Android学习之创建和使用数据库

Android使用的是SQLite数据库系统，为一个应用程序所创建的数据库只能被此应用程序访问，其他应用程序不能访问！

下面的程序片段将说明如何以编程的方式在Android应用程序中创建一个SQLite数据库。对于Android，在一个应用程序中以编程方式创建的SQLite数据库总是存储在/data/data/<pakage_name>/databases文件夹下。

创建数据库辅助类

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class DBAdapter {

static final String KEY_ROWID = "_id";
static final String KEY_NAME = "name";
static final String KEY_EMAIL = "email";
static final String TAG = "DBAdapter";

static final String DATABASE_NAME = "MyDB";
static final String DATABASE_TABLE = "contacts";
static final int DATABASE_VERSION = 1;

static final String DATABASE_CREATE =
"create table contacts( _id integer primary key autoincrement, " +
"name text not null, email text not null);";
final Context context;

DatabaseHelper DBHelper;
SQLiteDatabase db;

public DBAdapter(Context cxt)
{
this.context = cxt;
DBHelper = new DatabaseHelper(context);
}

private static class DatabaseHelper extends SQLiteOpenHelper
{

DatabaseHelper(Context context)
{
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
// TODO Auto-generated method stub
try
{
db.execSQL(DATABASE_CREATE);
}
catch(SQLException e)
{
e.printStackTrace();
}
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// TODO Auto-generated method stub
Log.wtf(TAG, "Upgrading database from version "+ oldVersion + "to "+
newVersion + ", which will destroy all old data");
db.execSQL("DROP TABLE IF EXISTS contacts");
onCreate(db);
}
}

//open the database
public DBAdapter open() throws SQLException
{
db = DBHelper.getWritableDatabase();
return this;
}
//close the database
public void close()
{
DBHelper.close();
}

//insert a contact into the database
public long insertContact(String name, String email)
{
ContentValues initialValues = new ContentValues();
initialValues.put(KEY_NAME, name);
initialValues.put(KEY_EMAIL, email);
return db.insert(DATABASE_TABLE, null, initialValues);
}
//delete a particular contact
public boolean deleteContact(long rowId)
{
return db.delete(DATABASE_TABLE, KEY_ROWID + "=" +rowId, null) > 0;
}
//retreves all the contacts
public Cursor getAllContacts()
{
return db.query(DATABASE_TABLE, new String[]{ KEY_ROWID,KEY_NAME,KEY_EMAIL}, null, null, null, null, null);
}
//retreves a particular contact
public Cursor getContact(long rowId) throws SQLException
{
Cursor mCursor =
db.query(true, DATABASE_TABLE, new String[]{ KEY_ROWID,
KEY_NAME, KEY_EMAIL}, KEY_ROWID + "=" + rowId, null, null, null, null, null);
if (mCursor != null)
mCursor.moveToFirst();
return mCursor;
}
//updates a contact
public boolean updateContact(long rowId, String name, String email)
{
ContentValues args = new ContentValues();
args.put(KEY_NAME, name);
args.put(KEY_EMAIL, email);
return db.update(DATABASE_TABLE, args, KEY_ROWID + "=" +rowId, null) > 0;
}
}

每个类的作用已经做了说明。本例子中创建一个名为MyDB的数据库，包含一个名为contacts的表，这个表有3个列：_id、name和email。

2、以编程的方式使用数据库

package com.wshouyou.database;

import android.os.Bundle;
import android.app.Activity;
import android.database.Cursor;
import android.view.Menu;
import android.widget.Toast;

public class DatabaseActivity extends Activity {

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

DBAdapter db = new DBAdapter(this);

//add a contact
/*db.open();
long id = db.insertContact("junli", "liujl07@126.com");
id = db.insertContact("juan zhan", "mary@autonavi.com");
db.close();*/

//get all contacts
/*db.open();
Cursor c = db.getAllContacts();
if (c.moveToFirst())
{
do {
DisplayContact(c);
}while (c.moveToNext());
}
db.close();*/

//get a contact
/*db.open();
Cursor c = db.getContact(2);
if (c.moveToFirst())
DisplayContact(c);
else
Toast.makeText(this, "No contact found!", Toast.LENGTH_SHORT).show();
db.close();*/

//update contact
/*db.open();
if ( db.updateContact(1, "Wei-Meng Lee", "weimenglee@gmail.com") )
Toast.makeText(this, "Update successful!", Toast.LENGTH_SHORT).show();
else
Toast.makeText(this, "Update failed!", Toast.LENGTH_SHORT).show();
db.close();*/

//delete a contact
db.open();
if (db.deleteContact(1))
Toast.makeText(this, "Delete successful!", Toast.LENGTH_LONG).show();
else
Toast.makeText(this, "Delete failed!", Toast.LENGTH_LONG).show();
db.close();
}

public void DisplayContact(Cursor c)
{
Toast.makeText(this ,
"id: "+c.getString(0) + "\n" +
"Name: "+c.getString(1) + "\n"+
"Email: "+c.getString(2),
Toast.LENGTH_SHORT).show();
}

}

操作数据库。