Android studio设计备忘录，要求实现增删改查功能的代码

以下是一个简单的备忘录实现增删改查功能的代码，使用的是SQLite数据库：

public class MemoDatabaseHelper extends SQLiteOpenHelper {

    private static final String DATABASE_NAME = "memo.db";
    private static final int DATABASE_VERSION = 1;

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

    @Override
    public void onCreate(SQLiteDatabase db) {
        String CREATE_MEMO_TABLE = "CREATE TABLE " + MemoContract.MemoEntry.TABLE_NAME + "("
                + MemoContract.MemoEntry._ID + " INTEGER PRIMARY KEY,"
                + MemoContract.MemoEntry.COLUMN_TITLE + " TEXT,"
                + MemoContract.MemoEntry.COLUMN_CONTENT + " TEXT,"
                + MemoContract.MemoEntry.COLUMN_TIMESTAMP + " DATETIME DEFAULT CURRENT_TIMESTAMP"
                + ")";
        db.execSQL(CREATE_MEMO_TABLE);
    }

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

    public long insertMemo(String title, String content) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(MemoContract.MemoEntry.COLUMN_TITLE, title);
        values.put(MemoContract.MemoEntry.COLUMN_CONTENT, content);
        long id = db.insert(MemoContract.MemoEntry.TABLE_NAME, null, values);
        db.close();
        return id;
    }

    public List<Memo> getAllMemos() {
        List<Memo> memos = new ArrayList<>();
        String selectQuery = "SELECT * FROM " + MemoContract.MemoEntry.TABLE_NAME + " ORDER BY " +
                MemoContract.MemoEntry.COLUMN_TIMESTAMP + " DESC";
        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);
        if (cursor.moveToFirst()) {
            do {
                Memo memo = new Memo();
                memo.setId(cursor.getInt(cursor.getColumnIndex(MemoContract.MemoEntry._ID)));
                memo.setTitle(cursor.getString(cursor.getColumnIndex(MemoContract.MemoEntry.COLUMN_TITLE)));
                memo.setContent(cursor.getString(cursor.getColumnIndex(MemoContract.MemoEntry.COLUMN_CONTENT)));
                memo.setTimestamp(cursor.getString(cursor.getColumnIndex(MemoContract.MemoEntry.COLUMN_TIMESTAMP)));
                memos.add(memo);
            } while (cursor.moveToNext());
        }
        db.close();
        return memos;
    }

    public Memo getMemoById(long id) {
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.query(MemoContract.MemoEntry.TABLE_NAME,
                new String[]{MemoContract.MemoEntry._ID, MemoContract.MemoEntry.COLUMN_TITLE,
                        MemoContract.MemoEntry.COLUMN_CONTENT, MemoContract.MemoEntry.COLUMN_TIMESTAMP},
                MemoContract.MemoEntry._ID + "=?",
                new String[]{String.valueOf(id)}, null, null, null, null);
        if (cursor != null)
            cursor.moveToFirst();
        Memo memo = new Memo(
                cursor.getInt(cursor.getColumnIndex(MemoContract.MemoEntry._ID)),
                cursor.getString(cursor.getColumnIndex(MemoContract.MemoEntry.COLUMN_TITLE)),
                cursor.getString(cursor.getColumnIndex(MemoContract.MemoEntry.COLUMN_CONTENT)),
                cursor.getString(cursor.getColumnIndex(MemoContract.MemoEntry.COLUMN_TIMESTAMP))
        );
        db.close();
        return memo;
    }

    public int updateMemo(Memo memo) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(MemoContract.MemoEntry.COLUMN_TITLE, memo.getTitle());
        values.put(MemoContract.MemoEntry.COLUMN_CONTENT, memo.getContent());
        return db.update(MemoContract.MemoEntry.TABLE_NAME, values, MemoContract.MemoEntry._ID + " = ?",
                new String[]{String.valueOf(memo.getId())});
    }

    public void deleteMemo(long id) {
        SQLiteDatabase db = this.getWritableDatabase();
        db.delete(MemoContract.MemoEntry.TABLE_NAME, MemoContract.MemoEntry._ID + " = ?",
                new String[]{String.valueOf(id)});
        db.close();
    }

}

Memo.java:

public class Memo {

    private int id;
    private String title;
    private String content;
    private String timestamp;

    public Memo() {
    }

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

    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;
    }

    public String getTimestamp() {
        return timestamp;
    }

    public void setTimestamp(String timestamp) {
        this.timestamp = timestamp;
    }
}

MemoContract.java:

public final class MemoContract {

    private MemoContract() {}

    public static class MemoEntry implements BaseColumns {
        public static final String TABLE_NAME = "memo";
        public static final String COLUMN_TITLE = "title";
        public static final String COLUMN_CONTENT = "content";
        public static final String COLUMN_TIMESTAMP = "timestamp";
    }

}

然后在你的Activity或Fragment中，你可以如下使用：

public class MemoActivity extends AppCompatActivity {

    private MemoDatabaseHelper dbHelper;

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

        dbHelper = new MemoDatabaseHelper(this);

        // 插入备忘录
        long memoId = dbHelper.insertMemo("Title", "Content");

        // 获取所有备忘录
        List<Memo> memos = dbHelper.getAllMemos();

        // 获取单个备忘录
        Memo memo = dbHelper.getMemoById(memoId);

        // 更新备忘录
        memo.setTitle("New Title");
        dbHelper.updateMemo(memo);

        // 删除备忘录
        dbHelper.deleteMemo(memoId);
    }

}

当然，你还需要在你的布局文件中添加相应的UI元素，例如EditText、ListView等等，以显示备忘录的内容。