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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 , without guarantees regarding their current valueswith unknown variables. The code must validate its data and then check whether array[i] is a valid index.

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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   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. Although correct, it is more verbose and could be more difficult to maintain.

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) {
      // Use array
    } else {
      // Handle error
    }
  } else {
    // Handle 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 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.

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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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