1. Persistence technology:
Data persistence: a technology that saves the data in the memory to a file so that the device can be used normally after shutdown or restart;
The cost of switching between memory state and persistent state: CPU, file reading, file writing, kernel state switching
2.Android built-in persistence technology:
(1) file system;
(2) Shared Preferences
(3)SQLite
3. How to use the file system: file persistence
How to save data to a file:
How to read data from a file:
Note: The data is placed in the data/data/packagename/files folder by default, and it will be warned by the system if it is placed in other paths
Example of file writing and reading:
(1) Layout file:
(2) Java program for writing and reading files:
2.1 Define variables and get layout ID:
public void write(){<!-- --><br> String data="Merry Christmas, I wish you all a smooth exam!";<br> FileOutputStream out=null;<br> BufferedWriter writer=null;<br> try{<!-- --><br> out=openFileOutput("data.log", Context.MODE_PRIVATE);<br> writer=new BufferedWriter(new OutputStreamWriter(out));<br> writer. write(data);<br> }catch (IOException e){<!-- --><br> e.printStackTrace();<br> }finally {<!-- --><br> try{<!-- --><br> if (writer!=null){<!-- --><br> writer. close();<br> }<br> }catch(IOException e){<!-- --><br> e.printStackTrace();<br> }<br> }<br>}
2.2 button binding write file function event:
WriteButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
write();
}
});
(3) Define the permission to write to the file:
(4) Java program for reading files:
public String Read(){ //return a String
FileInputStream in=null; //Read the data stream of the file
BufferedReader reader=null;
StringBuilder content=new StringBuilder();
try {
in=openFileInput("data. log");
reader=new BufferedReader(new InputStreamReader(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();
}
}
}
return content.toString();
}
3.1 Button binding to read file system data:
ReadButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
String str=Read();
Toast.makeText(MainActivity.this,str,Toast.LENGTH_LONG).show();
}
});
}
4. How to use SharedPreferences:
(1) Non-relational data
(2) Stored in the form of Key-Value key-value pairs;
(3) In essence, an XML file is maintained, and data is stored and read on this file;
Example: Just look at the Java program code directly, everything else is the same
public void write(){ //no return value
SharedPreferences.Editor editor=getSharedPreferences("data",MODE_PRIVATE).edit();
//The first parameter: the stored file name, the second parameter is the open mode
editor.putString("username","chunyu");
editor. putString("password","0321");
editor. apply();
}
Button bindings write events to SharedPreferences:
WriteButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
write();
}
});
The result of writing:
5. How to use SQLite:
(1) A relational database
(2) Android, windows and Linux can be used directly
(3) Free and open source
(4) In the case of a small amount of data, it is a good choice
example:
layout file:
JAVA program that uses SQLite to store data:
private Button createButton;
private DataBaseHelper dbHelper;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R. layout. activity_main);
//Database class instantiation, instantiate database access
dbHelper=new DataBaseHelper(this,"userdatabase,db",null,1); //The second parameter is the name of the database
createButton=(Button)findViewById(R.id.creatButton);
createButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
dbHelper.getWritableDatabase(); //
}
});
}
public class DataBaseHelper extends SQLiteOpenHelper{<!-- -->
public static final String Create_SQL="create table userinfo(id integer primary key autoincrement, username text, password text)";
private Context mContext;
public DataBaseHelper(Context context, String name, SQLiteDatabase. CursorFactory factory, int version){
super(context, name, factory, version);
mContext=context;
}
@Override
public void onCreate(SQLiteDatabase sqLiteDatabase) { // directly rewrite the method of the parent class
sqLiteDatabase.execSQL(Create_SQL);
Toast.makeText(mContext,"Create database successfully",Toast.LENGTH_LONG).show();
}
@Override
public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {
}
}
operation result:
