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

Exercise caution if 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 */
int max = /* initialize to maximum value */
if ( (i >= 0) & i < array.length & ( (i++) <= max) )array[i] >= 0) {
  // Use array
} else {
  //* codeHandle */error
}

Although the behavior is well defined, it is not immediately obvious whether i gets incremented or not. Even in the absence of side effects, it is possible to write confusing expressions that do not execute as desired. This guideline exemplifies both these conditions.

Noncompliant Code Example

This noncompliant example is designed to rename a given file if it is present, perform operations on it, and then delete it. However, the renameTo() method does not execute when the exists() method returns true, and an unsuspecting developer would incorrectly attempt to delete the fNew file instead of the original one. This problem is exacerbated by the fact that File.delete() does not throw an exception but returns an error code on failure, which is often silently ignored or perceived as unnecessary. (See EXP02-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(); // fNew does not exist as renameTo() was not executed
    }
  }
} 

Compliant Solution

Knowledge of the short-circuit behavior can be used to enforce the desired specification. This program 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 {
      // Handle error
 do something with fNew}
  }  if(!fNew.delete()) else {
      // handleHandle error
  }
} else {
  }
// Handle }error
} 

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 example differs from the previous one in that, there are no side effects in the right hand side operand. Nevertheless, an unaware programmer can get caught in the short-circuit behavior of the conditional AND and OR operators. The programmer has combined two expressions in the if statement. The first checks whether the d object is null and the second checks if the default security manager has been installed (by comparinf sm with null) depending on which the security check will be performed. This is a case of trying to combine together two null checks into one statement. A conditional && is used as using a conditional || would mean that whenever d is null, the complete expression can still succeed depending on the value of sm (see the next noncompliant example). This violates the invariants of d as it is desired that operations on it be prohibited if it is null. Unfortunately, when d is equal to null as shown, the current if expression evaluates to false and the security check is not executedcode 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.

// d = null
SecurityManager sm = System.getSecurityManager();	  
if(d != null && sm != null) {
  FilePermission perm = new FilePermission("file.dat", "read");
  sm.checkPermission(perm);
  /* do something with d */
}
/* read the file (skips security check) */

Noncompliant Code Example

In this example, the programmer switches the ordering of the two subexpressions and uses a || operator to ensure the security check is carried out but does not realize that the second check is evaluated if the first one succeeds. The result is the inadvertent modification of d.

SecurityManager sm = System.getSecurityManager();	  
if(sm != null || d != null) {
  FilePermission perm = new FilePermission("file.dat", "read");
  sm.checkPermission(perm);
  // do something with d
}
// read the file

Compliant Solution

Decouple distinct operations that use the conditional AND and OR operators from expressions constituting decision statements. When inevitable, be aware of the short-circuit behavior and code accordingly.

if(d != null) { /* do something with d */ }

if(sm != null) {
  FilePermission perm = new FilePermission("file.dat", "read");
  sm.checkPermission(perm);
}
/* read the file (default security check is carried out) */

Exceptions

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

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

This snippet sequentially executes the subexpressions while avoiding an array indexing exception resulting from the checks that execute prior to the last subexpression.

Risk Assessment

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

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[JLS 05|AA. Java References#JLS 05]\] 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 ||"
\[[Flanagan 05|AA. Java References#Flanagan 05]\] 2.5.6. Boolean Operators

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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