Kotlin 中的 Android SQLite 数据库

Android 带有一个数据库包的内置实现，它是 SQLite，这是一种开源 SQL 数据库，可在设备中以文本形式存储数据。在本文中，我们将看看 Android SQLite 在 Kotlin 中的实现。

SQLite 是一个独立的、高可靠性的、嵌入式的、功能齐全的、公共领域的 SQL 数据库引擎。它是世界上使用最广泛的数据库引擎。它是一个进程内库，其代码是公开的。它可以免费用于任何目的，商业或私人。它基本上是一个嵌入式 SQL 数据库引擎。 SQLite 可以轻松读写普通磁盘文件，因为它没有像 SQL 那样的任何单独的服务器。 SQLite 数据库文件格式是跨平台的，因此任何人都可以轻松地在 32 位和 64 位系统之间复制数据库。由于所有这些功能，它是作为应用程序文件格式的流行选择。

我们将在本文中构建什么？

我们将构建一个使用 SQLite 存储数据的简单应用程序，我们还将实现检索数据的方法。下面是一个示例视频，展示了我们将要做的事情。

分步实施

第 1 步：创建一个新项目

要在 Android Studio 中创建新项目，请参阅如何在 Android Studio 中创建/启动新项目。请注意，选择Kotlin作为编程语言。

第 2 步：授予访问 AndroidManifest.xml 文件中存储的权限

导航到app > AndroidManifest.xml并将以下代码添加到其中。

步骤 3：使用 activity_main.xml 文件

导航到app > res > layout > activity_main.xml 。将以下代码添加到您的文件中。下面是activity_main.xml的代码。

第 4 步：为 SQLite 操作创建一个新类

导航到app > Java > 您项目的包名称 > 右键单击它 > 新建 > Kotlin 类并将其命名为DBHelper并将以下代码添加到其中。为了使代码更易于理解，添加了注释。

科特林

package com.release.gfg1
 
import android.content.ContentValues
import android.content.Context
import android.database.Cursor
import android.database.sqlite.SQLiteDatabase
import android.database.sqlite.SQLiteOpenHelper
 
class DBHelper(context: Context, factory: SQLiteDatabase.CursorFactory?) :
    SQLiteOpenHelper(context, DATABASE_NAME, factory, DATABASE_VERSION) {
 
    // below is the method for creating a database by a sqlite query
    override fun onCreate(db: SQLiteDatabase) {
        // below is a sqlite query, where column names
        // along with their data types is given
        val query = ("CREATE TABLE " + TABLE_NAME + " ("
                + ID_COL + " INTEGER PRIMARY KEY, " +
                NAME_COl + " TEXT," +
                AGE_COL + " TEXT" + ")")
 
        // we are calling sqlite
        // method for executing our query
        db.execSQL(query)
    }
 
    override fun onUpgrade(db: SQLiteDatabase, p1: Int, p2: Int) {
        // this method is to check if table already exists
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME)
        onCreate(db)
    }
 
    // This method is for adding data in our database
    fun addName(name : String, age : String ){
 
        // below we are creating
        // a content values variable
        val values = ContentValues()
 
        // we are inserting our values
        // in the form of key-value pair
        values.put(NAME_COl, name)
        values.put(AGE_COL, age)
 
        // here we are creating a
        // writable variable of
        // our database as we want to
        // insert value in our database
        val db = this.writableDatabase
 
        // all values are inserted into database
        db.insert(TABLE_NAME, null, values)
 
        // at last we are
        // closing our database
        db.close()
    }
 
    // below method is to get
    // all data from our database
    fun getName(): Cursor? {
 
        // here we are creating a readable
        // variable of our database
        // as we want to read value from it
        val db = this.readableDatabase
 
        // below code returns a cursor to
        // read data from the database
        return db.rawQuery("SELECT * FROM " + TABLE_NAME, null)
 
    }
 
    companion object{
        // here we have defined variables for our database
 
        // below is variable for database name
        private val DATABASE_NAME = "GEEKS_FOR_GEEKS"
 
        // below is the variable for database version
        private val DATABASE_VERSION = 1
 
        // below is the variable for table name
        val TABLE_NAME = "gfg_table"
 
        // below is the variable for id column
        val ID_COL = "id"
 
        // below is the variable for name column
        val NAME_COl = "name"
 
        // below is the variable for age column
        val AGE_COL = "age"
    }
}

第 5 步：使用 MainActivity.kt 文件

转到MainActivity.kt文件并参考以下代码。下面是MainActivity.kt文件的代码。代码中添加了注释以更详细地理解代码。

科特林

package com.release.gfg1
 
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Toast
import kotlinx.android.synthetic.main.activity_main.*
 
class MainActivity : AppCompatActivity() {
 
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
 
        // below code is to add on click
        // listener to our add name button
        addName.setOnClickListener{
 
            // below we have created
            // a new DBHelper class,
            // and passed context to it
            val db = DBHelper(this, null)
 
            // creating variables for values
            // in name and age edit texts
            val name = enterName.text.toString()
            val age = enterAge.text.toString()
 
            // calling method to add
            // name to our database
            db.addName(name, age)
 
            // Toast to message on the screen
            Toast.makeText(this, name + " added to database", Toast.LENGTH_LONG).show()
 
            // at last, clearing edit texts
            enterName.text.clear()
            enterAge.text.clear()
        }
 
        // below code is to add on  click
        // listener to our print name button
        printName.setOnClickListener{
 
            // creating a DBHelper class
            // and passing context to it
            val db = DBHelper(this, null)
 
            // below is the variable for cursor
            // we have called method to get
            // all names from our database
            // and add to name text view
            val cursor = db.getName()
 
            // moving the cursor to first position and
            // appending value in the text view
            cursor!!.moveToFirst()
            Name.append(cursor.getString(cursor.getColumnIndex(DBHelper.NAME_COl)) + "\n")
            Age.append(cursor.getString(cursor.getColumnIndex(DBHelper.AGE_COL)) + "\n")
 
            // moving our cursor to next
            // position and appending values
            while(cursor.moveToNext()){
                Name.append(cursor.getString(cursor.getColumnIndex(DBHelper.NAME_COl)) + "\n")
                Age.append(cursor.getString(cursor.getColumnIndex(DBHelper.AGE_COL)) + "\n")
            }
 
            // at last we close our cursor
            cursor.close()
        }
    }
}

现在运行您的应用程序并查看输出。

输出：

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