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The operation of the remainder operator in Java is defined in the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\], Section 15.17.3 "Remainder Operator %":

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.

Although clearly defined in the Java specification, the behavior of the remainder operator's equivalent is undefined in several early C implementations. Programmers who are unaware of this distinction might always expect a positive remainder and code accordingly. This can result in vulnerabilities.

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Noncompliant Code Example

In this noncompliant code example, the integer hashKey references an element of the hash array. However, as the hash key is not guaranteed to be positive, the lookup function may fail, producing triggering a java.lang.ArrayIndexOutOfBoundsException on all negative inputs.

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This compliant solution calls a function method that returns a a modulus that is always positive.

/*/ remaindermethod functionimod() givinggives 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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