android studio简易记事本

以下是使用Android Studio实现简易记事本的步骤和代码示例：

1.创建一个新的Android Studio项目，并在布局文件中添加一个EditText和三个Button，分别用于添加、更新和删除笔记。

2.创建一个SQLite数据库来存储笔记。在MainActivity.java文件中，创建一个内部类NoteDatabase，该类继承自SQLiteOpenHelper，并实现onCreate()和onUpgrade()方法。

public class MainActivity extends AppCompatActivity {
    private EditText etNote;
    private Button btnAdd, btnUpdate, btnDelete;
    private NoteDatabase dbHelper;
    private SQLiteDatabase db;
    private Cursor cursor;
    private int selectedId = -1;

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

        etNote = findViewById(R.id.et_note);
        btnAdd = findViewById(R.id.btn_add);
        btnUpdate = findViewById(R.id.btn_update);
        btnDelete = findViewById(R.id.btn_delete);

        dbHelper = new NoteDatabase(this, "Note.db", null, 1);
        db = dbHelper.getWritableDatabase();

        btnAdd.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String note = etNote.getText().toString();
                ContentValues values = new ContentValues();
                values.put("content", note);
                db.insert("Note", null, values);
                refreshList();
            }
        });

        btnUpdate.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String note = etNote.getText().toString();
                ContentValues values = new ContentValues();
                values.put("content", note);
                db.update("Note", values, "id=?", new String[]{String.valueOf(selectedId)});
                refreshList();
            }
        });

        btnDelete.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                db.delete("Note", "id=?", new String[]{String.valueOf(selectedId)});
                refreshList();
            }
        });

        refreshList();
    }

    private void refreshList() {
        cursor = db.query("Note", null, null, null, null, null, null);
        List<String> notes = new ArrayList<>();
        while (cursor.moveToNext()) {
            int id = cursor.getInt(cursor.getColumnIndex("id"));
            String content = cursor.getString(cursor.getColumnIndex("content"));
            notes.add(id + ": " + content);
        }
        ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, notes);
        ListView listView = findViewById(R.id.list_view);
        listView.setAdapter(adapter);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                cursor.moveToPosition(position);
                selectedId = cursor.getInt(cursor.getColumnIndex("id"));                String content = cursor.getString(cursor.getColumnIndex("content"));
                etNote.setText(content);
            }
        });
    }

    class NoteDatabase extends SQLiteOpenHelper {
        public static final String CREATE_NOTE = "create table Note ("
                + "id integer primary key autoincrement, "
                + "content text)";

        public NoteDatabase(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
            super(context, name, factory, version);
        }

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

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        }
    }
}

3.在AndroidManifest.xml文件中添加以下权限：

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

4.运行应用程序并测试它是否可以添加、更新和删除笔记。