I ran into a problem when comparing the value of a byte. Theoretically, the following code snippet should return true, yet it returns false.
public static void main(String [] args) {
byte a = 3;
if (MpegUtil.isSlice(a) == true) {
System.out.print("Yes");
}
else System.out.print("No");
}
public static boolean isSlice(byte ID) {
if (ID >= 1 && ID <= (byte) 0xaf) return true;
else return false;
}
What the function isSlice does is: receive a byte and if it is between 1 and af (hex), it will return true, if it doesn't the returned value is False. Yet somehow it fails my test by answering false to 3.
Does anyone has an idea of why this function doesn't work?
EDIT: Thank-you for the suggestion of removing the 'byte' cast, it does indeed allow the code to function, however once the compared value is superior or equal to '0x80' the result change back to being a wrong 'false'. I know this is caused by the fact that Java considers the byte to be signed (now i'm really looking forward to Java 8), but would there be any "cleaner" version than changing the condition to:
if (ID != 0 && ID >= (byte) 0xaf && ID <= (byte) 0x7f) return true
Since this condition doesn't seem at first glance logical, is there any more obvious way to fix this?
It doesn't work because (byte) 0xAF is not what you think. Try printing the following, so you get the idea:
System.out.println(0xaf); // 175
System.out.println((byte) 0xaf); // -81
Why is (byte) 0xAF negative?
Print this:
System.out.println(Byte.MAX_VALUE); // 127
The range of a byte is [-128, 127] (256 numbers), so if you cast 175 to byte, you will get:
175 - 256 = -81
You can also try this:
System.out.println((byte) 128); // -128
System.out.println((byte) -129); // 127
System.out.println((byte) 256); // 0
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