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Noncompliant Code Example

This noncompliant code example shows how side-effects in expressions can lead to unanticipated outcomes. The programmer intends to write access control logic based on different threshold levels. Each user has a rating that must be above the threshold to be granted access. As shown, a simple function is can be used to calculate the rating. The get() method is used expected to provide return a non-zero factor when the user is authorized, and a zero value when the user is not authorized.

In this case, the programmer expects the rightmost subexpression to evaluate first because the * operator has a higher precedence than the + operator. The parentheses reinforce this belief. These ideas lead to the incorrect conclusion that the right hand side evaluates to zero whenever the get() method returns zero. The programmer expects number to be assigned 0 because of the rightmost number = get() subexpression. Consequently, the test in the left hand subexpression is expected to reject the unprivileged user because the rating value (number) is below the threshold of 10.

However, the program grants access to the unauthorized user. The reason is that evaluation of the side-effect infested subexpressions follows the left to right ordering rule and which should not be confused with the tenets of operator precedence, associativity and indicative parentheses.

class BadPrecedence {
  public static void main(String[] args) {
    int number = 17;
    int[] threshold = new int[20];
    threshold[0] = 10;
    number = (number > threshold[0]? 0 : -2) + ((31 * ++number) * (number = get()));
    // ... 
    if(number == 0) {
      System.out.println("Access granted");
    } else {
      System.out.println("Denied access"); // number = -2
    }
  }
  public static int get() {
    int number = 0;
    // assignAssign number to non zero value if authorized else 0
    return number;
  }
}

Compliant Solution

Diligently By diligently following the left to right evaluation order, a programmer can expect this compliant solution to evaluate to the expected final outcome depending on the value returned by the get() method.

number = ((31 * ++number) * (number=get())) + (number > threshold[0]? 0 : -2);

Although this solution solves the problem, in general it is advisable to avoid using expressions with more than one side-effect. It is also inadvisable to depend on the left-right ordering for evaluation of side-effects because operands are evaluated in place first, and then subject to laws of operator precedence and associativity.

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number = ((31 * (number + 1)) * get()) + (get() > threshold[0]? 0 : -2);

Risk Assessment

Failing to keep in mind the evaluation order of expressions containing side effects can result in unexpected output.

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