Android sq lite-chapter 22

Chapter 22
SQLite
By
Dr. Ramkumar Lakshminarayanan
Introduction
SQLite is available on every Android device. Using an SQLite database in Android
does not require any database setup or administration. You only have to define the SQL
statements for creating and updating the database. Afterwards the database is automatically
managed for you by the Android platform. In this chapter we will discuss about developing
application with SQLite.
SQLite
SQLite is an Open Source Database which is embedded into Android. SQLite supports
standard relational database features like SQL syntax, transactions and prepared statements. In
addition it requires only little memory at runtime (approx. 250 KByte).
SQLite supports the data types TEXT (similar to String in Java), INTEGER (similar to
long in Java) and REAL (similar to double in Java). All other types must be converted into one
of these fields before saving them in the database. SQLite itself does not validate if the types
written to the columns are actually of the defined type, e.g. you can write an integer into a
string column and vice versa.
SQLite in Android
SQLite is available on every Android device. Using an SQLite database in Android
does not require any database setup or administration. You only have to define the SQL
statements for creating and updating the database. Afterwards the database is automatically
managed for you by the Android platform. Access to an SQLite database involves accessing
the filesystem. This can be slow. Therefore it is recommended to perform database operations
asynchronously, for example inside the AsyncTask class.
If your application creates a database, this database is by default saved in the directory
DATA/data/APP_NAME/databases/FILENAME.
The parts of the above directory are constructed based on the following rules. DATA is
the path which the Environment.getDataDirectory() method returns. APP_NAME is your
application name. FILENAME is the name you specify in your application code for the
database.
SQLite Packages

The package android.database contains all general classes for working with databases.
android.database.sqlite contains the SQLite specific classes
SQLiteOpenHelper
To create and upgrade a database in your Android application you usually subclass
SQLiteOpenHelper. In the constructor of your subclass you call the super() method of
SQLiteOpenHelper, specifying the database name and the current database version.
In this class you need to override the onCreate() and onUpgrade() methods.
onCreate() is called by the framework, if the database does not exists.
onUpgrade() is called, if the database version is increased in your application code.
This method allows you to update the database schema.
Both methods receive an SQLiteDatabase object as parameter which represents the
database. SQLiteOpenHelper provides the methods getReadableDatabase() and
getWriteableDatabase() to get access to an SQLiteDatabase object; either in read or write
mode.
The database tables should use the identifier _id for the primary key of the table.
Several Android functions rely on this standard.
It is best practice to create a separate class per table. This class defines static onCreate()
and onUpgrade() methods. These methods are called in the corresponding methods of
SQLiteOpenHelper. This way your implementation of SQLiteOpenHelper will stay readable,
even if you have several tables
SQLiteDatabase
SQLiteDatabase is the base class for working with a SQLite database in Android and
provides methods to open, query, update and close the database.
More specifically SQLiteDatabase provides the insert(), update() and delete() methods.
In addition it provides the execSQL() method, which allows to execute an SQL statement
directly.
The object ContentValues allows to define key/values. The "key" represents the table
column identifier and the "value" represents the content for the table record in this column.
ContentValues can be used for inserts and updates of database entries.
Queries can be created via the rawQuery() and query() methods or via the
SQLiteQueryBuilder class .

rawQuery() directly accepts an SQL select statement as input.
rawQuery() Example
The following gives an example of a rawQuery() call.
Cursor cursor = getReadableDatabase().
rawQuery("select * from todo where _id = ?", new String[] { id });
query() Example
The following gives an example of a query() call.
return database.query(DATABASE_TABLE,
new String[] { KEY_ROWID, KEY_CATEGORY, KEY_SUMMARY, KEY_DESCRIPTION
},
null, null, null, null, null);
Cursor
A query returns a Cursor object. A Cursor represents the result of a query and basically
points to one row of the query result. This way Android can buffer the query results efficiently;
as it does not have to load all data into memory.
To get the number of elements of the resulting query use the getCount() method.
To move between individual data rows, you can use the moveToFirst() and
moveToNext() methods. The isAfterLast() method allows to check if the end of the query result
has been reached.
Cursor provides typed get*() methods, e.g. getLong(columnIndex),
getString(columnIndex) to access the column data for the current position of the result. The
"columnIndex" is the number of the column you are accessing.
Cursor also provides the getColumnIndexOrThrow(String) method which allows to get
the column index for a column name of the table.
A Cursor needs to be closed with the close() method call
query() providesastructuredinterface forspecifyingthe SQLquery.
SQLiteQueryBuilderisa convenience classthathelpstobuildSQLqueries.

ListViews, ListActivities and SimpleCursorAdapter
ListViews are Views which allow to display a list of elements. ListActivities are
specialized activities which make the usage of ListViews easier. To work with databases and
ListViews you can use the SimpleCursorAdapter. The SimpleCursorAdapter allows to set a
layout for each row of the ListViews. You also define an array which contains the column names
and another array which contains the IDs of Views which should be filled with the data. The
SimpleCursorAdapter class will map the columns to the Views based on the Cursor passed to it.
To obtain the Cursor you should use the Loader class.
Using SQLite Example
The following demonstrates how to work with an SQLite database. We will use a data
access object (DAO) to manage the data for us. The DAO is responsible for handling the
database connection and for accessing and modifying the data. It will also convert the database
objects into real Java Objects, so that our user interface code does not have to deal with the
persistence layer.
Create the new Android project with the name com.mjs.sqlite.first and an Activity called
TestDatabaseActivity.

Create the MySQLiteHelper class. This class is responsible for creating the database. The
onUpgrade() method will simply delete all existing data and re-create the table. It also defines
several constants for the table name and the table columns.
package com.mjs.sqlite.first;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;import android.util.Log;
public class MySQLiteHelper extends SQLiteOpenHelper {
public static final String TABLE_COMMENTS = "comments";
public static final String COLUMN_ID = "_id";
public static final String COLUMN_COMMENT = "comment";
private static final String DATABASE_NAME = "commments.db";
private static final int DATABASE_VERSION = 1;
// Database creation sql statement
private static final String DATABASE_CREATE = "create table "
+ TABLE_COMMENTS + "(" + COLUMN_ID
+ " integer primary key autoincrement, " + COLUMN_COMMENT
+ " text not null);"; public MySQLiteHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION); } @Override
public void onCreate(SQLiteDatabase database){
database.execSQL(DATABASE_CREATE); } @Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
Log.w(MySQLiteHelper.class.getName(),
"Upgrading database from version " + oldVersion + " to "
+ newVersion + ", which will destroy all old data");
db.execSQL("DROP TABLE IF EXISTS " + TABLE_COMMENTS); onCreate(db); }}

Create the Comment class. This class is our model and contains the data we will save in
the database and show in the user interface.
Create the CommentsDataSource class. This class is our DAO. It maintains the database
connection and supports adding new comments and fetching all comments.
public class Comment {
private long id;
private String comment;
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getComment() {
return comment;
}
public void setComment(String comment) {
this.comment = comment;
}
// Will be used by the ArrayAdapter in the ListView
@Override
public String toString() {
return comment;
}
}

import java.util.ArrayList;
import java.util.List;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
public class CommentsDataSource {
// Database fields
private SQLiteDatabase database;
private MySQLiteHelper dbHelper;
private String[] allColumns = { MySQLiteHelper.COLUMN_ID,
MySQLiteHelper.COLUMN_COMMENT };
public CommentsDataSource(Context context) {
dbHelper = new MySQLiteHelper(context);
}
public void open() throws SQLException {
database = dbHelper.getWritableDatabase();
}
public void close() {
dbHelper.close();
}
public Comment createComment(String comment) {
ContentValues values = new ContentValues();
values.put(MySQLiteHelper.COLUMN_COMMENT, comment);
long insertId = database.insert(MySQLiteHelper.TABLE_COMMENTS, null,
values);
Cursor cursor = database.query(MySQLiteHelper.TABLE_COMMENTS,
allColumns, MySQLiteHelper.COLUMN_ID + " = " + insertId, null,
null, null, null);
cursor.moveToFirst();
Comment newComment = cursorToComment(cursor);
cursor.close();
return newComment;
}
public void deleteComment(Comment comment) {
long id = comment.getId();
System.out.println("Comment deleted with id: " + id);
database.delete(MySQLiteHelper.TABLE_COMMENTS,
MySQLiteHelper.COLUMN_ID
+ " = " + id, null);
}
public List<Comment> getAllComments() {
List<Comment> comments = new ArrayList<Comment>();
Cursor cursor = database.query(MySQLiteHelper.TABLE_COMMENTS,
allColumns, null, null, null, null, null);
cursor.moveToFirst();
while (!cursor.isAfterLast()) {
Comment comment = cursorToComment(cursor);
comments.add(comment);
cursor.moveToNext();
}
// Make sure to close the cursor

User Interface
Change your main.xml layout file in the res/layout folder to the following. This layout
has two buttons for adding and deleting comments and a ListView which will be used to display
the existing comments. The comment text will be generated later in the Activity by a small
random generator.
Change your TestDatabaseActivity class. to the following. We use here a ListActivity for
displaying the data.
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent" android:layout_height="match_parent"
android:orientation="vertical" > <LinearLayout
android:id="@+id/group" android:layout_width="wrap_content"
android:layout_height="wrap_content" > <Button
android:id="@+id/add" android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Add New" android:onClick="onClick"/>
<Button android:id="@+id/delete"
android:layout_width="wrap_content"
android:text="Delete First" android:onClick="onClick"/>
</LinearLayout> <ListView android:id="@android:id/list"
android:layout_width="match_parent"
android:text="@string/hello" /></LinearLayout>

package de.vogella.android.sqlite.first;
import java.util.List;
import java.util.Random;
import android.app.ListActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
public class TestDatabaseActivity extends ListActivity {
privateCommentsDataSource datasource;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
datasource = new CommentsDataSource(this);
datasource.open();
List<Comment> values = datasource.getAllComments();
// Usethe SimpleCursorAdapter to show the
// elements in a ListView
ArrayAdapter<Comment> adapter = new ArrayAdapter<Comment>(this,
android.R.layout.simple_list_item_1, values);
setListAdapter(adapter);
}
// Will be called via theonClick attribute
// of thebuttons in main.xml
public void onClick(View view) {
@SuppressWarnings("unchecked")
ArrayAdapter<Comment> adapter = (ArrayAdapter<Comment>) getListAdapter();
Comment comment = null;
switch (view.getId()) {
case R.id.add:
String[] comments = new String[] { "Cool", "Very nice", "Hateit" };
int nextInt = new Random().nextInt(3);
// Save the new comment to thedatabase
comment = datasource.createComment(comments[nextInt]);
adapter.add(comment);
break;
case R.id.delete:
if (getListAdapter().getCount() > 0) {
comment = (Comment) getListAdapter().getItem(0);
datasource.deleteComment(comment);
adapter.remove(comment);
}
break;
}
adapter.notifyDataSetChanged();
}
@Override
protected void onResume() {
datasource.open();
super.onResume();
}
@Override
protected void onPause() {
datasource.close();
super.onPause();

Running the apps
Install your application and use the Add and Delete button. Restart your application to
validate that the data is still there.
Summary
Android provides several ways to store user and app data. SQLite is one way of storing
user data. SQLite is a very light weight database which comes with Android OS.

Android sq lite-chapter 22

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Android sq lite-chapter 22