getFilesDir() 方法用于獲取應用內部存儲的目錄，這個目錄下的文件默認是加密存儲的

如果你需要加密存儲文件，可以使用 Cipher 類來實現加密和解密操作。以下是一個簡單的示例：

import android.util.Base64;
import java.security.SecureRandom;
import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class EncryptionUtil {
    private static final String ALGORITHM = "AES";
    private static final String TRANSFORMATION = "AES/CBC/PKCS5Padding";
    private static final String CHARSET = "UTF-8";

    public static String encrypt(String data, String key) throws Exception {
        SecretKeySpec secretKeySpec = new SecretKeySpec(key.getBytes(CHARSET), ALGORITHM);
        IvParameterSpec ivParameterSpec = new IvParameterSpec(new byte[16]);
        Cipher cipher = Cipher.getInstance(TRANSFORMATION);
        cipher.init(Cipher.ENCRYPT_MODE, secretKeySpec, ivParameterSpec);
        byte[] encryptedData = cipher.doFinal(data.getBytes(CHARSET));
        return Base64.encodeToString(encryptedData, Base64.DEFAULT);
    }

    public static String decrypt(String data, String key) throws Exception {
        SecretKeySpec secretKeySpec = new SecretKeySpec(key.getBytes(CHARSET), ALGORITHM);
        IvParameterSpec ivParameterSpec = new IvParameterSpec(new byte[16]);
        Cipher cipher = Cipher.getInstance(TRANSFORMATION);
        cipher.init(Cipher.DECRYPT_MODE, secretKeySpec, ivParameterSpec);
        byte[] decodedData = Base64.decode(data, Base64.DEFAULT);
        return new String(cipher.doFinal(decodedData), CHARSET);
    }
}

使用這個工具類，你可以在 getFilesDir() 目錄下加密和解密文件。但請注意，你需要確保密鑰（key）的安全性，因為它是解密文件所必需的。