin = this.findViewById(R.id.login);         //登录按钮         btnreg = this.findViewById(R.id.reg);               //注册按钮         sp1 =  this.getSharedPreferences(useinfo,this.MODE_PRIVATE);         sp2 = this.getSharedPreferences(username,this.MODE_PRIVATE);          name.setText(sp1.getString(usname,null));         pwd.setText(sp1.getString(uspwd,null));         mysql = new Mysql(this,Userinfo,null,1);      //建数据库或者取数据库         db = mysql.getReadableDatabase();         btnlogin.setOnClickListener(new View.OnClickListener() {                //登录事件             @Override             public void onClick(View v) {                 String username = name.getText().toString();                 String password = pwd.getText().toString();                 //获取用户输入的用户名和密码                 //查询用户名和密码相同的数据                 Cursor cursor = db.query(logins,new String[]{usname,uspwd}, usname=? and uspwd=?,new String[]{username,password},null,null,null);                  int flag = cursor.getCount();                            //查询出来的记录项的条数，若没有该用户则为0条                 if(flag!=0){                                            //若查询出的记录不为0，则进行跳转操作                     Intent intent = new Intent();                     intent.setClass(MainActivity.this,Welcome.class);            //设置页面跳转                     SharedPreferences.Editor editor = sp2.edit();                     cursor.moveToFirst();                                   //将光标移动到position为0的位置，默认位置为-1                     String loginname = cursor.getString(0);                     editor.putString(Loginname,loginname);                     editor.commit();                                        //将用户名存到SharedPreferences中                     startActivity(intent);                 }                 else{                     Toast.makeText(MainActivity.this,用户名或密码错误！,Toast.LENGTH_LONG).show();             //提示用户信息错误或没有账号                 }              }         });          btnreg.setOnClickListener(new View.OnClickListener() {                  //注册事件             @Override             public void onClick(View v) {                  Intent intent = new Intent();                 intent.setClass(MainActivity.this,Register.class);          //跳转到注册页面                 startActivity(intent);                 Toast.makeText(MainActivity.this,前往注册！,Toast.LENGTH_SHORT).show();             }         });     } }