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;


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.

The following code works correctly as well as the previous example.

long value = -1;


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.