第七章：数据持久化存储
一．对Android来说，主要有三种基本方法存储数据：
Shared preferences 一种轻量级的数据存储机制，用来存储一些简单的数据
传统文件系统
关系型数据库管理系统，通过支持SQLite数据库实现
Android提供SharedPreferences对象帮助用户保存简单的应用数据二.保存和读取用户偏好 使用文件持久保存数据
<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.jfdimarzio.files"> <application android:allowBackup="true" android:icon="@mipmap/ic_launcher" android:label="@string/app_name" android:supportsRtl="true" android:theme="@style/AppTheme"> <activity android:name=".MainActivity"> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> </application> </manifest> <?xml version="1.0" encoding="utf-8"?> <android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools" android:id="@+id/activity_main" android:layout_width="match_parent" android:layout_height="match_parent" tools:context="com.jfdimarzio.files.MainActivity"> <TextView android:text="Please enter some text." android:layout_width="245dp" android:layout_height="wrap_content" android:id="@+id/textView" app:layout_constraintLeft_toLeftOf="@+id/activity_main" app:layout_constraintTop_toTopOf="@+id/activity_main" android:layout_marginTop="16dp" app:layout_constraintRight_toRightOf="@+id/activity_main" app:layout_constraintBottom_toTopOf="@+id/editText" android:layout_marginBottom="8dp" app:layout_constraintVertical_bias="0.28" /> <EditText android:layout_width="241dp" android:layout_height="wrap_content" android:inputType="text" android:ems="10" tools:layout_editor_absoluteY="82dp" android:id="@+id/editText" app:layout_constraintLeft_toLeftOf="@+id/activity_main" app:layout_constraintRight_toRightOf="@+id/activity_main" app:layout_constraintTop_toBottomOf="@+id/textView" android:layout_marginTop="136dp"/> <Button android:text="Save" android:layout_width="240dp" android:layout_height="wrap_content" android:id="@+id/btnSave" app:layout_constraintLeft_toLeftOf="@+id/activity_main" android:layout_marginStart="16dp" app:layout_constraintTop_toBottomOf="@+id/editText" android:layout_marginTop="136dp" app:layout_constraintRight_toRightOf="@+id/activity_main" android:layout_marginEnd="16dp" android:onClick="onClickSave" /> <Button android:text="Load" android:layout_width="241dp" android:layout_height="wrap_content" android:id="@+id/btnLoad" app:layout_constraintLeft_toLeftOf="@+id/activity_main" android:layout_marginStart="16dp" app:layout_constraintTop_toBottomOf="@+id/editText" android:layout_marginTop="48dp" app:layout_constraintRight_toRightOf="@+id/activity_main" android:layout_marginEnd="16dp" android:onClick="onClickLoad" /> </android.support.constraint.ConstraintLayout>
package
com.jfdimarzio.files;
import
android.support.v7.app.AppCompatActivity;
import
android.os.Bundle;
import
android.view.View;
import
android.widget.EditText;
import
android.widget.Toast;
import
java.io.FileInputStream;
import
java.io.FileOutputStream;
import
java.io.IOException;
import
java.io.InputStreamReader;
import
java.io.OutputStreamWriter;
public
class MainActivity extends AppCompatActivity {
EditText textBox;
static final int READ_BLOCK_SIZE = 100;
@Override
protected void onCreate(Bundle
savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
textBox = (EditText)
findViewById(R.id.editText);
}
public void onClickSave(View view) {
String str = textBox.getText().toString();
try {
FileOutputStream fOut =
openFileOutput("textfile.txt", MODE_PRIVATE);
OutputStreamWriter osw = new
OutputStreamWriter(fOut);
//---write the string to the
file---
try {
osw.write(str);
} catch (IOException e) {
e.printStackTrace();
}
osw.flush();
osw.close();
//---display file saved message---
Toast.makeText(getBaseContext(),
"File saved successfully!", Toast.LENGTH_SHORT).show();
//---clears the EditText---
textBox.setText("");
} catch (IOException ioe) {
ioe.printStackTrace();
}
}
public void onClickLoad(View view) {
try {
FileInputStream fIn =
openFileInput("textfile.txt");
InputStreamReader isr = new
InputStreamReader(fIn);
char[] inputBuffer = new
char[READ_BLOCK_SIZE];
String s = "";
int charRead;
while ((charRead =
isr.read(inputBuffer)) > 0) {
//---convert the chars to a
String---
String readString =
String.copyValueOf(inputBuffer, 0,
charRead);
s += readString;
inputBuffer = new
char[READ_BLOCK_SIZE];
}
//---set the EditText to the text
that has been
// read---
textBox.setText(s);
Toast.makeText(getBaseContext(),
"File loaded successfully!",
Toast.LENGTH_SHORT).show();
} catch (IOException ioe) {
ioe.printStackTrace();
}
}
} 三 .创建和使用数据库
<?xml version="1.0" encoding="utf-8"?> <android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools" android:id="@+id/activity_main" android:layout_width="match_parent" android:layout_height="match_parent" tools:context="com.jfdimarzio.databases.MainActivity"> <TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Hello World!" app:layout_constraintBottom_toBottomOf="@+id/activity_main" app:layout_constraintLeft_toLeftOf="@+id/activity_main" app:layout_constraintRight_toRightOf="@+id/activity_main" app:layout_constraintTop_toTopOf="@+id/activity_main" /> </android.support.constraint.ConstraintLayout>
package
com.jfdimarzio.databases;
import
android.content.ContentValues;
import
android.content.Context;
import
android.database.Cursor;
import
android.database.SQLException;
import
android.database.sqlite.SQLiteDatabase;
import
android.database.sqlite.SQLiteOpenHelper;
import
android.util.Log;
public
class DBAdapter {
static final String KEY_ROWID =
"_id";
static final String KEY_NAME =
"name";
static final String KEY_EMAIL =
"email";
static final String TAG =
"DBAdapter";
static final String DATABASE_NAME =
"MyDB";
static final String DATABASE_TABLE =
"contacts";
static final int DATABASE_VERSION = 1;
static final String DATABASE_CREATE =
"create table contacts (_id
integer primary key autoincrement, "
+ "name text not null,
email text not null);";
final Context context;
DatabaseHelper DBHelper;
SQLiteDatabase db;
public DBAdapter(Context ctx)
{
this.context = ctx;
DBHelper = new DatabaseHelper(context);
}
private static class DatabaseHelper extends
SQLiteOpenHelper
{
DatabaseHelper(Context context)
{
super(context, DATABASE_NAME, null,
DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db)
{
try {
db.execSQL(DATABASE_CREATE);
} catch (SQLException e) {
e.printStackTrace();
}
}
@Override
public void onUpgrade(SQLiteDatabase
db, int oldVersion, int newVersion)
{
Log.w(TAG, "Upgrading database
from version " + oldVersion + " to "
+ newVersion + ", which will
destroy all old data");
db.execSQL("DROP TABLE IF
EXISTS contacts");
onCreate(db);
}
}
//---opens the database---
public DBAdapter open() throws SQLException
{
db = DBHelper.getWritableDatabase();
return this;
}
//---closes the database---
public void close()
{
DBHelper.close();
}
//---insert a contact into the database---
public long insertContact(String name,
String email)
{
ContentValues initialValues = new
ContentValues();
initialValues.put(KEY_NAME, name);
initialValues.put(KEY_EMAIL, email);
return db.insert(DATABASE_TABLE, null,
initialValues);
}
//---deletes a particular contact---
public boolean deleteContact(long rowId)
{
return db.delete(DATABASE_TABLE,
KEY_ROWID + "=" + rowId, null) > 0;
}
//---retrieves all the contacts---
public Cursor getAllContacts()
{
return db.query(DATABASE_TABLE, new
String[] {KEY_ROWID, KEY_NAME,
KEY_EMAIL}, null, null, null,
null, null);
}
//---retrieves a particular contact---
public Cursor getContact(long rowId) throws
SQLException
{
Cursor mCursor =
db.query(true, DATABASE_TABLE,
new String[] {KEY_ROWID,
KEY_NAME,
KEY_EMAIL}, KEY_ROWID + "=" + rowId, null,
null, null, null,
null);
if (mCursor != null) {
mCursor.moveToFirst();
}
return mCursor;
}
//---updates a contact---
public boolean updateContact(long rowId,
String name, String email)
{
ContentValues args = new ContentValues();
args.put(KEY_NAME, name);
args.put(KEY_EMAIL, email);
return db.update(DATABASE_TABLE, args,
KEY_ROWID + "=" + rowId, null) > 0;
}
}
package
com.jfdimarzio.databases;
import
android.database.Cursor;
import
android.support.v7.app.AppCompatActivity;
import
android.os.Bundle;
import
android.widget.Toast;
public
class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle
savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
DBAdapter db = new DBAdapter(this);
//---add a contact---
db.open();
long id =
db.insertContact("Jennifer Ann",
"jenniferann@jfdimarzio.com");
id = db.insertContact("Oscar
Diggs", "oscar@oscardiggs.com");
db.close();
db.open();
Cursor c = db.getAllContacts();
if (c.moveToFirst())
{
do {
DisplayContact(c);
} while (c.moveToNext());
}
db.close();
}
public void DisplayContact(Cursor c)
{
Toast.makeText(this,
"id: " +
c.getString(0) + "\n" +
"Name: " + c.getString(1) +
"\n" +
"Email: " + c.getString(2),
Toast.LENGTH_LONG).show();
}
} 
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