In java/com.example.android.pets/data/PetContract.java
/**
* Inner class that defines constant values for the pets database table.
* Each entry in the table represents a single pet.
*/
public static final class PetEntry implements BaseColumns {
/** Name of database table for pets */
public final static String TABLE_NAME = "pets";
/**
* Unique ID number for the pet (only for use in the database table).
*
* Type: INTEGER
*/
public final static String _ID = BaseColumns._ID;
/**
* Name of the pet.
*
* Type: TEXT
*/
public final static String COLUMN_PET_NAME ="name";
/**
* Possible values for the gender of the pet.
*/
public static final int GENDER_UNKNOWN = 0;
public static final int GENDER_MALE = 1;
public static final int GENDER_FEMALE = 2;
}
注意 PetContract 文件放在应用包名 (com.example.android.pets) 下单独的 data 包内，与 Activity 文件区分开。这是因为 Android 是通过包 (package) 来组织代码的，一类包存放一类特定功能的代码，例如 data 包下就存放数据相关的代码。
In java/com.example.android.pets/data/PetDbHelper.java
public class PetDbHelper extends SQLiteOpenHelper {
private static final String DATABASE_NAME = "shelter.db";
private static final int DATABASE_VERSION = 1;
public PetDbHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
// Create a String that contains the SQL statement to create the pets table
String SQL_CREATE_PETS_TABLE = "CREATE TABLE " + PetEntry.TABLE_NAME + " ("
+ PetEntry._ID + " INTEGER PRIMARY KEY AUTOINCREMENT, "
+ PetEntry.COLUMN_PET_NAME + " TEXT NOT NULL, "
+ PetEntry.COLUMN_PET_BREED + " TEXT, "
+ PetEntry.COLUMN_PET_GENDER + " INTEGER NOT NULL, "
+ PetEntry.COLUMN_PET_WEIGHT + " INTEGER NOT NULL DEFAULT 0);";
// Execute the SQL statement
db.execSQL(SQL_CREATE_PETS_TABLE);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// The database is still at version 1, so there's nothing to do be done here.
}
}
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_SUBTITLE
};
// Filter results WHERE "title" = 'My Title'
String selection = FeedEntry.COLUMN_NAME_TITLE + " = ?";
String[] selectionArgs = { "My Title" };
// How you want the results sorted in the resulting Cursor
String sortOrder = FeedEntry.COLUMN_NAME_SUBTITLE + " DESC";
Cursor cursor = 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
);
将上述代码放入 try/finally 区块内，即使应用崩溃，也能保证执行 close method 关闭 Cursor。
如何提取应用的数据库文件
在 Android Studio 界面右上角搜索 Device File Explorer 打开模拟器的文件浏览器，打开目录 data > data > com.example.android.pets > databases 即可查看应用内的数据库文件。右键选择 "Save As..." 将数据库文件提取到电脑中，即可通过终端访问。
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_editor);
...
mGenderSpinner = (Spinner) findViewById(R.id.spinner_gender);
setupSpinner();
}
/**
* Setup the dropdown spinner that allows the user to select the gender of the pet.
*/
private void setupSpinner() {
// Create adapter for spinner. The list options are from the String array it will use
// the spinner will use the default layout
ArrayAdapter genderSpinnerAdapter = ArrayAdapter.createFromResource(this,
R.array.array_gender_options, android.R.layout.simple_spinner_item);
// Specify dropdown layout style - simple list view with 1 item per line
genderSpinnerAdapter.setDropDownViewResource(android.R.layout.simple_dropdown_item_1line);
// Apply the adapter to the spinner
mGenderSpinner.setAdapter(genderSpinnerAdapter);
// Set the integer mSelected to the constant values
mGenderSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
String selection = (String) parent.getItemAtPosition(position);
if (!TextUtils.isEmpty(selection)) {
if (selection.equals(getString(R.string.gender_male))) {
mGender = PetEntry.GENDER_MALE;
} elseif (selection.equals(getString(R.string.gender_female))) {
mGender = PetEntry.GENDER_FEMALE;
} else {
mGender = PetEntry.GENDER_UNKNOWN;
}
}
}
// Because AdapterView is an abstract class, onNothingSelected must be defined
@Override
public void onNothingSelected(AdapterView<?> parent) {
mGender = PetEntry.GENDER_UNKNOWN;
}
});
}
Spinner 适配器设置为一个 ArrayAdapter，通过其静态方法 createFromResource 创建，输入参数分别为应用环境 (Context)、Array 资源 ID、布局资源 ID。其中，布局资源 ID 使用 Android 提供的默认布局 simple_spinner_item；Array 资源 ID 则是在应用内定义的资源。
In res/values/arrays.xml
<resources>
<!-- These are the options displayed in the gender drop-down Spinner -->
<string-array name="array_gender_options">
<item>@string/gender_unknown</item>
<item>@string/gender_male</item>
<item>@string/gender_female</item>
</string-array>
</resources>
