Here is a code of encryption and decryption string.Similarly you can encrypt and decrypt the file data.

import java.security.*; 
import javax.crypto.*;
import javax.crypto.spec.DESKeySpec;

class EncryptAndDecrypt {

public static void main (String[] args) throws Exception{ 
    SecretKeyFactory key = SecretKeyFactory.getInstance("DES");
    SecureRandom random = SecureRandom.getInstance("SHA1PRNG", "SUN");

    byte[] desKeyData = { (byte)0x01, (byte)0x02, (byte)0x03, (byte)0x04, (byte)0x05, (byte)0x06, (byte)0x07, (byte)0x08 }; 
    DESKeySpec desKeySpec = new DESKeySpec(desKeyData);

    Key keys = key.generateSecret(desKeySpec);
    Cipher cipher = Cipher.getInstance("DES");
    cipher.init(Cipher.ENCRYPT_MODE, keys);
    String st= "Welcome";
    byte[] cleartext = null;
    cleartext = st.getBytes();
    byte[] ciphertext = null;
    ciphertext = cipher.doFinal(cleartext); 
    System.out.println("the encrypted text is: " + ciphertext.toString());

    cipher.init(Cipher.DECRYPT_MODE, keys); 
    byte[] cleartext1 = cipher.doFinal(ciphertext);
    System.out.println("the decrypted cleartext is: " + new String(cleartext1));
    }
}