EXP05-J. Do not write more than once to the same variable within an expression

According to the JLS, §15.7, "Evaluation Order" [JLS 2005]:

The Java programming language guarantees that the operands of operators appear to be evaluated in a specific evaluation order, namely, from left to right.

§15.7.3, "Evaluation Respects Parentheses and Precedence" adds:

Java programming language implementations must respect the order of evaluation as indicated explicitly by parentheses and implicitly by operator precedence.

When an expression contains side effects, these two requirements can yield unexpected results. Evaluation of the operands proceeds left to right, without regard to operator precedence rules and indicative parentheses; evaluation of the operators, however, obeys precedence rules and parentheses.

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.

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 because evaluation of the side-effect-infested subexpressions follows the left-to-right ordering rule.

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.

Although this code performs as expected, it still represents poor practice by writing to number three times in a single expression.

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.

EXP05-EX1: The logical operators || and && have well-understood short-circuit semantics, so expressions involving these operators do not violate this rule. Consider the following code:

public void exampleMethod(InputStream in) {
  int i;
  // Skip one char, process next
  while ((i = in.read()) != -1 && (i = in.read()) != -1) {
    // ...
  }

}

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

EXP04-J. Ensure that autoboxed values have the intended type      02. Expressions (EXP)      EXP06-J. Do not use side-effecting expressions in assertions