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

5 % 3 produces 2
5 % (-3) produces 2
(-5) % 3 produces -2
(-5) % (-3) produces -2

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

Noncompliant Code Example

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.

private int SIZE = 16;	
public int[] hash = new int[SIZE];
	
public int lookup(int hashKey) {
  return hash[hashKey % SIZE];
}

Compliant Solution

This compliant solution calls the imod() method that always returns a positive remainder.

// method imod() gives non-negative result
private int SIZE = 16;
public int[] hash = new int[SIZE];

private int imod(int i, int j) {
  int temp = i % j;
  return (temp < 0) ? -temp : temp; // unary - will succeed without overflow  
                                    // because temp cannot be Integer.MIN_VALUE
}
	
public int lookup(int hashKey) {
  return hash[imod(hashKey, SIZE)];
}

Risk Assessment

Incorrectly assuming a positive remainder from a remainder operation can result in erroneous code.

Automated Detection

Automated detection of uses of the % operator is straightforward. Sound determination of whether those uses correctly reflect the intent of the programmer is infeasible in the general case. Heuristic warnings could be useful.

Related Guidelines

Bibliography

03. Numeric Types and Operations (NUM)      03. Numeric Types and Operations (NUM)