Hey, this is a part of the series describing the situation where I encountered weird behavior in the programming :) Today is about Java. When I wrote a code to do bit manipulation in Java, the unexpected outcome shows up. Unfortunately, I could not find the official specification behind this behavior. Thus this aims to get a chance to find the answer from someone who read this article.
Masking the most significant bit
What we wanted to do was getting the most significant bit in the 2’s complement format. For instance, 1111_1111 is -1 in the signed 8-bit format. To get the most significant bit in the number, we can use the mask of the signed bit.
byte value = -1;
long byteSignMask = 0b1000_0000;;
assertEquals(0b1000_0000, value & byteSignMask);
Yes, it properly works to get only the bit representing the sign of the number. Using binary literal and shift operation, constructing the mask gives us an identical result.
long byteSign1 = 1L << 7;
long byteSign2 = 0b1000_0000;
// byteSign1 = 10000000
System.out.println("byteSign1 = " + Long.toBinaryString(byteSign1));
// byteSign2 = 10000000
System.out.println("byteSign2 = " + Long.toBinaryString(byteSign2));
// OK
assertEquals(byteSign1, byteSign2);
But when I do the same thing for integer, it does not work.
Implicit padding
The following code works correctly as well as the previous example.
long value = -1;
long intSignMask = 0b1000_0000_0000_0000_0000_0000_0000_0000;
assertEquals(0b1000_0000_0000_0000_0000_0000_0000_0000, value & intSignMask);
Okay, let me check the mask is the same with 1L << 31.
long intSign1 = 1L << 31;
long intSign2 = 0b1000_0000_0000_0000_0000_0000_0000_0000;
// intSign1 = 10000000000000000000000000000000
System.out.println("intSign1 = " + Long.toBinaryString(intSign1));
// intSign2 = 1111111111111111111111111111111110000000000000000000000000000000
System.out.println("intSign2 = " + Long.toBinaryString(intSign2));
// Fail: expected:<2147483648> but was:<-2147483648>
assertEquals(intSign1, intSign2);
It’s interesting. Why is the mask constructed by the shift operation 1L << 31 results in a different outcome from the binary literal? Why is the binary literal automatically left-padded with 1?
I asked in the StackOverflow as before to get the answer. Please let me know if you an explanation for it.
Thanks.