android编写笔记app的代码

这个问题比较广泛，需要分步骤讲解。以下是一些可能的步骤和代码示例：

1. 创建一个笔记类（Note）：

public class Note {
    private int id;
    private String title;
    private String content;
    private Date createdDate;
    private Date modifiedDate;

    public Note() {
        // 构造函数
    }

    // getter 和 setter 方法

    // 重写 toString 方法，方便调试
    @Override
    public String toString() {
        return "Note{" +
                "id=" + id +
                ", title='" + title + '\'' +
                ", content='" + content + '\'' +
                ", createdDate=" + createdDate +
                ", modifiedDate=" + modifiedDate +
                '}';
    }
}

2. 创建一个数据库帮助类（DbHelper）：

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

    private static final String TABLE_NAME = "notes";
    private static final String COLUMN_ID = "_id";
    private static final String COLUMN_TITLE = "title";
    private static final String COLUMN_CONTENT = "content";
    private static final String COLUMN_CREATED_DATE = "created_date";
    private static final String COLUMN_MODIFIED_DATE = "modified_date";

    // SQL 语句
    private static final String SQL_CREATE_ENTRIES =
            "CREATE TABLE " + TABLE_NAME + " (" +
                    COLUMN_ID + " INTEGER PRIMARY KEY," +
                    COLUMN_TITLE + " TEXT," +
                    COLUMN_CONTENT + " TEXT," +
                    COLUMN_CREATED_DATE + " INTEGER," +
                    COLUMN_MODIFIED_DATE + " INTEGER)";

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

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(SQL_CREATE_ENTRIES);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // 数据库升级
    }
}

3. 创建一个数据访问层（NoteDao）：

public class NoteDao {
    private SQLiteDatabase db;
    private DbHelper dbHelper;

    public NoteDao(Context context) {
        dbHelper = new DbHelper(context);
        db = dbHelper.getWritableDatabase();
    }

    public long add(Note note) {
        ContentValues values = new ContentValues();
        values.put(DbHelper.COLUMN_TITLE, note.getTitle());
        values.put(DbHelper.COLUMN_CONTENT, note.getContent());
        values.put(DbHelper.COLUMN_CREATED_DATE, note.getCreatedDate().getTime());
        values.put(DbHelper.COLUMN_MODIFIED_DATE, note.getModifiedDate().getTime());
        return db.insert(DbHelper.TABLE_NAME, null, values);
    }

    public int update(Note note) {
        ContentValues values = new ContentValues();
        values.put(DbHelper.COLUMN_TITLE, note.getTitle());
        values.put(DbHelper.COLUMN_CONTENT, note.getContent());
        values.put(DbHelper.COLUMN_MODIFIED_DATE, note.getModifiedDate().getTime());
        String whereClause = DbHelper.COLUMN_ID + "=?";
        String[] whereArgs = { String.valueOf(note.getId()) };
        return db.update(DbHelper.TABLE_NAME, values, whereClause, whereArgs);
    }

    public int delete(Note note) {
        String whereClause = DbHelper.COLUMN_ID + "=?";
        String[] whereArgs = { String.valueOf(note.getId()) };
        return db.delete(DbHelper.TABLE_NAME, whereClause, whereArgs);
    }

    public List<Note> getAll() {
        List<Note> notes = new ArrayList<>();
        Cursor cursor = db.query(DbHelper.TABLE_NAME, null, null, null, null, null,
                DbHelper.COLUMN_MODIFIED_DATE + " DESC");
        while (cursor.moveToNext()) {
            Note note = new Note();
            note.setId(cursor.getInt(cursor.getColumnIndex(DbHelper.COLUMN_ID)));
            note.setTitle(cursor.getString(cursor.getColumnIndex(DbHelper.COLUMN_TITLE)));
            note.setContent(cursor.getString(cursor.getColumnIndex(DbHelper.COLUMN_CONTENT)));
            note.setCreatedDate(new Date(cursor.getLong(cursor.getColumnIndex(DbHelper.COLUMN_CREATED_DATE))));
            note.setModifiedDate(new Date(cursor.getLong(cursor.getColumnIndex(DbHelper.COLUMN_MODIFIED_DATE))));
            notes.add(note);
        }
        cursor.close();
        return notes;
    }
}

4. 创建一个笔记列表界面（NoteListActivity）：

public class NoteListActivity extends AppCompatActivity {
    private static final int REQUEST_CODE_ADD = 1;
    private static final int REQUEST_CODE_EDIT = 2;

    private ListView listView;
    private List<Note> notes;
    private NoteDao noteDao;
    private ArrayAdapter<Note> adapter;

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

        noteDao = new NoteDao(this);

        listView = findViewById(R.id.list_view);
        notes = noteDao.getAll();
        adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, notes);
        listView.setAdapter(adapter);

        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Note note = notes.get(position);
                Intent intent = new Intent(NoteListActivity.this, NoteEditActivity.class);
                intent.putExtra("note", note);
                startActivityForResult(intent, REQUEST_CODE_EDIT);
            }
        });
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (resultCode == RESULT_OK) {
            switch (requestCode) {
                case REQUEST_CODE_ADD:
                    Note note = (Note) data.getSerializableExtra("note");
                    long id = noteDao.add(note);
                    note.setId((int) id);
                    notes.add(0, note);
                    adapter.notifyDataSetChanged();
                    break;
                case REQUEST_CODE_EDIT:
                    note = (Note) data.getSerializableExtra("note");
                    noteDao.update(note);
                    adapter.notifyDataSetChanged();
                    break;
            }
        }
    }

    public void onAddButtonClick(View view) {
        Intent intent = new Intent(this, NoteEditActivity.class);
        startActivityForResult(intent, REQUEST_CODE_ADD);
    }
}

5. 创建一个笔记编辑界面（NoteEditActivity）：

public class NoteEditActivity extends AppCompatActivity {
    private EditText titleEditText;
    private EditText contentEditText;
    private Note note;

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

        titleEditText = findViewById(R.id.title_edit_text);
        contentEditText = findViewById(R.id.content_edit_text);

        note = (Note) getIntent().getSerializableExtra("note");
        if (note == null) {
            note = new Note();
        } else {
            titleEditText.setText(note.getTitle());
            contentEditText.setText(note.getContent());
        }
    }

    public void onSaveButtonClick(View view) {
        note.setTitle(titleEditText.getText().toString());
        note.setContent(contentEditText.getText().toString());
        note.setModifiedDate(new Date());
        Intent intent = new Intent();
        intent.putExtra("note", note);
        setResult(RESULT_OK, intent);
        finish();
    }
}

这些代码只是一个简单的示例，实际开发中需要根据具体需求进行修改和扩展。