如何使用 SQLite 数据库
在安卓系统中预填充数据库
通常，需要用已经存在的数据库启动一个安卓应用程序。这叫做预填充数据库。在本文中，我们将看到如何使用 SQLite 数据库在安卓系统中预填充数据库。本例使用的数据库可以下载为演示数据库。
进场:
-
在 build.gradle 文件中添加支持库，在依赖项部分添加 Recycler View 依赖项。
java dependencies{ implementation 'androidx.recyclerview:recyclerview:1.1.0' } -
确保将数据库添加到资产文件夹中。要创建资产文件夹，右键单击应用目录- >新建- >文件夹- >(选择)资产文件夹。然后简单的粘贴你的。数据库文件在资产文件夹中。
-
在 activity_main.xml 中，添加以下代码。
```java <?xml version="1.0" encoding="utf-8"?>
```
-
使用以下代码创建一个新的 custom_layout.xml 文件。
java <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_marginTop="3dp" > <TextView android:textStyle="bold" android:layout_margin="5dp" android:textSize="20sp" android:layout_width="match_parent" android:layout_height="wrap_content" android:id="@+id/textView"/> </LinearLayout> -
创建一个数据库助手类并添加以下代码。
```java package org.geeksforgeeks.dictionary;
import android.app.Activity; import android.content.Context; import android.database.Cursor; import android.database.sqlite.SQLiteDatabase; import android.database.sqlite.SQLiteException; import android.database.sqlite.SQLiteOpenHelper; import android.util.Log;
import java.io.FileOutputStream; import java.io.IOException; import java.io.InputStream; import java.io.OutputStream; import java.sql.SQLException; import java.util.ArrayList; import java.util.List;
public class DatabaseHelper extends SQLiteOpenHelper {
// The Android's default system path // of your application database. private static String DB_PATH = ""; private static String DB_NAME = "database.db"; private SQLiteDatabase myDataBase; private final Context myContext; private SQLiteOpenHelper sqLiteOpenHelper;
// Table name in the database. public static final String ALGO_TOPICS = "algo_topics";
/* * Constructor * Takes and keeps a reference of * the passed context in order * to access the application assets and resources. / public DatabaseHelper(Context context) {
super(context, DB_NAME, null, 1); this.myContext = context; DB_PATH = myContext.getDatabasePath(DB_NAME) .toString(); }
// Creates an empty database // on the system and rewrites it // with your own database. public void createDataBase() throws IOException {
boolean dbExist = checkDataBase();
if (dbExist) { // do nothing - database already exist } else { // By calling this method and // the empty database will be // created into the default system // path of your application // so we are gonna be able // to overwrite that database // with our database. this.getWritableDatabase(); try { copyDataBase(); } catch (IOException e) { throw new Error( "Error copying database"); } } } // Check if the database already exist // to avoid re-copying the file each // time you open the application // return true if it exists // false if it doesn't. private boolean checkDataBase() { SQLiteDatabase checkDB = null; try { String myPath = DB_PATH; checkDB = SQLiteDatabase .openDatabase( myPath, null, SQLiteDatabase.OPEN_READONLY); } catch (SQLiteException e) {
// database doesn't exist yet. Log.e("message", "" + e); } if (checkDB != null) { checkDB.close(); } return checkDB != null; }
/* * Copies your database from your * local assets-folder to the just * created empty database in the * system folder, from where it * can be accessed and handled. * This is done by transferring bytestream. * / private void copyDataBase() throws IOException { // Open your local db as the input stream InputStream myInput = myContext.getAssets() .open(DB_NAME);
// Path to the just created empty db String outFileName = DB_PATH;
// Open the empty db as the output stream OutputStream myOutput = new FileOutputStream(outFileName);
// transfer bytes from the // inputfile to the outputfile byte[] buffer = new byte[1024]; int length; while ((length = myInput.read(buffer)) > 0) { myOutput.write(buffer, 0, length); }
// Close the streams myOutput.flush(); myOutput.close(); myInput.close(); }
public void openDataBase() throws SQLException { // Open the database String myPath = DB_PATH; myDataBase = SQLiteDatabase .openDatabase( myPath, null, SQLiteDatabase.OPEN_READONLY); }
@Override public synchronized void close() { // close the database. if (myDataBase != null) myDataBase.close(); super.close(); }
@Override public void onCreate(SQLiteDatabase db) { // It is an abstract method // but we define our own method here. }
@Override public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { // It is an abstract method which is // used to perform different task // based on the version of database. }
// This method is used to get the // algorithm topics from the database. public List getAlgorithmTopics( Activity activity) { sqLiteOpenHelper = new DatabaseHelper(activity); SQLiteDatabase db = sqLiteOpenHelper .getWritableDatabase();
List list = new ArrayList<>();
// query help us to return all data // the present in the ALGO_TOPICS table. String query = "SELECT * FROM " + ALGO_TOPICS; Cursor cursor = db.rawQuery(query, null);
if (cursor.moveToFirst()) { do { list.add(cursor.getString(1)); } while (cursor.moveToNext()); } return list; } } ```
-
创建一个MyAdapter.java类，并添加以下代码。
```java package org.geeksforgeeks.dictionary;
import android.app.Activity; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import android.widget.TextView;
import androidx.annotation.NonNull; import androidx.recyclerview.widget.RecyclerView; import java.util.List;
public class MyAdapter extends RecyclerView.Adapter {
private List data; Activity activity;
public MyAdapter(Activity activity, List data) { this.data = data; this.activity = activity; }
// This method is used to attach // custom layout to the recycler view @NonNull @Override public ViewHolder onCreateViewHolder( @NonNull ViewGroup parent, int viewType) { LayoutInflater LI = activity.getLayoutInflater(); View vw = LI.inflate( R.layout.custom_layout, null); return new ViewHolder(vw); }
// This method is used to set the action // to the widgets of our custom layout. @Override public void onBindViewHolder( @NonNull ViewHolder holder, int position) { holder.topic_name .setText(data.get(position)); }
@Override public int getItemCount() { return data.size(); }
class ViewHolder extends RecyclerView.ViewHolder { TextView topic_name; public ViewHolder(View itemView) { super(itemView); this.topic_name = itemView.findViewById(R.id.textView); } } } ```
-
最后，在MainActivity.java中添加以下代码。
```java package org.geeksforgeeks.algorithmTopics;
import androidx.appcompat.app.AppCompatActivity; import androidx.recyclerview.widget.DefaultItemAnimator; import androidx.recyclerview.widget.LinearLayoutManager; import androidx.recyclerview.widget.RecyclerView; import android.os.Bundle; import java.util.ArrayList; import java.util.List;
public class MainActivity extends AppCompatActivity {
private static RecyclerView.Adapter adapter; private static RecyclerView recyclerView; public static List data; DatabaseHelper db;
@Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); recyclerView = findViewById(R.id.my_recycler_view);
db = new DatabaseHelper(this); recyclerView.setLayoutManager( new LinearLayoutManager(this)); data = new ArrayList<>(); fetchData(); }
public void fetchData() { // Before fetching the data // directly from the database. // first we have to creates an empty // database on the system and // rewrites it with your own database. // Then we have to open the // database to fetch the data from it. db = new DatabaseHelper(this); try { db.createDataBase(); db.openDataBase(); } catch (Exception e) { e.printStackTrace(); }
data = db.getAlgorithmTopics(this); adapter = new MyAdapter(this, data); recyclerView.setAdapter(adapter); } } ```
输出:
版权属于：月萌API www.moonapi.com，转载请注明出处
