用androidstudio写一个记事本至少3个Acitvity以上；必须使用到sqlite数据库；

好的，以下是一个简单的记事本应用，包含三个Activity，其中使用了SQLite数据库存储数据：

1. MainActivity：显示所有已创建的笔记列表，可点击进入编辑笔记页面或新建笔记页面。
2. EditNoteActivity：编辑已有笔记或新建笔记页面，可输入笔记标题和内容，并保存至数据库。
3. ViewNoteActivity：查看已有笔记页面，显示笔记标题和内容。

首先，在Android Studio中创建一个新项目，并添加以下依赖项：

implementation 'androidx.appcompat:appcompat:1.3.1'
implementation 'androidx.recyclerview:recyclerview:1.2.1'
implementation 'androidx.cardview:cardview:1.0.0'
implementation 'androidx.constraintlayout:constraintlayout:2.1.0'
implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'
implementation 'androidx.room:room-runtime:2.3.0'
annotationProcessor 'androidx.room:room-compiler:2.3.0'

然后，在`AndroidManifest.xml`文件中添加以下权限：

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

接下来，创建一个SQLiteOpenHelper类和一个Note实体类，用于在数据库中保存笔记数据：

public class NoteOpenHelper extends SQLiteOpenHelper {
    private static final String DATABASE_NAME = "notes.db";
    private static final int DATABASE_VERSION = 1;

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

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE notes (_id INTEGER PRIMARY KEY AUTOINCREMENT, title TEXT, content TEXT)");
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS notes");
        onCreate(db);
    }
}

@Entity(tableName = "notes")
public class Note {
    @PrimaryKey(autoGenerate = true)
    private int id;
    private String title;
    private String content;

    public Note() {
    }

    public Note(int id, String title, String content) {
        this.id = id;
        this.title = title;
        this.content = content;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }
}

接下来，创建一个NoteDao接口和一个NoteDatabase类，用于访问数据库：

@Dao
public interface NoteDao {
    @Query("SELECT * FROM notes ORDER BY _id DESC")
    LiveData<List<Note>> getAllNotes();

    @Query("SELECT * FROM notes WHERE _id = :noteId")
    LiveData<Note> getNoteById(int noteId);

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    void insert(Note note);

    @Update
    void update(Note note);

    @Delete
    void delete(Note note);
}

@Database(entities = {Note.class}, version = 1)
public abstract class NoteDatabase extends RoomDatabase {
    private static NoteDatabase instance;

    public abstract NoteDao noteDao();

    public static synchronized NoteDatabase getInstance(Context context) {
        if (instance == null) {
            instance = Room.databaseBuilder(context.getApplicationContext(),
                    NoteDatabase.class, "note_database")
                    .fallbackToDestructiveMigration()
                    .build();
        }
        return instance;
    }
}

接下来，创建MainActivity类，用于显示笔记列表和跳转到其他页面：

public class MainActivity extends AppCompatActivity {
    private RecyclerView mRecyclerView;
    private NoteAdapter mAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mRecyclerView = findViewById(R.id.recycler_view);
        mAdapter = new NoteAdapter(this, new ArrayList<>());
        mRecyclerView.setAdapter(mAdapter);
        mRecyclerView.setLayoutManager(new LinearLayoutManager(this));

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(MainActivity.this, EditNoteActivity.class);
                startActivity(intent);
            }
        });

        NoteDatabase.getInstance(this).noteDao().getAllNotes().observe(this, new Observer<List<Note>>() {
            @Override
            public void onChanged(List<Note> notes) {
                mAdapter.setNotes(notes);
            }
        });
    }

    public void editNoteClicked(Note note) {
        Intent intent = new Intent(MainActivity.this, EditNoteActivity.class);
        intent.putExtra(EditNoteActivity.EXTRA_NOTE_ID, note.getId());
        startActivity(intent);
    }

    public void viewNoteClicked(Note note) {
        Intent intent = new Intent(MainActivity.this, ViewNoteActivity.class);
        intent.putExtra(ViewNoteActivity.EXTRA_NOTE_ID, note.getId());
        startActivity(intent);
    }
}

接下来，创建EditNoteActivity类，用于编辑或新建笔记：

public class EditNoteActivity extends AppCompatActivity {
    public static final String EXTRA_NOTE_ID = "com.example.notepad.EXTRA_NOTE_ID";

    private EditText mTitleEditText;
    private EditText mContentEditText;

    private int mNoteId = -1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_edit_note);

        mTitleEditText = findViewById(R.id.title_edit_text);
        mContentEditText = findViewById(R.id.content_edit_text);

        Intent intent = getIntent();
        if (intent != null && intent.hasExtra(EXTRA_NOTE_ID)) {
            mNoteId = intent.getIntExtra(EXTRA_NOTE_ID, -1);
            NoteDatabase.getInstance(this).noteDao().getNoteById(mNoteId).observe(this, new Observer<Note>() {
                @Override
                public void onChanged(Note note) {
                    if (note != null) {
                        mTitleEditText.setText(note.getTitle());
                        mContentEditText.setText(note.getContent());
                    }
                }
            });
        }

        Button saveButton = findViewById(R.id.save_button);
        saveButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String title = mTitleEditText.getText().toString().trim();
                String content = mContentEditText.getText().toString().trim();
                if (TextUtils.isEmpty(title) || TextUtils.isEmpty(content)) {
                    Toast.makeText(EditNoteActivity.this, R.string.error_empty_fields, Toast.LENGTH_SHORT).show();
                } else {
                    if (mNoteId == -1) {
                        Note note = new Note();
                        note.setTitle(title);
                        note.setContent(content);
                        NoteDatabase.getInstance(EditNoteActivity.this).noteDao().insert(note);
                    } else {
                        NoteDatabase.getInstance(EditNoteActivity.this).noteDao().getNoteById(mNoteId).observe(EditNoteActivity.this, new Observer<Note>() {
                            @Override
                            public void onChanged(Note note) {
                                if (note != null) {
                                    note.setTitle(title);
                                    note.setContent(content);
                                    NoteDatabase.getInstance(EditNoteActivity.this).noteDao().update(note);
                                }
                            }
                        });
                    }
                    finish();
                }
            }
        });
    }
}

最后，创建ViewNoteActivity类，用于查看已有笔记：

public class ViewNoteActivity extends AppCompatActivity {
    public static final String EXTRA_NOTE_ID = "com.example.notepad.EXTRA_NOTE_ID";

    private TextView mTitleTextView;
    private TextView mContentTextView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_view_note);

        mTitleTextView = findViewById(R.id.title_text_view);
        mContentTextView = findViewById(R.id.content_text_view);

        Intent intent = getIntent();
        if (intent != null && intent.hasExtra(EXTRA_NOTE_ID)) {
            int noteId = intent.getIntExtra(EXTRA_NOTE_ID, -1);
            NoteDatabase.getInstance(this).noteDao().getNoteById(noteId).observe(this, new Observer<Note>() {
                @Override
                public void onChanged(Note note) {
                    if (note != null) {
                        mTitleTextView.setText(note.getTitle());
                        mContentTextView.setText(note.getContent());
                    }
                }
            });
        }
    }
}

这样，一个简单的记事本应用就完成了。当用户新建或编辑笔记时，应用会将笔记数据保存至SQLite数据库中，并在主页面上显示所有已创建的笔记列表。用户可以点击任何一个笔记进入查看笔记页面。