java类型转换不够用高位补

                byte b = (byte)0xFF;

System.out.println("b===" + b);//-1

int i = b;//位不够就用用高位补，故是负值

System.out.println("i===" + i);//-1

i = (int)b;//位不够就用用高位补，故是负值

System.out.println("i===" + i);//-1

i = 0xFF;//位够

System.out.println("i===" + i);//255

i = (int)0xFF;//位够

System.out.println("i===" + i);//255

i = b & 0xFF;

System.out.println("i===" + i);//255