写个android的登陆注册页面，使用sqlite数据库

好的，下面是一个简单的 Android 登录注册页面的示例代码，使用 SQLite 数据库：

首先是布局文件 `activity_main.xml`：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <!-- 登录表单 -->
    <LinearLayout
        android:id="@+id/login_form"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:orientation="vertical"
        android:visibility="visible">

        <EditText
            android:id="@+id/username"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="用户名" />

        <EditText
            android:id="@+id/password"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="密码"
            android:inputType="textPassword" />

        <Button
            android:id="@+id/login_button"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="登录" />

        <TextView
            android:id="@+id/register_link"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="16dp"
            android:gravity="center"
            android:text="没有账号？点击注册"
            android:textColor="#00f" />
    </LinearLayout>

    <!-- 注册表单 -->
    <LinearLayout
        android:id="@+id/register_form"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:orientation="vertical"
        android:visibility="gone">

        <EditText
            android:id="@+id/register_username"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="用户名" />

        <EditText
            android:id="@+id/register_password"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:hint="密码"
            android:inputType="textPassword" />

        <Button
            android:id="@+id/register_button"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="注册" />

        <TextView
            android:id="@+id/login_link"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="16dp"
            android:gravity="center"
            android:text="已有账号？点击登录"
            android:textColor="#00f" />
    </LinearLayout>

</LinearLayout>

其中包含两个表单，一个用于登录，一个用于注册。

接下来是 MainActivity 的代码：

import android.content.ContentValues;
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.LinearLayout;
import android.widget.TextView;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

    private EditText mUsernameView;
    private EditText mPasswordView;
    private EditText mRegisterUsernameView;
    private EditText mRegisterPasswordView;

    private LinearLayout mLoginForm;
    private LinearLayout mRegisterForm;

    private SQLiteDatabase mDatabase;

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

        mUsernameView = findViewById(R.id.username);
        mPasswordView = findViewById(R.id.password);
        mRegisterUsernameView = findViewById(R.id.register_username);
        mRegisterPasswordView = findViewById(R.id.register_password);

        mLoginForm = findViewById(R.id.login_form);
        mRegisterForm = findViewById(R.id.register_form);

        Button loginButton = findViewById(R.id.login_button);
        loginButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                attemptLogin();
            }
        });

        Button registerButton = findViewById(R.id.register_button);
        registerButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                attemptRegister();
            }
        });

        TextView registerLink = findViewById(R.id.register_link);
        registerLink.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                showRegisterForm();
            }
        });

        TextView loginLink = findViewById(R.id.login_link);
        loginLink.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                showLoginForm();
            }
        });

        mDatabase = new DatabaseHelper(this).getWritableDatabase();
    }

    private void attemptLogin() {
        String username = mUsernameView.getText().toString();
        String password = mPasswordView.getText().toString();

        Cursor cursor = mDatabase.query(
                DatabaseHelper.TABLE_USERS,
                null,
                DatabaseHelper.COLUMN_USERNAME + " = ? AND " + DatabaseHelper.COLUMN_PASSWORD + " = ?",
                new String[] {username, password},
                null, null, null);

        if (cursor.moveToFirst()) {
            // 登录成功
            Toast.makeText(this, "登录成功", Toast.LENGTH_SHORT).show();
        } else {
            // 登录失败
            Toast.makeText(this, "用户名或密码错误", Toast.LENGTH_SHORT).show();
        }

        cursor.close();
    }

    private void attemptRegister() {
        String username = mRegisterUsernameView.getText().toString();
        String password = mRegisterPasswordView.getText().toString();

        if (username.isEmpty() || password.isEmpty()) {
            Toast.makeText(this, "用户名和密码不能为空", Toast.LENGTH_SHORT).show();
            return;
        }

        Cursor cursor = mDatabase.query(
                DatabaseHelper.TABLE_USERS,
                null,
                DatabaseHelper.COLUMN_USERNAME + " = ?",
                new String[] {username},
                null, null, null);

        if (cursor.moveToFirst()) {
            // 用户名已存在
            Toast.makeText(this, "用户名已存在", Toast.LENGTH_SHORT).show();
        } else {
            // 注册新用户
            ContentValues values = new ContentValues();
            values.put(DatabaseHelper.COLUMN_USERNAME, username);
            values.put(DatabaseHelper.COLUMN_PASSWORD, password);
            mDatabase.insert(DatabaseHelper.TABLE_USERS, null, values);

            // 注册成功，自动登录
            mUsernameView.setText(username);
            mPasswordView.setText(password);
            attemptLogin();
        }

        cursor.close();
    }

    private void showLoginForm() {
        mLoginForm.setVisibility(View.VISIBLE);
        mRegisterForm.setVisibility(View.GONE);
    }

    private void showRegisterForm() {
        mLoginForm.setVisibility(View.GONE);
        mRegisterForm.setVisibility(View.VISIBLE);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        mDatabase.close();
    }
}

其中 `DatabaseHelper` 是一个辅助类，用于创建和升级数据库：

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

public class DatabaseHelper extends SQLiteOpenHelper {

    private static final String DATABASE_NAME = "myapp.db";
    private static final int DATABASE_VERSION = 1;

    public static final String TABLE_USERS = "users";
    public static final String COLUMN_USERNAME = "username";
    public static final String COLUMN_PASSWORD = "password";

    private static final String SQL_CREATE_USERS_TABLE =
            "CREATE TABLE " + TABLE_USERS + " (" +
                    COLUMN_USERNAME + " TEXT PRIMARY KEY," +
                    COLUMN_PASSWORD + " TEXT)";

    public DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

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

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // 升级数据库的代码
    }
}

以上就是一个简单的 Android 登录注册页面的示例代码，使用 SQLite 数据库。