android连接数据库

activity_main.xml:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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">
<TextView
    android:id="@+id/tv1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="用户名："
    android:textSize="30dp"
    android:layout_marginTop="60dp"
    />
<EditText
    android:id="@+id/et1"
    android:layout_width="600dp"
    android:layout_height="wrap_content"
    android:layout_toRightOf="@+id/tv1"
    android:layout_marginTop="50dp"
    android:textSize="30dp"
    />
<TextView
    android:id="@+id/tv2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="密码："
    android:textSize="30dp"
    android:layout_marginTop="50dp"
    android:layout_below="@+id/tv1"
    />
<EditText
    android:id="@+id/et2"
    android:layout_width="600dp"
    android:layout_height="wrap_content"
    android:layout_toRightOf="@+id/tv2"
    android:layout_marginTop="30dp"
    android:textSize="30dp"
    android:layout_below="@+id/et1"
    />
    <Button
        android:layout_marginTop="80dp"
        android:layout_width="300dp"
        android:layout_height="wrap_content"
        android:text="注册"
        android:textSize="50dp"
        android:layout_below="@+id/et2"
        android:layout_centerHorizontal="true"
        android:onClick="click"
        />
</RelativeLayout>

MainActivity.java:
package com.example.test;
import android.content.ContentValues;
import android.os.Bundle;
import android.app.Activity;
import android.database.sqlite.SQLiteDatabase;
import android.view.Menu;
import android.view.View;
import android.widget.EditText;
public class MainActivity extends Activity {
    private String username;
    private String password;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
 }
    public void click(View view){
        username =((EditText) findViewById(R.id.et1)).getText().toString();
        password =((EditText) findViewById(R.id.et2)).getText().toString();
        StuOpenHelper helper = new StuOpenHelper(this);
        SQLiteDatabase db =helper.getReadableDatabase();
        ContentValues cv = new ContentValues();
        cv.put("username",username);
        cv.put("password",password);
        db.insert("stu",null,cv);
        db.close();
    }
}

helper.java:
package com.example.test;
import android.content.ContentValues;
import android.os.Bundle;
import android.app.Activity;
import android.database.sqlite.SQLiteDatabase;
import android.view.Menu;
import android.view.View;
import android.widget.EditText;
public class MainActivity extends Activity {
    private String username;
    private String password;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
 }
    public void click(View view){
        username =((EditText) findViewById(R.id.et1)).getText().toString();
        password =((EditText) findViewById(R.id.et2)).getText().toString();
        StuOpenHelper helper = new StuOpenHelper(this);
        SQLiteDatabase db =helper.getReadableDatabase();
        ContentValues cv = new ContentValues();
        cv.put("username",username);
        cv.put("password",password);
        db.insert("stu",null,cv);
        db.close();
    }
}