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Expressions must not write to memory that they subsequently read and also must not write to any memory twice. Note that memory reads and writes can occur either directly in the expression from assignments or indirectly through side effects in methods called in the expression.

Noncompliant Code Example (Order of Evaluation)

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 method can calculate the rating. The get() method is expected to return a non-zero factor for users who are authorized and a zero value for those who are unauthorized.

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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;
    // Assign number to non zero value if authorized else 0
    return number;
  }
}

Noncompliant Code Example (Order of Evaluation)

This noncompliant code example reorders the previous expression so that the left-to-right evaluation order of the operands corresponds with the programmer's intent.

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int number = 17;

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

Compliant Solution (Order of Evaluation)

This compliant solution uses equivalent code with no side effects and performs not more than one write per expression. The resulting expression can be reordered without concern for the evaluation order of the component expressions, making the code easier to understand and maintain.

int number = 17;

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

Exceptions

EXP05-EX0: The increment and decrement operators (++) and (--) read a numeric variable, and then assign a new value to the variable. These are well-understood and are an exception to this rule.

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Although the conditional expression appears to violate this rule, this code is compliant because the subexpressions on either side of the && operator do not violate it. Each subexpression has exactly one assignment and one side effect (the reading of a character from in).

Risk Assessment

Failure to understand the evaluation order of expressions containing side effects can result in unexpected output.

Automated Detection

Detection of all expressions involving both side effects and multiple operator precedence levels is straightforward. Determining the correctness of such uses is infeasible in the general case; heuristic warnings could be useful.

Related Guidelines

Bibliography

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