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The conditional AND and OR operators (&& and ||, respectively) exhibit short-circuit behavior. That is, the second operand is evaluated only when the result of the conditional operator cannot be deduced solely by evaluating the first operand.

Exercise caution when the operands following the first operand contain side effects. In the following code, the value of i is incremented only when i >= 0.

Consequently, when the result of the conditional operator can be deduced solely from the result of the first operand, the second operand will remain unevaluated; its side effects, if any, will never occur.

The bitwise AND and OR operators (& and |) lack short-circuit behavior. Similar to most Java operators, they evaluate both operands. They return the same Boolean result as && and || respectively but can have different overall effects depending on the presence or absence of side effects in the second operand.

Consequently, either the & or the && operator can be used when performing Boolean logic. However, there are times when the short-circuiting behavior is preferred and other times when the short-circuiting behavior causes subtle bugs.

Noncompliant Code Example (Improper &)

This noncompliant code example, derived from Flanagan [Flanagan 2005], has two variables with unknown variables. The code must validate its data and then check whether array[i] is a valid index.

int array[]; // May be null
int i;       // May be an invalid index for array
if (array != null & 

int i = // initialize to user supplied value 
if ((i >= 0) & i < array.length & ((i++) <= Integer.MAX_VALUE))array[i] >= 0) {
  // Use array
} else {
  // ...Handle error
}

Although the behavior is well defined, it is unclear whether i gets incremented.

Noncompliant Code Example

This noncompliant code example attempts to rename a given file if it exists, perform operations on the renamed file, and then delete the renamed file. However, because of the short-circuit behavior of the || operator, the renameTo() method does not execute when the exists() method returns true. Because of this, the renamed file could or could not exist, which can result in an attempt to use and then delete a nonexistent file. This problem is exacerbated by the fact that File.delete() does not throw an exception but returns an error code on failure, which is sometimes silently ignored or perceived as unnecessary. (See guideline EXP00-J. Do not ignore values returned by methods.)

class BadRenameFile {
  public static void main(String[] args) {
    File fOriginal = new File("original.txt");
    File fNew = new File("new.txt");
    if (fOriginal.exists() || fOriginal.renameTo(fNew)) {
      // do something with fNew
      fNew.delete();
    }
  }
} 

Compliant Solution

Knowledge of the short-circuit behavior can be used to enforce the desired specification. This compliant solution traps an error if the file does not exist or when it cannot be renamed to the new file name. Operations on the new file follow.

This code can fail as a result of the same errors it is trying to prevent. When array is NULL or i is not a valid index, the reference to array and array[i] will cause either a NullPointerException or an ArrayIndexOutOfBoundsException to be thrown. The exception occurs because the & operator fails to prevent evaluation of its right operand even when evaluation of its left operand proves that the right operand is inconsequential.

Compliant Solution (Use &&)

This compliant solution mitigates the problem by using &&, which causes the evaluation of the conditional expression to terminate immediately if any of the conditions fail, thereby preventing a runtime exception:

int array[]; // May be null
int i;       // May be an invalid index for array
if (array != null && i >= 0 &&
    i < array.length && array[i] >= 0) {
  // Handle array
} else {
  // Handle error
}

Compliant Solution (Nested if Statements)

This compliant solution uses multiple if statements to achieve the proper effect.

int array[]; // May be null
int i;       // May be a valid index for array
if (array != null) {
  if (i >= 0 && i < array.length) {
    if (array[i] >= 0

class RenameFile {
  public static void main(String[] args) {
    File fOriginal = new File("original.txt");
    File fNew = new File("new.txt");

    if (!fOriginal.exists() || !fOriginal.renameTo(fNew)) {
      // handleUse errorarray
    } else {
      // doHandle somethingerror
 with fNew
  }
  if (!fNew.delete())} else {
      // handleHandle error
  }
} else {
  }
// Handle }error
} 

Exceptions

*EXP06-EX1:* Sometimes programmers who are aware of the short-circuit behavior use it to their advantage, as Flanagan \[[Flanagan 2005|AA. Bibliography#Flanagan 05]\] exemplifies in the following example:

if (data != null && i < data.length && data[i] != -1) ... 

This code snippet sequentially checks for potential error conditions before allowing the main computation to proceed. The short-circuit behavior of && guarantees that the first error condition encountered will terminate the checking process.

Risk Assessment

Failure to understand the short-circuit behavior of the logical AND and OR operators can cause unintended program behavior.

Automated Detection

Detection of short-circuit operators is straightforward, but sound error checking for this guideline is not feasible in the general case. Heuristic warnings could be useful.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

\[[Flanagan 2005|AA. Bibliography#Flanagan 05]\] 2.5.6. Boolean Operators\[[JLS 2005|AA. Bibliography#JLS 05]\] Sections [15.23|http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.23] "Conditional-And Operator &&" and [15.24|http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.24] "Conditional-Or Operator ||"

Although correct, this solution is more verbose and could be more difficult to maintain. Nevertheless, this solution is preferable when the error-handling code for each potential failure condition is different.

Noncompliant Code Example (Improper &&)

This noncompliant code example demonstrates code that compares two arrays for ranges of members that match. Here i1 and i2 are valid array indices in array1 and array2 respectively. Variables end1 and end2 are the indices of the ends of the matching ranges in the two arrays.

if (end1 >= 0 & i2 >= 0) {
  int begin1 = i1;
  int begin2 = i2;
  while (++i1 < array1.length &&
         ++i2 < array2.length &&
         array1[i1] == array2[i2]) {
    // Arrays match so far
  }
  int end1 = i1;
  int end2 = i2;
  assert end1 - begin1 == end2 - begin2;
}

The problem with this code is that when the first condition in the while loop fails, the second condition does not execute. That is, once i1 has reached array1.length, the loop terminates after i1 is incremented. Consequently, the apparent range over array1 is larger than the apparent range over array2, causing the final assertion to fail.

Compliant Solution (Use &)

This compliant solution mitigates the problem by judiciously using &, which guarantees that both i1 and i2 are incremented regardless of the outcome of the first condition:

public void exampleFunction() {  
  while (++i1 < array1.length &     // Not &&
         ++i2 < array2.length &&
         array1[i1] == array2[i2]){
 //doSomething
  }
}

Applicability

Failure to understand the behavior of the bitwise and conditional operators can cause unintended program behavior.

Bibliography

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Image Added Image Added Image AddedEXP06-J. Use parentheses for precedence of operation      04. Expressions (EXP)      EXP08-J. Understand the evaluation of expressions containing non-short-circuit operators