What is the meaning of double tilde (~~) in Java?

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When browsing the source code of Guava, I came across the following piece of code (part of the implementation of hashCode for the inner class CartesianSet):

int adjust = size() - 1;
for (int i = 0; i < axes.size(); i++) {
    adjust *= 31;
    adjust = ~~adjust;
    // in GWT, we have to deal with integer overflow carefully
}
int hash = 1;
for (Set<E> axis : axes) {
    hash = 31 * hash + (size() / axis.size() * axis.hashCode());

    hash = ~~hash;
}
hash += adjust;
return ~~hash;

Both of adjust and hash are ints. From what I know about Java, ~ means bitwise negation, so adjust = ~~adjust and hash = ~~hash should leave the variables unchanged. Running the small test (with assertions enabled, of course),

for (int i = Integer.MIN_VALUE; i < Integer.MAX_VALUE; i++) {
    assert i == ~~i;
}

confirms this. Assuming that the Guava guys know what they are doing, there must be a reason for them to do this. The question is what?

EDIT As pointed out in the comments, the test above doesn’t include the case where i equals Integer.MAX_VALUE. Since i <= Integer.MAX_VALUE is always true, we will need to check that case outside the loop to prevent it from looping forever. However, the line

assert Integer.MAX_VALUE == ~~Integer.MAX_VALUE;

yields the compiler warning “Comparing identical expressions”, which pretty much nails it.

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