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The Java Language Specification (JLS), §15.17.3, "Remainder Operator %," states

The remainder operation for operands that are integers after binary numeric promotion (§5.6.2) produces a result value such that (a/b)*b+(a%b) is equal to a. This identity holds even in the special case that the dividend is the negative integer of largest possible magnitude for its type and the divisor is -?1 (the remainder is 0). It follows from this rule that the result of the remainder operation can be negative only if the dividend is negative, and can be positive only if the dividend is positive; moreover, the magnitude of the result is always less than the magnitude of the divisor.

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As a result, code that depends upon on the remainder operation to always return a positive result is erroneous.

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This noncompliant code example uses the integer hashKey as an index into the hash array. A negative hash key produces a negative result from the remainder operator, causing the lookup() method to throw java.lang.ArrayIndexOutOfBoundsException.

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