Java Security

First of all Thank You in Advance.

I am converting a byte array to String and then convert same String to byte array. If my byte array contain these values (-127,-115,-113,-112,-99) then the output will not correct why? and tell me why all these element encoded to 63.

take a look a this program only for five number Test.

public class TestBytes
{
public static void main(String args[])
{
byte b[] = {-127,-115,-113,-112,-99,-11,65,66}; /* initialize a byte array b. */
String s = new String(b); /* convert byte array b to String s. */
byte a[] = s.getBytes();
/* reverse the process i.e convert String s to byte array a and then contents of byte array a must equal to contents of byte array b. */
System.out.println(s); // String Contents
for(int i = 0 ; i < a.length ; i++)
{
System.out.print(a [ i ] + ",");
/* but the contents of byte array a are: 63,63,63,63,63,-11,65,66, */
}
}
}

Same Program But Cover All the 256 elements -128 to 127

public class TestBytes
{
public static void main(String args[])
{
byte b[] = new byte[256];
for(int i = -128,j = 0 ; i < 128 && j < 256 ; i++,j++)
{
b [ j++ ] = (byte)i; /* fill the byte array with -128 to +127*/
}
String s = new String(b); /* convert byte array b to String s. */
byte a[] = s.getBytes(); /* reverse the process i.e convert String s to byte array a and then contents of byte array a must equal to contents of byte array b. */
s = ""; /* empty the String s */
for(int i = 0 ; i < a.length ; i++)
{

/* Now compare hole array it will compare all numbers(-128 to +127) but these five (-127,-115,-113,-112,-99) will be replaced by 63 '?'. why these numbers can not converted, means convert all negative numbers except these (-127,-115,-113,-112,-99). what is the problem with these numbers.*/

if(a[ i ] != b[ i ])
{
s += "" + a[ i ] + " , " + b[ i ] + " \n\n";
/* a is converted array and b is orignal array*/
}
}
System.out.println(s);
}
}

Basically I am developing Encryption/Decryption program which will apply to any file. so I am reading a File(txt,doc,jpg or etc.) Using FileInputStream class, after reading the all the data is in byte array. I will convert it to String, to apply my Encryption algoritm on it. After this i am converting Encrypted String to byte array to write the data in file using FileOutputStream class.
Decrption Process is just reverse of above process that why i need to convert a byte array to String and vice versa.

Yahya Kamran.