bit manipulation - Java: right shift on negative number

Question

I am very confused on right shift operation on negative number, here is the code.

int n = -15;
System.out.println(Integer.toBinaryString(n));
int mask = n >> 31;
System.out.println(Integer.toBinaryString(mask));

And the result is:

11111111111111111111111111110001
11111111111111111111111111111111

Why right shifting a negative number by 31 not 1 (the sign bit)?

Answer 1

Because in Java there are no unsigned datatypes, there are two types of right shifts: arithmetic shift >> and logical shift >>>. http://docs.oracle.com/javase/tutorial/java/nutsandbolts/op3.html

Arithmetic shift >> will keep the sign bit.

Unsigned shift >>> will not keep the sign bit (thus filling 0s).

(images from Wikipedia)

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By the way, both arithmetic left shift and logical left shift have the same result, so there is only one left shift <<.