public string this[string columnName] { get { string result = ""; // 检查是否通过验证 switch (columnName) { case "Min": result = GetResult(Min); break; case "Average": result = GetResult(Average); break; case "Max": result = GetResult(Max); break; } return result; } }

这段代码是一个索引器（indexer），它允许你通过列名来访问类中的某个属性。在这个代码中，索引器接受一个字符串类型的参数 columnName，然后根据不同的 columnName 返回不同的结果。具体来说，当 columnName 为 "Min" 时，返回 Min 属性的验证结果；当 columnName 为 "Average" 时，返回 Average 属性的验证结果；当 columnName 为 "Max" 时，返回 Max 属性的验证结果。通过这种方式，可以方便地对类中的多个属性进行验证，并且可以通过统一的方式来访问它们。

相关问题

Android 打开/storage/emulated/0/DCIM/Camera/ 从中选择一张照片返回

以下是Android打开相册并选择照片的代码示例：

// 在Activity中调用以下方法
private static final int REQUEST_CODE = 1;
private static final int PERMISSION_REQUEST_CODE = 2;

// 检查是否有读取存储权限
private boolean checkPermission() {
    int result = ContextCompat.checkSelfPermission(MainActivity.this, Manifest.permission.READ_EXTERNAL_STORAGE);
    return result == PackageManager.PERMISSION_GRANTED;}

// 请求读取存储权限
private void requestPermission() {
    ActivityCompat.requestPermissions(MainActivity.this, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, PERMISSION_REQUEST_CODE);
}

// 处理权限请求结果
@Override
public void onRequestPermissionsResult(int requestCode, String permissions[], int[] grantResults) {
    switch (requestCode) {
        case PERMISSION_REQUEST_CODE:
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                Toast.makeText(MainActivity.this, "Permission granted successfully!", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(MainActivity.this, "Permission denied, please grant the permission to access the gallery.", Toast.LENGTH_SHORT).show();
            }
            break;
    }
}

// 打开相册
private void openGallery() {
    Intent intent = new Intent(Intent.ACTION_PICK, android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);
    startActivityForResult(intent, REQUEST_CODE);
}

// 处理选择照片的结果
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (requestCode == REQUEST_CODE && resultCode == RESULT_OK && data != null) {
        Uri selectedImage = data.getData();
        String[] filePathColumn = {MediaStore.Images.Media.DATA};
        Cursor cursor = getContentResolver().query(selectedImage, filePathColumn, null, null, null);
        cursor.moveToFirst();
        int columnIndex = cursor.getColumnIndex(filePathColumn[0]);
        String picturePath = cursor.getString(columnIndex);
        cursor.close();
        // 在这里可以使用picturePath来处理选择的照片
    }
}

请注意，为了访问存储，您需要在AndroidManifest.xml文件中添加以下权限：

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

如果您的应用程序运行在Android 10或更高版本上，则还需要在AndroidManifest.xml文件中添加以下代码：

<application
    android:requestLegacyExternalStorage="true"
    ...>
    ...
</application>

三、简答题 1、简述如何使用 SharedPreferences 读写数据。 2、简述 Handler 消息机制的工作原理。 3、简述使用HttpURConnection访问网络的步骤。 四、编程题 1、现有两个Activity，分别为UserinfoActivity和ShowActivity，现在要从UserinfoActivity中通过一个“查看”按钮切换界面到ShowActivity，并传递如下三个参数 id=1:username="eric”;state=True，且ShowActivity使用 Toast 显示接收到的参数，请写出两个Activity中的核心代码 2、在网络接口 http://192.168.1.100:8080/news/getnewslist中提供了如下所示的JSON数组格式的返回数据:[f"id":1001,"title”:“"程序员因写代码太乱被杀害"，"detail”:"凶手是死者同事，维护死者代码时完全看不懂而痛下杀手":"comment" 16359,"image”:"http://192.168.1.100:8080/news/images/6.jpg) 请编写程序，访问该接口获取数据，解析后将数据显示到界面的控件上，同时将数据保存到本地的SQLite 数据库中。

三、简答题
1、使用SharedPreferences读写数据的步骤：
① 获取SharedPreferences对象：SharedPreferences sharedPreferences = getSharedPreferences("name", Context.MODE_PRIVATE);
② 获取SharedPreferences.Editor对象：SharedPreferences.Editor editor = sharedPreferences.edit();
③ 写入数据：editor.putString("key", value);
④ 提交数据：editor.commit();
⑤ 读取数据：sharedPreferences.getString("key", defaultValue);

2、Handler消息机制的工作原理：
Handler是Android中消息机制的基础，它可以实现不同线程之间的通信。Handler包括两个主要部分：消息队列和消息处理器。消息队列是一个存储消息的FIFO队列，消息处理器则是用来处理消息的对象。当Handler创建时，它会自动与当前线程的消息队列关联起来。当调用Handler的sendMessage()方法，会向消息队列中添加一条消息。当Handler处理消息时，会从消息队列中取出一条消息，交给消息处理器处理，如果消息处理器处理完消息后需要更新UI，可以使用Handler的post()或者postDelayed()方法将消息发送到UI线程中执行。

3、使用HttpURLConnection访问网络的步骤：
① 创建URL对象：URL url = new URL("http://www.example.com");
② 打开URLConnection连接：HttpURLConnection connection = (HttpURLConnection) url.openConnection();
③ 设置请求方式：connection.setRequestMethod("GET");
④ 设置连接超时时间：connection.setConnectTimeout(5000);
⑤ 设置读取超时时间：connection.setReadTimeout(5000);
⑥ 发送请求：connection.connect();
⑦ 获取响应码：int responseCode = connection.getResponseCode();
⑧ 获取输入流：InputStream inputStream = connection.getInputStream();
⑨ 读取数据：BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
StringBuilder stringBuilder = new StringBuilder();
String line;
while ((line = reader.readLine()) != null) {
stringBuilder.append(line);
}
String result = stringBuilder.toString();
⑩ 关闭连接：connection.disconnect();

四、编程题
1、UserinfoActivity中的代码：

public void onClick(View view) {
    Intent intent = new Intent(UserinfoActivity.this, ShowActivity.class);
    intent.putExtra("id", 1);
    intent.putExtra("username", "eric");
    intent.putExtra("state", true);
    startActivity(intent);
}

ShowActivity中的代码：

Intent intent = getIntent();
int id = intent.getIntExtra("id", 0);
String username = intent.getStringExtra("username");
boolean state = intent.getBooleanExtra("state", false);
Toast.makeText(ShowActivity.this, "id=" + id + " username=" + username + " state=" + state, Toast.LENGTH_SHORT).show();

2、代码如下：

public void getData() {
    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                URL url = new URL("http://192.168.1.100:8080/news/getnewslist");
                HttpURLConnection connection = (HttpURLConnection) url.openConnection();
                connection.setRequestMethod("GET");
                connection.setConnectTimeout(5000);
                connection.setReadTimeout(5000);
                connection.connect();
                int responseCode = connection.getResponseCode();
                if (responseCode == HttpURLConnection.HTTP_OK) {
                    InputStream inputStream = connection.getInputStream();
                    BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
                    StringBuilder stringBuilder = new StringBuilder();
                    String line;
                    while ((line = reader.readLine()) != null) {
                        stringBuilder.append(line);
                    }
                    String result = stringBuilder.toString();
                    Gson gson = new Gson();
                    List<News> newsList = gson.fromJson(result, new TypeToken<List<News>>(){}.getType());
                    for (News news : newsList) {
                        // 将数据保存到SQLite数据库中
                        dbHelper.insertNews(news.getId(), news.getTitle(), news.getDetail(), news.getComment(), news.getImage());
                    }
                    // 更新UI
                    Message message = Message.obtain();
                    message.what = UPDATE_UI;
                    message.obj = newsList;
                    handler.sendMessage(message);
                    reader.close();
                    inputStream.close();
                }
                connection.disconnect();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }).start();
}

private void updateUI(List<News> newsList) {
    // 将数据显示到界面的控件上
}

// News类
public class News {
    private int id;
    private String title;
    private String detail;
    private int comment;
    private String image;

    public News(int id, String title, String detail, int comment, String image) {
        this.id = id;
        this.title = title;
        this.detail = detail;
        this.comment = comment;
        this.image = image;
    }

    public int getId() {
        return id;
    }

    public String getTitle() {
        return title;
    }

    public String getDetail() {
        return detail;
    }

    public int getComment() {
        return comment;
    }

    public String getImage() {
        return image;
    }
}

// DBHelper类
public class DBHelper extends SQLiteOpenHelper {

    private static final String DB_NAME = "news.db";
    private static final int DB_VERSION = 1;
    private static final String TABLE_NAME = "news";
    private static final String COLUMN_ID = "id";
    private static final String COLUMN_TITLE = "title";
    private static final String COLUMN_DETAIL = "detail";
    private static final String COLUMN_COMMENT = "comment";
    private static final String COLUMN_IMAGE = "image";
    private static final String CREATE_TABLE = "CREATE TABLE " + TABLE_NAME + " (" +
            COLUMN_ID + " INTEGER PRIMARY KEY, " +
            COLUMN_TITLE + " TEXT, " +
            COLUMN_DETAIL + " TEXT, " +
            COLUMN_COMMENT + " INTEGER, " +
            COLUMN_IMAGE + " TEXT)";

    public DBHelper(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
    }

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

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

    public void insertNews(int id, String title, String detail, int comment, String image) {
        SQLiteDatabase db = getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(COLUMN_ID, id);
        values.put(COLUMN_TITLE, title);
        values.put(COLUMN_DETAIL, detail);
        values.put(COLUMN_COMMENT, comment);
        values.put(COLUMN_IMAGE, image);
        db.insert(TABLE_NAME, null, values);
        db.close();
    }

    public List<News> getNewsList() {
        List<News> newsList = new ArrayList<>();
        SQLiteDatabase db = getReadableDatabase();
        Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_NAME, null);
        if (cursor.moveToFirst()) {
            do {
                int id = cursor.getInt(cursor.getColumnIndex(COLUMN_ID));
                String title = cursor.getString(cursor.getColumnIndex(COLUMN_TITLE));
                String detail = cursor.getString(cursor.getColumnIndex(COLUMN_DETAIL));
                int comment = cursor.getInt(cursor.getColumnIndex(COLUMN_COMMENT));
                String image = cursor.getString(cursor.getColumnIndex(COLUMN_IMAGE));
                News news = new News(id, title, detail, comment, image);
                newsList.add(news);
            } while (cursor.moveToNext());
        }
        cursor.close();
        db.close();
        return newsList;
    }
}

// Handler类
private static final int UPDATE_UI = 1;
private Handler handler = new Handler(new Handler.Callback() {
    @Override
    public boolean handleMessage(@NonNull Message message) {
        switch (message.what) {
            case UPDATE_UI:
                List<News> newsList = (List<News>) message.obj;
                updateUI(newsList);
                break;
        }
        return false;
    }
});