- 37. Do not shadow or obscure identifiers in subscopes
- 38. Do not declare more than one variable per declaration
- 39. Use meaningful symbolic constants to represent literal values in program logic
- 40. Properly encode relationships in constant definitions
- 41. Return an empty array or collection instead of a null value for methods that return an array or collection
- 42. Use exceptions only for exceptional conditions
- 43. Use a try-with-resources statement to safely handle closeable resources
- 44. Do not use assertions to verify the absence of runtime errors
- 45. Use the same type for the second and third operands in conditional expressions
- 46. Do not serialize direct handles to system resources
- 47. Prefer using iterators over enumerations
- 48. Do not use direct buffers for short-lived, infrequently used objects
- 49. Remove short-lived objects from long-lived container objects
This chapter is from the book
This chapter is from the book
This chapter is from the book
45. Use the same type for the second and third operands in conditional expressions
The conditional operator ?: uses the boolean value of its first operand to decide which of the other two expressions will be evaluated. (See §15.25, “Conditional Operator ? :,” of the JLS [JLS 2013].)
The general form of a Java conditional expression is operand1 ? operand2 : operand3.
- If the value of the first operand (operand1) is true, then the second operand expression (operand2) is chosen.
- If the value of the first operand is false, then the third operand expression (operand3) is chosen.
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 rules for determining the result type of a conditional expression (see Table 3–1) are complicated; programmers could be surprised by the type conversions required for expressions they have written.
Table 3–1. Determining the result type of a conditional expression
|
Rule |
Operand 2 |
Operand 3 |
Resultant Type |
|
1 |
Type T |
Type T |
Type T |
|
2 |
boolean |
Boolean |
boolean |
|
3 |
Boolean |
boolean |
boolean |
|
4 |
null |
reference |
reference |
|
5 |
reference |
null |
reference |
|
6 |
byte or Byte |
short or Short |
short |
|
7 |
short or Short |
byte or Byte |
short |
|
8 |
byte, short, char, Byte, Short, Character |
constant int |
byte, short, char if value of int is representable |
|
9 |
constant int |
byte, short, char, Byte, Short, Character |
byte, short, char if value of int is representable |
|
10 |
Other numeric |
Other numeric |
Promoted type of the second and third operands |
|
11 |
T1 = boxing conversion(S1) |
T2 = boxing conversion(S2) |
Apply capture conversion to the least upper bound of T1 and T2 |
Result type determination begins from the top of the table; the compiler applies the first matching rule. The Operand 2 and Operand 3 columns refer to operand2 and operand3 (from the previous definition) respectively. In the table, constant int refers to constant expressions of type int (such as '0' or variables declared final).
For the final table row, S1 and S2 are the types of the second and third operands, respectively. T1 is the type that results from applying boxing conversion to S1, and T2 is the type that results from applying boxing conversion to S2. The type of the conditional expression is the result of applying capture conversion to the least upper bound of T1 and T2. See §5.1.7, “Boxing Conversion,” §5.1.10, “Capture Conversion,” and §15.12.2.7, “Inferring Type Arguments Based on Actual Arguments” of the JLS for additional information [JLS 2013].
The complexity of the rules that determine the result type of a conditional expression can result in unintended type conversions. Consequently, the second and third operands of each conditional expression should have identical types. This recommendation also applies to boxed primitives.
Noncompliant Code Example
In this noncompliant code example, the programmer expects that both print statements will print the value of alpha as a char:
public class Expr {
public static void main(String[] args) {
char alpha = 'A';
int i = 0;
// Other code. Value of i may change
boolean trueExp = true; // Expression that evaluates to true
System.out.print(trueExp ? alpha : 0); // Prints A
System.out.print(trueExp ? alpha : i); // Prints 65
}
}
The first print statement prints A because the compiler applies rule 8 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 is rule 10. Consequently, the compiler promotes the value of alpha to type int.
Compliant Solution
This compliant solution uses identical types for the second and third operands of each conditional expression; the explicit casts specify the type expected by the programmer:
public class Expr {
public static void main(String[] args) {
char alpha = 'A';
int i = 0;
boolean trueExp = true; // Expression that evaluates to true
System.out.print(trueExp ? alpha : 0); // Prints A
// Deliberate narrowing cast of i; possible truncation OK
System.out.print(trueExp ? alpha : ((char) i)); // Prints A
}
}
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 NUM12-EX0 of NUM12-J, “Ensure conversions of numeric types to narrower types do not result in lost or misinterpreted data” in The CERT® Oracle® Secure Coding Standard for Java™ [Long 2012].
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):
public class ShortSet {
public static void main(String[] args) {
HashSet<Short> s = new HashSet<Short>();
for (short i = 0; i < 100; i++) {
s.add(i);
// Cast of i-1 is safe,
// because value is always representable
Short workingVal = (short) (i-1);
// ... Other code may update workingVal
s.remove(((i % 2) == 1) ? i-1 : workingVal);
}
System.out.println(s.size());
}
}
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 integer promotion rules. Consequently, the Short object in the third argument is unboxed into a short, which is then promoted to 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.
Compliant Solution
This compliant solution casts the second operand to type short, then explicitly invokes the Short.valueOf() method to create a Short instance whose value is i-1:
public class ShortSet {
public static void main(String[] args) {
HashSet<Short> s = new HashSet<Short>();
for (short i = 0; i < 100; i++) {
s.add(i);
// Cast of i-1 is safe, because the
// resulting value is always representable
Short workingVal = (short) (i-1);
// ... other code may update workingVal
// Cast of i-1 is safe, because the
// resulting value is always representable
s.remove(((i % 2) == 1) ? Short.valueOf((short) (i-1)) :
workingVal);
}
System.out.println(s.size());
}
}
As a result of the cast, the second and third operands of the conditional expression both have type Short, and the remove() call has the expected result.
Writing the conditional expression as ((i % 2) == 1) ? (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 unboxing of workingVal on each even iteration of the loop and autoboxing of the result of the conditional expression (from short to Short) on every iteration of the loop.
Applicability
When the second and third operands of a conditional expression have different types, they can be subject to unexpected type conversions.
Automated detection of condition expressions whose second and third operands are of different types is straightforward.
Bibliography
[Bloch 2005] |
Puzzle 8, “Dos Equis” |
[Findbugs 2008] |
“Bx: Primitive Value Is Unboxed and Coerced for Ternary Operator” |
[JLS 2013] |
§15.25, “Conditional Operator ? :” |
[Long 2012] |
NUM12-J. Ensure conversions of numeric types to narrower types do not result in lost or misinterpreted data |