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Get a Handle to a SharedPreferences
1.1 getSharedPreferences()
Use this if you need multiple shared preference files identified by name, which you specify with the first parameter. You can call this from any Context
調(diào)用這個(gè)方法可以創(chuàng)建多個(gè)shared preference 文件,并且可以通過(guò)任何的上下文content來(lái)調(diào)用
1.2 getPreferences()
Use this from an Activity if you need to use only one shared preference file
for the activity. Because this retrieves a default shared preference file that
belongs to the activity, you don't need to supply a name.
創(chuàng)建當(dāng)前Activity的shared preference文件
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Save a Fire on Internal Storage
2.1 getFilesDir() : 為你的應(yīng)用程序返回一個(gè)內(nèi)存文件
2.2 getCacheDir() : 返回一個(gè)緩存文件. "當(dāng)文件不需要的時(shí)候應(yīng)該將其刪除并且在使用的時(shí)候應(yīng)該給予限制內(nèi)存的大小"
2.3 如何使用
To create a new file in one of these directories, you can use the File()
constructor, passing the File provided by one of the above methods that specifies your internal storage directory. For example:
File file = new File(context.getFilesDir(), filename);
Alternatively, you can call openFileOutput() to get a FileOutputStream
that writes to a file in your internal directory. For example, here's how to write some text to a file:
String filename = "myfile";
String string = "Hello world!";
FileOutputStream outputStream;
try {
outputStream = openFileOutput(filename,Context.MODE_PRIVATE);
outputStream.write(string.getBytes());
outputStream.close();
} catch (Exception e) { e.printStackTrace();}
Or, if you need to cache some files, you should instead use createTempFile()
. For example, the following method extracts the file name from a URL
and creates a file with that name in your app's internal cache directory:
public File getTempFile(Context context, String url) {
File file;
try {
String fileName = Uri.parse(url).getLastPathSegment();
file = File.createTempFile(fileName, null, context.getCacheDir());
catch (IOException e) { // Error while creating file
}
return file;
}
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Saving Data in SQL Databases
3.1 Define a Schema and Contract
創(chuàng)建一個(gè)常量類(lèi),在該常量類(lèi)中創(chuàng)建一個(gè)嵌套類(lèi)實(shí)現(xiàn)"BaseColumns"接口用來(lái)構(gòu)建一種表.
public final class FeedReaderContract {
// To prevent someone from accidentally instantiating the contract class,
// give it an empty constructor.
public FeedReaderContract() {}
/* Inner class that defines the table contents */
public static abstract class FeedEntry implements BaseColumns {
public static final String TABLE_NAME = "entry";
public static final String COLUMN_NAME_ENTRY_ID = "entryid";
public static final String COLUMN_NAME_TITLE = "title";
public static final String COLUMN_NAME_SUBTITLE = "subtitle";
...
}}
在這里有一處不太明白,那如果有多張表是不是在里面寫(xiě)多個(gè)嵌套類(lèi)呢?
3.2 Create a Database Using a SQL Helper
創(chuàng)建一個(gè)類(lèi)繼承于"SQLiteOpenHelper",用于來(lái)創(chuàng)建和更新數(shù)據(jù)表
public class FeedReaderDbHelper extends SQLiteOpenHelper {
/* If you change the database schema, you must increment the database
version. */
private static final String TEXT_TYPE = " TEXT";
private static final String COMMA_SEP = ",";
private static final String SQL_CREATE_ENTRIES =
"CREATE TABLE " + FeedEntry.TABLE_NAME + " (" +
FeedEntry._ID + " INTEGER PRIMARY KEY,"
+ FeedEntry.COLUMN_NAME_ENTRY_ID
+ TEXT_TYPE + COMMA_SEP +
FeedEntry.COLUMN_NAME_TITLE + TEXT_TYPE + COMMA_SEP +
... // Any other options for the CREATE command
" )";
private static final String SQL_DELETE_ENTRIES =
"DROP TABLE IF EXISTS " + FeedEntry.TABLE_NAME;
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME = "FeedReader.db";
public FeedReaderDbHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
public void onCreate(SQLiteDatabase db) {
db.execSQL(SQL_CREATE_ENTRIES);
}
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// This database is only a cache for online data, so its upgrade policy is
// to simply to discard the data and start over
db.execSQL(SQL_DELETE_ENTRIES);
onCreate(db);
}
public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
onUpgrade(db, oldVersion, newVersion);
}}
3.3 Put Information into a Database
// Gets the data repository in write mode
SQLiteDatabase db = mDbHelper.getWritableDatabase();
// Create a new map of values, where column names are the keysContentValues values = new ContentValues();
values.put(FeedEntry.COLUMN_NAME_ENTRY_ID, id);
values.put(FeedEntry.COLUMN_NAME_TITLE, title);
values.put(FeedEntry.COLUMN_NAME_CONTENT, content);
// Insert the new row, returning the primary key value of the new row
long newRowId;
newRowId = db.insert(
FeedEntry.TABLE_NAME,
FeedEntry.COLUMN_NAME_NULLABLE,
values
);
3.4 Read Information from a Database
SQLiteDatabase db = mDbHelper.getReadableDatabase();
// Define a *projection* that specifies which columns from the database// you will actually use after this query.
String[] projection = { FeedEntry._ID, FeedEntry.COLUMN_NAME_TITLE, FeedEntry.COLUMN_NAME_UPDATED,
...
};
// How you want the results sorted in the resulting Cursor
String sortOrder = FeedEntry.COLUMN_NAME_UPDATED + " DESC";
Cursor c = db.query(
FeedEntry.TABLE_NAME, // The table to query
projection, // The columns to return
selection, // The columns for the WHERE clause
selectionArgs, // The values for the WHERE clause
null, // don't group the rows
null, // don't filter by row groups
sortOrder // The sort order
);
3.5 Delete Information from a Database
// Define 'where' part of query.
String selection = FeedEntry.COLUMN_NAME_ENTRY_ID + " LIKE ?";
// Specify arguments in placeholder order.
String[] selectionArgs = { String.valueOf(rowId) };
// Issue SQL statement.
db.delete(table_name, selection, selectionArgs);
