SQLiteOpenHelper是一個助手類,可以幫助我們訪問SQLite。它有以下功能:
1.通過getReadableDatabase()和getWritableDatabase()兩個函數可以獲得SQLiteDatabase對象,然後對該對象進行操作;
2.提供了onCreate()和onUpgrade()兩個回調函數,允許我們在創建和升級數據庫時,進行我們自己的操作(例如在升級數據庫時進行表結構的更改)。
怎樣使用SQLiteOpenHelper助手類?
1.首先自己寫一個助手類來繼承SQLiteOpenHelper類,重寫該類的構造函數、onCreate()方法以及onUpgrade()方法,並在onCreate()方法中執行SQL語句來創建表。這是我寫的類:
DatabaseHelper.java:
package com.mycompany.sqlite;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
/**
* Created by Administrator on 2016/8/12.
*/
public class DatabaseHelper extends SQLiteOpenHelper{
private static final int VERSION = 1;
public DatabaseHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version){
super(context,name,factory,version);
}
public DatabaseHelper(Context context, String name){
this(context,name,VERSION);
}
public DatabaseHelper(Context context,String name,int version){
this(context,name,null,version);
}
//該函數是在第一次創建數據庫的時候執行,實際上是在第一次得到SQLiteDatabase對象的時候,
//纔會調用這個方法
@Override
public void onCreate(SQLiteDatabase db) {
System.out.println("創建數據庫");
//execSQL函數用於執行SQL語句
db.execSQL("create table user(id int,name varchar(20))");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
System.out.println("更新數據庫");
}
}
2.然後進行各種操作:
創建數據庫:首先生成我們寫的助手類的對象(注意參數),然後調用該對象的getReadableDatabase()方法得到數據庫,並將該數據庫賦給新生成的SQLiteDatabase對象。
<pre name="code" class="java"> <span style="font-family: Arial, Helvetica, sans-serif;">// 創建一個DatabaseHelper對象</span>
DatabaseHelper dbHelper = new DatabaseHelper(MainActivity.this,"my_database.db"); // 只有調用了DatabaseHelper對象的getReadableDatabase()方法,或者是getWritableDatabase()方法 // 之後纔會真正創建一個數據庫 SQLiteDatabase db = dbHelper.getReadableDatabase();
升級數據庫:依然首先生成助手類對象(系統會根據形參版本號的改變來調用DatabaseHelper類的onUpgrade()方法),然後調用getReadableDatabase()方法。
// 根據版本號的改變來調用DatabaseHelper類的onUpgrade()方法
DatabaseHelper dbHelper = new DatabaseHelper(MainActivity.this,"my_database.db",2);
SQLiteDatabase db = dbHelper.getReadableDatabase();
插入數據:首先生成一個ContentValues對象(注意:SQLite對數據的新建、修改都是通過ContentValues來進行的。),然後用該對象的put()方法將信息按列賦給該對象,接着生成助手類,調用getWritableDatabase()方法得到數據庫,然後調用SQLiteDatabase對象的insert()方法插入數據。
// 生成ContentValues對象
ContentValues values = new ContentValues();
// ContentValues對象用來保存鍵值對,其中鍵是列名,值是值,值必須
// 與列值類型匹配。由此可以向數據庫中插入數據
values.put("id",001);
values.put("name","劉德華");
DatabaseHelper dbHelper = new DatabaseHelper(MainActivity.this,"my_database.db",2);
SQLiteDatabase db = dbHelper.getWritableDatabase();
db.insert("user", null, values);
System.out.println("插入信息成功");
更新數據:首先生成助手類,用getWritableDatabase()方法得到數據庫,然後用ContentValues對象的put()方法保存想要更新的列以及數據,最後用SQLiteDatabase對象的update()方法更新數據。
DatabaseHelper databaseHelper = new DatabaseHelper(MainActivity.this, "my_database.db", 2);
SQLiteDatabase db = databaseHelper.getWritableDatabase();
ContentValues values = new ContentValues();
values.put("name", "liudehua");
db.update("user", values, "id = ?", new String[]{"001"});
System.out.println("更新信息成功");
查詢數據:首先生成助手類,用getReadableDatabase()方法得到數據庫,然後用SQLiteDatabase對象的query()方法查詢數據,並賦值給Cursor對象。接着利用while(cursor對象.moveToNext())循環遍歷數據,並輸出數據。
DatabaseHelper databaseHelper = new DatabaseHelper(MainActivity.this, "my_database.db", 2);
SQLiteDatabase db = databaseHelper.getReadableDatabase();
Cursor cursor = db.query("user", new String[]{"id","name"}, "id = ?", new String[]{"001"}, null, null, null);
while(cursor.moveToNext()){
String name = cursor.getString(cursor.getColumnIndex("name"));
System.out.println("query--->" + name);
刪除數據:首先生成助手類,用getReadableDatabase()方法得到數據庫,然後用SQLiteDatabase對象的delete()方法刪除數據。
DatabaseHelper databaseHelper = new DatabaseHelper(MainActivity.this, "my_database.db", 2);
SQLiteDatabase db = databaseHelper.getReadableDatabase();
db.delete("user", "id = ?", new String[]{"001"});
System.out.println("刪除信息成功");
最後是整個MainActivity.java:
package com.mycompany.sqlite;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.provider.ContactsContract;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class MainActivity extends AppCompatActivity {
private Button createDatabase;
private Button updateDatabase;
private Button insert;
private Button update;
private Button query;
private Button delete;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
createDatabase = (Button)findViewById(R.id.createDatabase);
updateDatabase = (Button)findViewById(R.id.updateDatabase);
insert = (Button)findViewById(R.id.insert);
update = (Button)findViewById(R.id.update);
query = (Button)findViewById(R.id.query);
delete = (Button)findViewById(R.id.delete);
createDatabase.setOnClickListener(new createDatabaseListener());
updateDatabase.setOnClickListener(new updateDatabaseListener());
insert.setOnClickListener(new insertListener());
update.setOnClickListener(new updateListener());
query.setOnClickListener(new queryListener());
delete.setOnClickListener(new deleteListener());
}
class createDatabaseListener implements View.OnClickListener{
@Override
public void onClick(View v) {
// 創建一個DatabaseHelper對象
DatabaseHelper dbHelper = new DatabaseHelper(MainActivity.this,"my_database.db");
// 只有調用了DatabaseHelper對象的getReadableDatabase()方法,或者是getWritableDatabase()方法
// 之後纔會真正創建一個數據庫
SQLiteDatabase db = dbHelper.getReadableDatabase();
}
}
class updateDatabaseListener implements View.OnClickListener{
@Override
public void onClick(View v) {
// 根據版本號的改變來調用DatabaseHelper類的onUpgrade()方法
DatabaseHelper dbHelper = new DatabaseHelper(MainActivity.this,"my_database.db",2);
SQLiteDatabase db = dbHelper.getReadableDatabase();
}
}
class insertListener implements View.OnClickListener{
@Override
public void onClick(View v) {
// 生成ContentValues對象
ContentValues values = new ContentValues();
// ContentValues對象用來保存鍵值對,其中鍵是列名,值是值,值必須
// 與列值類型匹配。由此可以向數據庫中插入數據
values.put("id",001);
values.put("name","劉德華");
DatabaseHelper dbHelper = new DatabaseHelper(MainActivity.this,"my_database.db",2);
SQLiteDatabase db = dbHelper.getWritableDatabase();
db.insert("user", null, values);
System.out.println("插入信息成功");
}
}
class updateListener implements View.OnClickListener{
@Override
public void onClick(View v) {
DatabaseHelper databaseHelper = new DatabaseHelper(MainActivity.this, "my_database.db", 2);
SQLiteDatabase db = databaseHelper.getWritableDatabase();
ContentValues values = new ContentValues();
values.put("name", "liudehua");
db.update("user", values, "id = ?", new String[]{"001"});
System.out.println("更新信息成功");
}
}
class queryListener implements View.OnClickListener{
@Override
public void onClick(View v) {
DatabaseHelper databaseHelper = new DatabaseHelper(MainActivity.this, "my_database.db", 2);
SQLiteDatabase db = databaseHelper.getReadableDatabase();
Cursor cursor = db.query("user", new String[]{"id","name"}, "id = ?", new String[]{"001"}, null, null, null);
while(cursor.moveToNext()){
String name = cursor.getString(cursor.getColumnIndex("name"));
System.out.println("query--->" + name);
}
}
}
class deleteListener implements View.OnClickListener{
@Override
public void onClick(View v) {
DatabaseHelper databaseHelper = new DatabaseHelper(MainActivity.this, "my_database.db", 2);
SQLiteDatabase db = databaseHelper.getReadableDatabase();
db.delete("user", "id = ?", new String[]{"001"});
System.out.println("刪除信息成功");
}
}
}