How would I write a program that encrypts or decrypts a file using the following substitution/encryption method as shown below?

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

substituting the above with the below

F E A T H R Z Y X W V U S Q P O N M L K J I G D C B

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So far I have this as my code below

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import java.io.InputStream;

import java.io.OutputStream;

import java.io.IOException;

/**

This class encrypts files using the Caesar cipher.

For decryption, use an encryptor whose key is the

negative of the encryption key.

*/

public class Cipher

{

/**

Constructs a cipher object with a given key.

@param aKey the encryption key

*/

public Cipher(int aKey)

{

key = aKey;

}

/**

Encrypts the contents of a stream.

@param in the input stream

@param out the output stream

*/

public void encryptStream(InputStream in, OutputStream out)

throws IOException

{

boolean done = false;

while (!done)

{

int next = in.read();

if (next == -1) done = true;

else

{

char b = (char) next;

char c = encrypt(b);

out.write(c);

}

}

}

/**

Encrypts a char.

@param b the char to encrypt

@return the encrypted char

*/

public char encrypt(char c)

{

if ( key < 0 )

key = key + 26;

if ( Character.isUpperCase(c) )

return (char)((c - 'A' + key) % 26 + 'A');

if ( Character.isLowerCase(c) )

return (char)((c - 'a' + key) % 26 + 'a');

return c;

}

private int key;

}

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I'm lost on to deciding which part of the program is reusable and which part is not. What methods and instance variables do I need to add?