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The operation of the modulo remainder operator in Java is defined in the Java Language Specification [[JLS 05]], Section 15.17.3:

The modulo 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 modulo remainder is 0). It follows from this rule that the result of the modulo 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.

Although clearly defined in the Java specification, the behavior is undefined in several early C implementations and it is represented by the same symbol as the modulus operator, which always returns a positive value. Therefore, it is possible to have unintended behavior from use of this operator.

The result of the modulo remainder operator implies the following behavior:

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The result has the same sign as the dividend (the first operand in the expression).

% is also known as the modulo operator.

Noncompliant Code Example

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/* moduloremainder function giving non-negative result */
private int SIZE = 16;
public int[] hash = new int[SIZE];

private int imod(int i, int j) {
  return (i < 0) ? ((-i) % j) : (i % j);
}
	
public int lookup(int hashKey) {
  return hash[imod(hashKey, size)];
}

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Note that providing a well documented imod method is a better choice as it improves readability and makes it clear that its sole purpose is to return positive values when required and not to "fix" the unintuitive behavior of the modulo remainder operator, as defined by the specification.

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