0xFF is hexidecimal, you can Wikipedia that part. 

FF is a representation of 

00000000 00000000 00000000 11111111

(a 32-bit integer)

& means bit-wise "and", and so when you use it on two ints, each pair of bits from those two ints is and-ed and the result is placed in the resultant int:

Example (showing 16 bits only)

0101 1100 1010 1100
&0000 0000 1111 1111
-----------------
=0000 0000 1010 1100



So as you can see, 0xFF is often used to to isolate a byte in an integer, by truncating the integer to one byte's worth of 1's.

For more information, see this WP article:

------解决方案--------------------
与 0xff 做 & 运算会将 byte 值变成 int 类型的值，也将 -128～0 间的负值都转成正值了。

------解决方案--------------------
Java code
char c = (char)-1 & 0xFF;
char d = (char)-1;
System.out.println((int)c);
System.out.println((int)d);

------解决方案--------------------
1) 位运算不限制为int, long也行的。
2)  
3) 负数进行&操作时需要转成补码，-2 的补码是0xFFFFFFFE