android randomaccess 在應用中如何運用

RandomAccessFile 是 Android 中提供的一個類，它允許你在文件的任意位置讀取或寫入數據。這個類在處理大文件時非常有用，因為它不需要一次性將整個文件加載到內存中。你可以在應用中使用 RandomAccessFile 來實現以下功能：

1. 讀取文件的一部分：

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;

public class MainActivity extends AppCompatActivity {

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

        File file = new File("path/to/your/file.txt");
        try {
            RandomAccessFile randomAccessFile = new RandomAccessFile(file, "r");
            long fileSize = randomAccessFile.length();
            int bufferSize = 1024;
            byte[] buffer = new byte[bufferSize];
            int bytesRead;

            // 從文件的指定位置開始讀取
            long position = 50;
            randomAccessFile.seek(position);

            while ((bytesRead = randomAccessFile.read(buffer)) != -1) {
                // 處理讀取到的數據
                String data = new String(buffer, 0, bytesRead);
                System.out.println(data);
            }

            randomAccessFile.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

2. 寫入文件的一部分：

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;

public class MainActivity extends AppCompatActivity {

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

        File file = new File("path/to/your/file.txt");
        try {
            RandomAccessFile randomAccessFile = new RandomAccessFile(file, "rw");
            long fileSize = randomAccessFile.length();
            int bufferSize = 1024;
            byte[] buffer = new byte[bufferSize];
            int bytesRead;

            // 從文件的指定位置開始寫入
            long position = 50;
            randomAccessFile.seek(position);

            String dataToWrite = "This is a new line of text.";
            byte[] dataBytes = dataToWrite.getBytes();

            randomAccessFile.write(dataBytes);
            randomAccessFile.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

3. 更新文件的一部分：

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;

public class MainActivity extends AppCompatActivity {

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

        File file = new File("path/to/your/file.txt");
        try {
            RandomAccessFile randomAccessFile = new RandomAccessFile(file, "rw");
            long fileSize = randomAccessFile.length();
            int bufferSize = 1024;
            byte[] buffer = new byte[bufferSize];
            int bytesRead;

            // 從文件的指定位置開始更新
            long position = 50;
            randomAccessFile.seek(position);

            String dataToWrite = "This is an updated line of text.";
            byte[] dataBytes = dataToWrite.getBytes();

            // 將原數據讀取到緩沖區
            randomAccessFile.read(buffer);

            // 將緩沖區的數據寫入到指定位置
            System.arraycopy(dataBytes, 0, buffer, 0, dataBytes.length);
            randomAccessFile.write(buffer, 0, dataBytes.length);

            randomAccessFile.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

4. 刪除文件的一部分：

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;

public class MainActivity extends AppCompatActivity {

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

        File file = new File("path/to/your/file.txt");
        try {
            RandomAccessFile randomAccessFile = new RandomAccessFile(file, "rw");
            long fileSize = randomAccessFile.length();
            int bufferSize = 1024;
            byte[] buffer = new byte[bufferSize];
            int bytesRead;

            // 從文件的指定位置開始刪除
            long position = 50;
            randomAccessFile.seek(position);

            // 將后面的數據寫入到當前位置，從而實現刪除操作
            String remainingData = readRemainingData(randomAccessFile, position + 10);
            randomAccessFile.write(remainingData.getBytes());

            randomAccessFile.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private String readRemainingData(RandomAccessFile randomAccessFile, long position) throws IOException {
        StringBuilder remainingData = new StringBuilder();
        long fileSize = randomAccessFile.length();
        int bufferSize = 1024;
        byte[] buffer = new byte[bufferSize];
        int bytesRead;

        randomAccessFile.seek(position);

        while (position < fileSize) {
            bytesRead = randomAccessFile.read(buffer);
            if (bytesRead == -1) {
                break;
            }
            remainingData.append(new String(buffer, 0, bytesRead));
            position += bytesRead;
        }

        return remainingData.toString();
    }
}

這些示例展示了如何在 Android 應用中使用 RandomAccessFile 來讀取、寫入、更新和刪除文件的一部分。你可以根據自己的需求調整代碼以滿足你的應用場景。