在Java中实现文件加密与解密可以使用各种加密算法和IO流来实现。下面是一个示例代码，使用AES对称加密算法和文件IO流来实现文件的加密和解密：
import javax.crypto.Cipher;import javax.crypto.CipherInputStream;import javax.crypto.CipherOutputStream;import javax.crypto.spec.SecretKeySpec;import java.io.*;public class FileEncryption {public static void encrypt(String key, File inputFile, File outputFile) throws Exception {doCrypto(Cipher.ENCRYPT_MODE, key, inputFile, outputFile);}public static void decrypt(String key, File inputFile, File outputFile) throws Exception {doCrypto(Cipher.DECRYPT_MODE, key, inputFile, outputFile);}private static void doCrypto(int cipherMode, String key, File inputFile, File outputFile) throws Exception {SecretKeySpec secretKey = new SecretKeySpec(key.getBytes(), "AES");Cipher cipher = Cipher.getInstance("AES");cipher.init(cipherMode, secretKey);FileInputStream inputStream = new FileInputStream(inputFile);FileOutputStream outputStream = new FileOutputStream(outputFile);if (cipherMode == Cipher.ENCRYPT_MODE) {CipherOutputStream cipherOutputStream = new CipherOutputStream(outputStream, cipher);writeBytes(inputStream, cipherOutputStream);cipherOutputStream.close();} else if (cipherMode == Cipher.DECRYPT_MODE) {CipherInputStream cipherInputStream = new CipherInputStream(inputStream, cipher);writeBytes(cipherInputStream, outputStream);cipherInputStream.close();}inputStream.close();outputStream.close();}private static void writeBytes(InputStream inputStream, OutputStream outputStream) throws IOException {byte[] buffer = new byte[1024];int bytesRead;while ((bytesRead = inputStream.read(buffer)) != -1) {outputStream.write(buffer, 0, bytesRead);}}public static void main(String[] args) {try {File inputFile = new File("input.txt");File encryptedFile = new File("encrypted.txt");File decryptedFile = new File("decrypted.txt");String key = "ThisIsASecretKey";encrypt(key, inputFile, encryptedFile);System.out.println("File encrypted successfully.");decrypt(key, encryptedFile, decryptedFile);System.out.println("File decrypted successfully.");} catch (Exception e) {e.printStackTrace();}}}在上述代码中，我们定义了两个方法encrypt和decrypt，分别用于加密和解密文件。在doCrypto方法中，我们创建了一个AES加密算法的Cipher对象，并使用一个密钥初始化该对象。然后根据加密或解密模式，使用CipherInputStream或CipherOutputStream来读取和写入文件内容。加密和解密过程中使用的密钥长度必须是16、24或32字节，所以我们使用密钥的字节数组来创建一个SecretKeySpec对象。
在main方法中，我们定义了输入文件、加密后文件和解密后文件的路径，并指定一个密钥。然后调用encrypt方法对输入文件进行加密，再调用decrypt方法对加密后的文件进行解密。
请注意，这只是一个简单的示例，实际使用时需要根据具体需求进行优化和安全性考虑。
