EXP53-J. Use parentheses for precedence of operation

Programmers frequently make errors regarding the precedence of operators because of the unintuitive low-precedence levels of &, |, ^, <<, and >>. Avoid mistakes regarding precedence through the suitable use of parentheses. This also improves code readability unless taken to excess. The precedence of operations by the order of the subclauses are defined in the Java Tutorials [[Tutorials 2008]].

Although it advises against depending on parentheses for specifying evaluation order, guideline EXP08-J. Do not write more than once to the same variable within an expression applies only to expressions that contain side effects.

Noncompliant Code Example

The intent of the expression in this noncompliant code example is to add the variable OFFSET to the result of the bitwise logical & between x and MASK.

public static final int MASK = 1337;
public static final int OFFSET = -1337;

public static int computeCode(int x) {
  return x & MASK + OFFSET;
}

According to the operator precedence guidelines, the expression is parsed as the following:

x & (MASK + OFFSET)

This expression gets evaluated, as follows, resulting in the value 0.

x & (1337 - 1337)

Compliant Solution

This compliant solution uses parentheses to ensure that the expression evaluates as intended.

public static final int MASK = 1337;
public static final int OFFSET = -1337;

public static int computeCode(int x) {
  return (x & MASK) + OFFSET;
}

Exceptions

EXP06-EX0: Parentheses may be omitted from mathematical expressions that follow the algebraic precedence rules. For instance, consider the expression:

x + y * z

By mathematical convention, multiplication is performed before addition; parentheses are redundant in this case.

x + (y * z)

Risk Assessment

Mistakes regarding precedence guidelines can cause an expression to be evaluated in an unintended way. This can lead to unexpected and abnormal program behavior.

Automated Detection

Detection of all expressions using low-precedence operators without parentheses is straightforward. Determining the correctness of such uses is infeasible in the general case; heuristic warnings could be useful.

Related Guidelines

C Coding Standard: EXP00-C. Use parentheses for precedence of operation

C++ Secure Coding Standard: EXP00-CPP. Use parentheses for precedence of operation

Bibliography

EXP51-J. Do not perform assignments in conditional statements      02. Expressions (EXP)