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The conditional operator ?: uses the boolean value of its first operand to decide which of the other two expressions will be evaluated (see JLS Section 15. (See §15.25, "Conditional Operator ? :" of the Java Language Specification [JLS 2005].)

The general form of a Java conditional expression is operand1 ? operand2 : operand3.

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The conditional operator is syntactically right-associative; for example, a?b:c?d:e?f:g is equivalent to a?b:(c?d:(e?f:g)).

The JLS-defined Java Language Specification rules for determining the type of the result of a conditional expression (tabulated below) are complicated; programmers could be surprised by the type conversions required for expressions they have written.

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See JLS Section 5§5.1.7, "Boxing Conversion"; JLS Section 5§5.1.10, "Capture Conversion"; and JLS Section 15§15.12.2.7, "Inferring Type Arguments Based on Actual Arguments" of the Java Language Specification for additional information on the final table entry.

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In this noncompliant code example, the programmer expects that both print statements will print the value of alpha as a char — A. The first print statement does print A, because the compiler applies the eighth rule from the result type determination table to determine that the second and third operands of the conditional expression are, or are converted to, type char. However, the second print statement prints 65 — the value of alpha as an int. The first matching rule from the table above is the tenth rule; consequently, the compiler promotes the value of alpha to type int.

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Note that the explicit cast in the first conditional expression is redundant; that is, the value printed remains identical whether the cast is present or absent. Nevertheless, use of the redundant cast is good practice; it serves as an explicit indication of the programmer's intent and, and consequently, improves maintainability. When the value of i in the second conditional expression falls outside the range that can be represented as a char, the explicit cast will truncate its value. This usage complies with exception EXP13-EX1 of guideline NUM15"NUM00-J. Ensure conversions of numeric types to narrower types do not result in lost or misinterpreted data."

Noncompliant Code Example

This noncompliant code example prints 100 as the size of the HashSet rather than the expected result (some value between 0 and 50). The combination of values of types short and int in the second argument of the conditional expression (the operation i-1) causes the result to be an int, as specified by the normal integer promotion rules. Consequently, the Short object in the third argument is autounboxed into a short, which is then promoted into an int. The result of the conditional expression is then autoboxed into an object of type Integer. Because the HashSet contains only values of type Short, the call to HashSet.remove() has no effect.

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Writing the conditional expression as ((i & 1) == 0) ? (short) (i-1)) : workingVal also complies with this guideline , because both the second and third operands in this form have type short. However, this alternative is less efficient , because it forces both autounboxing of workingVal on each even iteration of the loop and also autoboxing of the result of the conditional expression (from short to Short) on every iteration of the loop.

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Automated detection of condition expressions whose second and third operands are of different types is straightforward.

Related Vulnerabilities

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

Bibliography

\[[Bloch 2005|AA. Bibliography#Bloch 05]\] Puzzle 8: Dos Equis
\[[Findbugs 2008|AA. Bibliography#Findbugs 08]\] "Bx: Primitive value is unboxed and coerced for ternary operator"
\[[JLS 2005|AA. Bibliography#JLS 05]\] [Section 15§15.25|http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.25], "Conditional Operator {{? :}}"

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