如何在Android中使用合同类？ [英] How to use a contract class in android?
问题描述
我有点困惑，因为我不知道我应该如何解释教程： http://developer.android.com/training/basics/data-storage/databases.html#DbHelper
我的代码到现在为止看起来像这样：
public final class DatabaseContract {
//为了防止有人意外地实例化合同类，
//给它一个空的构造函数。
public DatabaseContract（）{}
public static abstract class Table1 implements BaseColumns {
public static final String TABLE_NAME =nameOfTable;
public static final String COLUMN_NAME_COL1 =column1;
public static final String COLUMN_NAME_COL2 =column2;
public static final String COLUMN_NAME_COL3 =column3;
}
public class DatabaseHelper extends SQLiteOpenHelper {
//如果更改数据库模式，必须增加数据库版本。
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME =database.db;
private static final String TEXT_TYPE =TEXT;
private static final String COMMA_SEP =，;
private static final String SQL_CREATE_ENTRIES =CREATE TABLE+
Table1.TABLE_NAME +（+
Table1._ID +INTEGER PRIMARY KEY+
Table1.COLUMN_NAME_COL1 + TEXT_TYPE + COMMA_SEP +
Table1.COLUMN_NAME_COL2 + TEXT_TYPE + COMMA_SEP +
Table1.COLUMN_NAME_COL3 + TEXT_TYPE + COMMA_SEP +）;
private static final String SQL_DELETE_ALL_ENTRIES =DROP TABLE IF EXISTS+ Table1.TABLE_NAME;
public DatabaseHelper（Context context）{
super（context，DATABASE_NAME，null，DATABASE_VERSION）;
}
//在创建数据库时调用方法
@Override
public void onCreate（SQLiteDatabase db）{
db.execSQL（SQL_CREATE_ENTRIES ）;
}
//在数据库升级期间调用方法
@Override
public void onUpgrade（SQLiteDatabase db，int oldVersion，int newVersion）{
Log.w（DatabaseHelper.class.getName（），从版本升级数据库+ oldVersion +到+ newVersion +，这将销毁所有旧数据）;
db.execSQL（SQL_DELETE_ALL_ENTRIES）; $ c $ b onCreate（db）;
}
}
}
将Helper类中的前6个变量放在Contract类中？
或者辅助类不应该是合同类的内部类吗？
希望您能帮助我。
数据库以及人们应如何通过内容提供者与其进行交互。
契约类定义常数，帮助应用程序使用
的内容URI，列名，意图动作和其他功能
a内容提供商。合同类不包括自动
与提供者;
说到这里，你不需要定义它们必须需要内容提供者才能使用Contract类。我的示例包含内容提供程序使用的常量（MIME和URI部分）。如果您不使用内容提供商，则不需要这些部分。
我喜欢将契约类当作数据库模式，或者换句话说，它定义了数据库的设置方式。您可能会注意到，合同类中的所有内容都声明为静态。这是因为你永远不会实例化一个Contract类，而只是引用它定义的常量。你可以看到在我的例子中，我的Contract类只有一堆静态final变量声明。该合同类可以是其自己的文件，例如我的文件名为TransitContract.java。
例如，您想更改其中一个列的名称。而不是对多个文件进行更改，所有您需要做的是更改合同类中的列的值。
你不在合同类中进行任何类型的计算工作。
另一方面，SQLLiteOpenhelper类是Google提供的一种方法，可以更轻松地处理数据库。这是您实现创建和设置初始数据库的方法的位置。请参见 http://developer.android.com/reference/android/database/ sqlite / SQLiteOpenHelper.html 。实现这些方法之后，你所要做的就是实例化一个helper类的实例，然后调用helperClassInstance.getWriteableDatabase（）（或getReadableDataBase（）），然后你的helper类会自动创建一个新的数据库，或返回已经存在的类等。
此助手通常实现为内部类，但可以是其自己的独立类。但是你想要实现它。
我强烈建议您查看Google提供的记事本示例，因为它提供了一个很好的示例，用于说明如何设置合同类。请注意，他们还使用内容提供商。如果您有兴趣了解内容提供商，我们建议您阅读 http://developer.android.com/guide/topics/providers/content-provider-basics.html 。
这是一个使用你的代码的例子。我没有实际测试这个代码，所以它可能有错误。正如你可以看到的，你可以在你认为必要的任何地方实例化你的数据库助手。在这个例子中，我在主活动的onCreate中做，但实际上这是不好的做法。
DatabaseContract.java
public final class DatabaseContract {
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME =database.db;
private static final String TEXT_TYPE =TEXT;
private static final String COMMA_SEP =，;
//为了防止有人意外地实例化合同类，
//给它一个空的构造函数。
private DatabaseContract（）{}
public static abstract class Table1 implements BaseColumns {
public static final String TABLE_NAME =nameOfTable;
public static final String COLUMN_NAME_COL1 =column1;
public static final String COLUMN_NAME_COL2 =column2;
public static final String COLUMN_NAME_COL3 =column3;
public static final String CREATE_TABLE =CREATE TABLE+
TABLE_NAME +（+
_ID +INTEGER PRIMARY KEY+
COLUMN_NAME_COL1 + TEXT_TYPE + COMMA_SEP +
COLUMN_NAME_COL2 + TEXT_TYPE + COMMA_SEP +
COLUMN_NAME_COL3 + TEXT_TYPE +）;
public static final String DELETE_TABLE =DROP TABLE IF EXISTS+ TABLE_NAME;
}
}
DatabaseHelper.java
public class DatabaseHelper extends SQLiteOpenHelper {
public DatabaseHelper（Context context）{
super（context，DatabaseContract.DATABASE_NAME，null，DatabaseContract.DATABASE_VERSION ）;
}
//在创建数据库时调用方法
@Override
public void onCreate（SQLiteDatabase db）{
db.execSQL（DatabaseContract .Table1.CREATE_TABLE）;
}
//在数据库升级期间调用方法
@Override
public void onUpgrade（SQLiteDatabase db，int oldVersion，int newVersion）{
db.execSQL（DatabaseContract.Table1.DELETE_TABLE）;
onCreate（db）;
}
}
MainActivity.java
public class MainActivity extends Activity {
public void onCreate（Bundle savedInstanceState）{
super.onCreate（savedInstanceState）;
setContentView（R.layout.activity_main）;
//创建新助手
DatabaseHelper dbHelper = new DatabaseHelper（getContext（））;
//获取数据库。如果它不存在，这是
//也将被创建的地方。
SQLiteDatabase db = dbHelper.getWriteableDatabase（）;
//创建插入条目
ContentValues values = new ContentValues（）;
values.put（DatabaseContract.Table1.COLUMN_NAME_COL1，value1）;
values.put（DatabaseContract.Table1.COLUMN_NAME_COL2，value2）;
values.put（DatabaseContract.Table1.COLUMN_NAME_COL3，value3）;
//插入新行，返回新行的主键值
long newRowId;
newRowId = db.insert（
DatabaseContract.Table1.TABLE_NAME，
null，
values）;
}
}
和我的例子
public final class TransitContract {
public static final String AUTHORITY =com.example.TransitProvider;
public static final String SCHEME =content：//;
public static final String SLASH =/;
public static final String DATABASE_NAME =transit.db;
/ *所有SQL创建表语句的数组列表* /
public static final String [] SQL_CREATE_TABLE_ARRAY = {
Agency.CREATE_TABLE，
CalendarDates.CREATE_TABLE ，
Calendar.CREATE_TABLE，
Routes.CREATE_TABLE，
Shapes.CREATE_TABLE，
Stops.CREATE_TABLE，
StopTimes.CREATE_TABLE，
Trips.CREATE_TABLE
};
/ **
*将加载到
*数据库中的每个GTFS数据文件的资源ID数组
* /
public static final int [] RAW_IDS = {
R.raw.agency，
R.raw.calendar_dates，
R.raw.calendar，
R.raw.routes，
R .raw.shapes，
R.raw.stops，
R.raw.stop_times，
R.raw.trips，
};
/ *不允许这个类实例化* /
private TransitContract（）{}
public static final class Agency implements BaseColumns {
/ *不允许这个类被实例化* /
private Agency（）{}
public static final String TABLE_NAME =Agency;
public static final String KEY_AGENCY_ID =AgencyId;
public static final String KEY_NAME =Name;
public static final String KEY_URL =Url;
public static final String KEY_TIMEZONE =Timezone;
public static final String KEY_LANG =Language;
public static final String KEY_PHONE =PhoneNumber;
public static final String KEY_FARE_URL =FareUrl;
/ *
* URI定义
* /
/ **
*内容样式URI
* /
public static final Uri CONTENT_URI = Uri.parse（SCHEME + AUTHORITY + SLASH + TABLE_NAME）;
/ **
*单行的内容URI基础。必须附加ID。
* /
public static final Uri CONTENT_ID_URI_BASE = Uri.parse（SCHEME + AUTHORITY + SLASH + TABLE_NAME + SLASH）;
/ **
*表的默认排序顺序
* /
public static final String DEFAULT_SORT_ORDER = KEY_AGENCY_ID +ASC
/ *
* MIME类型定义
* /
/ **
* {@link #CONTENT_URI}的MIME类型提供行
* /
public static final String CONTENT_TYPE = ContentResolver.CURSOR_DIR_BASE_TYPE +
/vnd.com.marylandtransitcommuters.agency;
/ **
* {@link #CONTENT_URI}单行的MIME类型
* /
public static final String CONTENT_ITEM_TYPE = ContentResolver.CURSOR_ITEM_BASE_TYPE +
/vnd.com.marylandtransitcommuters.agency;
/ **
* SQL语句创建路由表
* /
public static final String CREATE_TABLE =CREATE TABLE+ TABLE_NAME + b $ b + _ID +INTEGER PRIMARY KEY，
+ KEY_AGENCY_ID +TEXT，
+ KEY_NAME +TEXT，
+ KEY_URL +TEXT KEY_TIMEZONE +TEXT，
+ KEY_LANG +TEXT，
+ KEY_PHONE +TEXT，
+ KEY_FARE_URL +TEXT
+）;
/ **
*删除表的SQL语句
* /
public static final String DELETE_TABLE =DROP TABLE IF EXISTS+ TABLE_NAME;
/ **
*所有列的数组。使更干净的代码
* /
public static final String [] KEY_ARRAY = {
KEY_AGENCY_ID，
KEY_NAME，
KEY_URL，
KEY_TIMEZONE，
KEY_LANG，
KEY_PHONE，
KEY_FARE_URL
};
}
I'm a little bit confused because I don't know how I should interpret the tutorial here: http://developer.android.com/training/basics/data-storage/databases.html#DbHelper
My code until now looks like this:
public final class DatabaseContract {
// To prevent someone from accidentally instantiating the contract class,
// give it an empty constructor.
public DatabaseContract() {}
public static abstract class Table1 implements BaseColumns {
public static final String TABLE_NAME = "nameOfTable";
public static final String COLUMN_NAME_COL1 = "column1";
public static final String COLUMN_NAME_COL2 = "column2";
public static final String COLUMN_NAME_COL3 = "column3";
}
public class DatabaseHelper extends SQLiteOpenHelper {
// If you change the database schema, you must increment the database version.
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME = "database.db";
private static final String TEXT_TYPE = " TEXT";
private static final String COMMA_SEP = ",";
private static final String SQL_CREATE_ENTRIES = "CREATE TABLE " +
Table1.TABLE_NAME + " (" +
Table1._ID + " INTEGER PRIMARY KEY," +
Table1.COLUMN_NAME_COL1 + TEXT_TYPE + COMMA_SEP +
Table1.COLUMN_NAME_COL2 + TEXT_TYPE + COMMA_SEP +
Table1.COLUMN_NAME_COL3 + TEXT_TYPE + COMMA_SEP + " )";
private static final String SQL_DELETE_ALL_ENTRIES = "DROP TABLE IF EXISTS " + Table1.TABLE_NAME;
public DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
// Method is called during creation of the database
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(SQL_CREATE_ENTRIES);
}
// Method is called during an upgrade of the database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
Log.w(DatabaseHelper.class.getName(), "Upgrading database from version " + oldVersion + " to " + newVersion + ", which will destroy all old data");
db.execSQL(SQL_DELETE_ALL_ENTRIES);
onCreate(db);
}
}
}
Did I interpreted it right or have the first 6 variables in the Helper class to be outside in the Contract class? Or should the Helper Class not be a inner class of the contract class?
Hope you can help me
Your contract basically defines your database and how people should interact with it through the Content Provider.
A contract class defines constants that help applications work with the content URIs, column names, intent actions, and other features of a content provider. Contract classes are not included automatically with a provider; the provider's developer has to define them and then make them available to other developers.
Having said that, you do not necessarily need a Content Provider to use a Contract class. My example contains constants that are used by the Content Provider (The MIME and URI parts). If you are not using a Content Provider, you do not need these sections.
I like to think of the contract class as a database schema, or in other words something that defines how your database is set up. You might notice that everything within the contract class is declared as static. That is because you will never be instantiating a Contract class, but only referring to the constants defined in it. You can see in my example that my Contract class just has a bunch of static final variables declared. This Contract class can be its own file, e.g. my file is called TransitContract.java.
Say for example you wanted to change the name of one of your columns. Rather than making changes to multiple files, all you have to do is change the value for the column in the contract class. You are not doing any kind of computation work inside of the contract class.
The SQLLiteOpenhelper class on the other hand is something that was provided by Google to make working with databases easier. This is where you implement methods that create and set up the initial database. See http://developer.android.com/reference/android/database/sqlite/SQLiteOpenHelper.html. After you implement these methods, all you have to do is instantiate an instance of your helper class, and then call helperClassInstance.getWriteableDatabase() (or getReadableDataBase()) and then your helper class automatically takes care of creating a new database if necessary, or returning the one that already exists, etc.
This helper is generally implemented as an inner class but could be its own standalone class. It's however you want to implement it.
I highly recommend looking at the Notepad example that Google provides as it has a pretty good example of how you could possibly set a contract class up. Please note that they also use a Content Provider. If you are interested in learning about Content Providers I recommend you read up some more at http://developer.android.com/guide/topics/providers/content-provider-basics.html. It goes into much more depth about Content Providers and Contract classes.
Here is an example using your code. I did not actually test this code so it might have errors. As you can see, you can instantiate your db helper anywhere you deem necessary. In this example I do it in the main activity's onCreate, but in reality this is bad practice.
DatabaseContract.java
public final class DatabaseContract {
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME = "database.db";
private static final String TEXT_TYPE = " TEXT";
private static final String COMMA_SEP = ",";
// To prevent someone from accidentally instantiating the contract class,
// give it an empty constructor.
private DatabaseContract() {}
public static abstract class Table1 implements BaseColumns {
public static final String TABLE_NAME = "nameOfTable";
public static final String COLUMN_NAME_COL1 = "column1";
public static final String COLUMN_NAME_COL2 = "column2";
public static final String COLUMN_NAME_COL3 = "column3";
public static final String CREATE_TABLE = "CREATE TABLE " +
TABLE_NAME + " (" +
_ID + " INTEGER PRIMARY KEY," +
COLUMN_NAME_COL1 + TEXT_TYPE + COMMA_SEP +
COLUMN_NAME_COL2 + TEXT_TYPE + COMMA_SEP +
COLUMN_NAME_COL3 + TEXT_TYPE + " )";
public static final String DELETE_TABLE = "DROP TABLE IF EXISTS " + TABLE_NAME;
}
}
DatabaseHelper.java
public class DatabaseHelper extends SQLiteOpenHelper {
public DatabaseHelper(Context context) {
super(context, DatabaseContract.DATABASE_NAME, null, DatabaseContract.DATABASE_VERSION);
}
// Method is called during creation of the database
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(DatabaseContract.Table1.CREATE_TABLE);
}
// Method is called during an upgrade of the database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL(DatabaseContract.Table1.DELETE_TABLE);
onCreate(db);
}
}
MainActivity.java
public class MainActivity extends Activity {
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Create new helper
DatabaseHelper dbHelper = new DatabaseHelper(getContext());
// Get the database. If it does not exist, this is where it will
// also be created.
SQLiteDatabase db = dbHelper.getWriteableDatabase();
// Create insert entries
ContentValues values = new ContentValues();
values.put(DatabaseContract.Table1.COLUMN_NAME_COL1, "value1");
values.put(DatabaseContract.Table1.COLUMN_NAME_COL2, "value2");
values.put(DatabaseContract.Table1.COLUMN_NAME_COL3, "value3");
// Insert the new row, returning the primary key value of the new row
long newRowId;
newRowId = db.insert(
DatabaseContract.Table1.TABLE_NAME,
null,
values);
}
}
And my example
public final class TransitContract {
public static final String AUTHORITY = "com.example.TransitProvider";
public static final String SCHEME = "content://";
public static final String SLASH = "/";
public static final String DATABASE_NAME = "transit.db";
/* An array list of all the SQL create table statements */
public static final String[] SQL_CREATE_TABLE_ARRAY = {
Agency.CREATE_TABLE,
CalendarDates.CREATE_TABLE,
Calendar.CREATE_TABLE,
Routes.CREATE_TABLE,
Shapes.CREATE_TABLE,
Stops.CREATE_TABLE,
StopTimes.CREATE_TABLE,
Trips.CREATE_TABLE
};
/**
* Array of resource ids for each GTFS data file that will be loaded into
* database
*/
public static final int[] RAW_IDS = {
R.raw.agency,
R.raw.calendar_dates,
R.raw.calendar,
R.raw.routes,
R.raw.shapes,
R.raw.stops,
R.raw.stop_times,
R.raw.trips,
};
/* Do not allow this class to be instantiated */
private TransitContract() {}
public static final class Agency implements BaseColumns {
/* Do not allow this class to be instantiated */
private Agency() {}
public static final String TABLE_NAME = "Agency";
public static final String KEY_AGENCY_ID = "AgencyId";
public static final String KEY_NAME = "Name";
public static final String KEY_URL = "Url";
public static final String KEY_TIMEZONE = "Timezone";
public static final String KEY_LANG = "Language";
public static final String KEY_PHONE = "PhoneNumber";
public static final String KEY_FARE_URL = "FareUrl";
/*
* URI definitions
*/
/**
* The content style URI
*/
public static final Uri CONTENT_URI = Uri.parse(SCHEME + AUTHORITY + SLASH + TABLE_NAME);
/**
* The content URI base for a single row. An ID must be appended.
*/
public static final Uri CONTENT_ID_URI_BASE = Uri.parse(SCHEME + AUTHORITY + SLASH + TABLE_NAME + SLASH);
/**
* The default sort order for this table
*/
public static final String DEFAULT_SORT_ORDER = KEY_AGENCY_ID + " ASC";
/*
* MIME type definitions
*/
/**
* The MIME type of {@link #CONTENT_URI} providing rows
*/
public static final String CONTENT_TYPE = ContentResolver.CURSOR_DIR_BASE_TYPE +
"/vnd.com.marylandtransitcommuters.agency";
/**
* The MIME type of a {@link #CONTENT_URI} single row
*/
public static final String CONTENT_ITEM_TYPE = ContentResolver.CURSOR_ITEM_BASE_TYPE +
"/vnd.com.marylandtransitcommuters.agency";
/**
* SQL Statement to create the routes table
*/
public static final String CREATE_TABLE = "CREATE TABLE " + TABLE_NAME + " ("
+ _ID + " INTEGER PRIMARY KEY,"
+ KEY_AGENCY_ID + " TEXT,"
+ KEY_NAME + " TEXT,"
+ KEY_URL + " TEXT,"
+ KEY_TIMEZONE + " TEXT,"
+ KEY_LANG + " TEXT,"
+ KEY_PHONE + " TEXT,"
+ KEY_FARE_URL + " TEXT"
+ ");";
/**
* SQL statement to delete the table
*/
public static final String DELETE_TABLE = "DROP TABLE IF EXISTS " + TABLE_NAME;
/**
* Array of all the columns. Makes for cleaner code
*/
public static final String[] KEY_ARRAY = {
KEY_AGENCY_ID,
KEY_NAME,
KEY_URL,
KEY_TIMEZONE,
KEY_LANG,
KEY_PHONE,
KEY_FARE_URL
};
}
这篇关于如何在Android中使用合同类？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
