Contract Class的定義:
    Contract Class，又可以叫做Companion Class。
    Android Developer的幫助文檔是這么說的:
   < A contract class is a container for constants that define names for URIs,
     tables, and columns. The contract class allows you to use the same constants
     across all the other classes in the same package. This lets you change a
     column name in one place and have it propagate throughout your code.>
     Contact 類是定義URI、表、列的名字的容器。這個類允許我們在同一包的不同類下使用相同的常量。
     我們在一處修改了列名，同時傳播到我們代碼的每個地方。

package com.example.sqlitetest;
//Contract類
public class Contact {
   
    int _id;
    String _name;
    String _phone_number;
   
    public Contact(){
       
    }
    public Contact(int id, String name, String _phone_number){
        this._id = id;
        this._name = name;
        this._phone_number = _phone_number;
    }
   
    public Contact(String name, String _phone_number){
        this._name = name;
        this._phone_number = _phone_number;
    }
    public int getID(){
        return this._id;
    }
   
    public void setID(int id){
        this._id = id;
    }
   
    public String getName(){
        return this._name;
    }
   
    public void setName(String name){
        this._name = name;
    }
   
    public String getPhoneNumber(){
        return this._phone_number;
    }

    public void setPhoneNumber(String phone_number){
        this._phone_number = phone_number;
    }
}

SQLiteDatabase db = this.getWritableDatabase();
 
ContentValues values = new ContentValues();
values.put(KEY_NAME, contact.getName());
values.put(KEY_PH_NO, contact.getPhoneNumber());
 
db.insert(TABLE_CONTACTS, null, values);
db.close();

SQLiteDatabase db = this.getWritableDatabase();
db.delete(TABLE_CONTACTS, KEY_ID + " = ?",
        new String[] { String.valueOf(contact.getID()) });
db.close();

 SQLiteDatabase db = this.getWritableDatabase();

ContentValues values = new ContentValues();
values.put(KEY_NAME, contact.getName());
values.put(KEY_PH_NO, contact.getPhoneNumber());

return db.update(TABLE_CONTACTS, values, KEY_ID + " = ?",
          ew String[] { String.valueOf(contact.getID()) });
 

SQLiteDatabase db = this.getReadableDatabase();

Cursor cursor = db.query(TABLE_CONTACTS, new String[] { KEY_ID,
                KEY_NAME, KEY_PH_NO }, KEY_ID + "=?",
                new String[] { String.valueOf(id) }, null, null, null, null);
if (cursor != null)
cursor.moveToFirst();

Contact contact = new Contact(Integer.parseInt(cursor.getString(0)),
                cursor.getString(1), cursor.getString(2));

return contact;

package com.example.sqlitetest;

import java.util.ArrayList;
import java.util.List;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

public class DatabaseHelper extends SQLiteOpenHelper {

    // 數(shù)據(jù)庫版本
    private static final int DATABASE_VERSION = 1;

    // 數(shù)據(jù)庫名
    private static final String DATABASE_NAME = "contactsManager";

    //Contact表名
    private static final String TABLE_CONTACTS = "contacts";

    //Contact表的列名
    private static final String KEY_ID = "id";
    private static final String KEY_NAME = "name";
    private static final String KEY_PH_NO = "phone_number";

    public DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    // 創(chuàng)建表
    @Override
    public void onCreate(SQLiteDatabase db) {
        String CREATE_CONTACTS_TABLE = "CREATE TABLE " + TABLE_CONTACTS + "("
                + KEY_ID + " INTEGER PRIMARY KEY," + KEY_NAME + " TEXT,"
                + KEY_PH_NO + " TEXT" + ")";
        db.execSQL(CREATE_CONTACTS_TABLE);
    }

    // 更新表
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // 刪除舊表
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_CONTACTS);

        //再次創(chuàng)建表
        onCreate(db);
    }

    /**
     *增刪改查操作
     */

    // 增加新的聯(lián)系人
    void addContact(Contact contact) {
        SQLiteDatabase db = this.getWritableDatabase();

        ContentValues values = new ContentValues();
        values.put(KEY_NAME, contact.getName());
        values.put(KEY_PH_NO, contact.getPhoneNumber());

        // 插入行
        db.insert(TABLE_CONTACTS, null, values);
        db.close(); // 關(guān)閉數(shù)據(jù)庫的連接
    }

    // 獲取聯(lián)系人
    Contact getContact(int id) {
        SQLiteDatabase db = this.getReadableDatabase();

        Cursor cursor = db.query(TABLE_CONTACTS, new String[] { KEY_ID,
                KEY_NAME, KEY_PH_NO }, KEY_ID + "=?",
                new String[] { String.valueOf(id) }, null, null, null, null);
        if (cursor != null)
            cursor.moveToFirst();

        Contact contact = new Contact(Integer.parseInt(cursor.getString(0)),
                cursor.getString(1), cursor.getString(2));
        return contact;
    }
   
    // 獲取所有聯(lián)系人
    public List<Contact> getAllContacts() {
        List<Contact> contactList = new ArrayList<Contact>();
        // Select All Query
        String selectQuery = "SELECT  * FROM " + TABLE_CONTACTS;

        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        if (cursor.moveToFirst()) {
            do {
                Contact contact = new Contact();
                contact.setID(Integer.parseInt(cursor.getString(0)));
                contact.setName(cursor.getString(1));
                contact.setPhoneNumber(cursor.getString(2));
                contactList.add(contact);
            } while (cursor.moveToNext());
        }

        return contactList;
    }

    // 更新單個聯(lián)系人
    public int updateContact(Contact contact) {
        SQLiteDatabase db = this.getWritableDatabase();

        ContentValues values = new ContentValues();
        values.put(KEY_NAME, contact.getName());
        values.put(KEY_PH_NO, contact.getPhoneNumber());

        //更新行
        return db.update(TABLE_CONTACTS, values, KEY_ID + " = ?",
                new String[] { String.valueOf(contact.getID()) });
    }

    // 刪除單個聯(lián)系人
    public void deleteContact(Contact contact) {
        SQLiteDatabase db = this.getWritableDatabase();
        db.delete(TABLE_CONTACTS, KEY_ID + " = ?",
                new String[] { String.valueOf(contact.getID()) });
        db.close();
    }

    // 獲取聯(lián)系人數(shù)量
    public int getContactsCount() {
        String countQuery = "SELECT  * FROM " + TABLE_CONTACTS;
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery(countQuery, null);
        cursor.close();

        return cursor.getCount();
    }
}