1. 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

调用这个方法可以创建多个shared preference 文件,并且可以通过任何的上下文content来调用

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.

创建当前Activity的shared preference文件

2. Save a Fire on Internal Storage

2.1 getFilesDir() : 为你的应用程序返回一个内存文件
2.2 getCacheDir() : 返回一个缓存文件. "当文件不需要的时候应该将其删除并且在使用的时候应该给予限制内存的大小"
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;
}

3. Saving Data in SQL Databases

3.1 Define a Schema and Contract
创建一个常量类,在该常量类中创建一个嵌套类实现"BaseColumns"接口用来构建一种表.

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";       
 ...    
}}

在这里有一处不太明白,那如果有多张表是不是在里面写多个嵌套类呢?

3.2 Create a Database Using a SQL Helper
创建一个类继承于"SQLiteOpenHelper",用于来创建和更新数据表

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);