Android Learning notes (14)

Last Update:2015-09-13 Source: Internet

Author: User

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Data storage in AndroidData persistence means that the instantaneous data in memory is saved to the storage device. There are three main ways in which Android systems are designed to simply implementData persistence, which is file storage, sharedpreferences storage, and database Storage.1. File storage is a basic method of storage in android, it does not do any formatting of the stored content, all the data is intactSaved to a File.1.1A Openfileoutput () method is provided in the context class that can be used to store data to the specified File.This method receives two Parameters:1. The first parameter is the file name, (the file name is not included in the path, all files are saved by default in The/data/data/<package name>/files/directory).2. The second parameter is the operating mode of the file (there are two main modes of Operation: 1. Mode_private and mode_append, of whichMode_private is the default mode of operation, indicating that when the same file name is specified, the content written will overwrite the contents of the original file;Mode_append indicates that if the file already exists, append content to the file and create a new file if it does not exist. ）The Openfileoutput () method returns a FileOutputStream object that can be used to write the data in a file using the Java Stream.The following code shows how to write a piece of text in a file:<pre> public voidSave (string Inputtext) {string data= "Data to Save"; FileOutputStream out=NULL; BufferedWriter writer=NULL; Try{ out= Openfileoutput ("data", context.mode_private); Writer=NewBufferedWriter (NewOutputStreamWriter (out)); Writer.write (inputtext); } Catch(ioexception E) {e.printstacktrace (); }finally { Try { if(writer! =NULL) {writer.close (); } } Catch(ioexception E) {e.printstacktrace (); } } }</pre>In the code above, a FileOutputStream object can be obtained through the Openfileoutput () method, which is then used to build aOutputStreamWriter object, and then uses OutputStreamWriter to build a BufferedWriter object so that it can beBufferedWriter to write text content to a File.1.2A Openfileinput () method is also provided in the context class for reading data from a File.This method takes only one parameter, which is the name of the file to be read, and then the system will automatically go to The/data/data/<package name>/files/directoryTo load the file and return a FileInputStream object that can be read by the Java stream after the object is Obtained.The following is a code example that shows how to read text data from a file:<pre>FileInputStream in =NULL; BufferedReader Reader=NULL; StringBuilder content=NewStringBuilder (); Try{ in= Openfileinput ("data"); Reader=NewBufferedReader (NewInputStreamReader (in)); String line= ""; while(line = Reader.readline ())! =NULL) {content.append (line); } } Catch(ioexception E) {e.printstacktrace (); }finally { if(reader! =NULL) { Try{reader.close (); } Catch(ioexception E) {e.printstacktrace (); } } } returncontent.tostring (); }</pre>In the above code, first obtains a FileInputStream object through the Openfileinput () method, then constructs a InputStreamReader object with it,Then use InputStreamReader to build a bufferedreader object, so that you can use the BufferedReader to read a line, the fileThe contents of the StringBuilder are all read out and present in a single object, and then the contents of the read are Returned.2.SharedPreferences is the way to store data using Key-value Pairs.2.1To store data with sharedpreferences, you first need to get to the sharedpreferences Object. There are three main methods available in Android forGet the Sharedpreferences Object.1). The Getsharedpreferences () method in the context classThe method receives two parameters, the first parameter specifies the name of the Sharedpreferences file, and if the specified file name does not exist, it creates aSharedpreferences files are present in The/data/data/<package name>/shared_prefs/directory;The second parameter specifies the mode of operation, and there are two main modes to choose from, mode_private and Mode_multi_process.Where mode_private is the default mode of operation, which means that only the current application can read and write to this sharedpreferences file.Mode_multi_process is typically used to read and write to the same sharedpreferences file in a process.2). The Getpreferences () method in the Activity class3). getdefaultsharedpreferences () method in the Preferencemanager classAfter the Sharedpreferences object is obtained, the data can be stored in the file, which can be divided into three implementations:1). Call the edit () method of the Sharedpreferences object to get a sharedpreferences.editor object.2). add data to the Sharedpreferences.editor Object. For example, Add a string and use the putstring () method.3). Call the Commit () method to submit the added Data.The specific implementation is shown in the following code:<pre><LinearLayoutxmlns:android= "http://schemas.android.com/apk/res/android"Android:layout_width= "match_parent"Android:layout_height= "match_parent"android:orientation= "vertical"> <ButtonAndroid:id= "@+id/save_data"Android:layout_width= "match_parent"Android:layout_height= "wrap_content"Android:text= "@string/save_data"/></LinearLayout></pre>Set a button to store the data in the File.<pre> protected voidonCreate (Bundle Savedinstancestate) {Super. OnCreate (savedinstancestate); Setcontentview (r.layout.activity_main); SaveData=(Button) Findviewbyid (r.id.save_data); //setting up listeners for SaveDataSavedata.setonclicklistener (NewView.onclicklistener () {@Override public voidOnClick (View V) {//Call the edit () method of the Sharedpreferences object to get a sharedpreferences.editor object. //the getsharedpreferences () method in the context class to get the Sharedpreferences object, which receives two parameters//1. The first parameter specifies the name of the Sharedpreferences file, and if the specified file does not exist, one is Created. //2. The second parameter is used to specify the operating mode, there are two main modes of Operation (mode_private and Mode_multi_process)Sharedpreferences.editor Editor = getsharedpreferences ("data", mode_private). Edit (); //adds data to the Sharedpreferences.editor Object. For example, Add a string and use the Putstring () methodEditor.putstring ("name", "Tom"); Editor.putint ("age", 22); //Call the Commit () method to submit the added dataEditor.commit (); } });</pre>The above code, click the button through the Getsharedpreferences () method to get the Sharedpreferences file name of data, and got theThe Sharedpreferences.editor Object. Then add the data and finally call the commit () method to Commit.2.2Reading data from the SharedpreferencesThe Sharedpreferences object provides a series of get methods for reading stored data, each of which corresponds to theA put method in Sharepreferences.editor.The specific implementation is shown in the following code:<pre><LinearLayoutxmlns:android= "http://schemas.android.com/apk/res/android"Android:layout_width= "match_parent"Android:layout_height= "match_parent"android:orientation= "vertical"> <ButtonAndroid:id= "@+id/save_data"Android:layout_width= "match_parent"Android:layout_height= "wrap_content"Android:text= "@string/save_data"/> <ButtonAndroid:id= "@+id/restore_data"Android:layout_width= "match_parent"Android:layout_height= "wrap_content"Android:text= "@string/restore_data"/></LinearLayout></pre>Modify the code in the mainactivity as shown in:<pre>Restoredata =(Button) Findviewbyid (r.id.restore_data); //set the listener on the Restoredata buttonRestoredata.setonclicklistener (NewView.onclicklistener () {@Override public voidOnClick (View V) {//get Sharedpreferences Object by Getsharedpreferences () methodSharedpreferences Sharedpre = getsharedpreferences ("data"), mode_private); //get the data through the Get methodString name = sharedpre.getstring ("name", "" "); intAge = Sharedpre.getint ("age", 0); LOG.D ("mainactivity", "name is" +name); LOG.D ("mainactivity", "age is" +age ); } });</pre>In the above code, the Sharedpreferences object is first obtained through the getsharedpreferences () method, and then the Get method is called to get the Data.If no corresponding value is found, the default value passed in the method is used Instead.Android Learning notes (14)

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Android Learning notes (14)

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