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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. :
| Code Block | ||
|---|---|---|
| ||
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; // Some expression that evaluates to true
System.out.print(trueExp ? alpha : 0); // prints A
System.out.print(trueExp ? alpha : i); // prints 65
}
}
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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.:
| Code Block | ||
|---|---|---|
| ||
public class Expr {
public static void main(String[] args) {
char alpha = 'A';
int i = 0;
boolean trueExp = true; // Some expression that evaluates to true
System.out.print(trueExp ? alpha : ((char) 0)); // prints A
// Deliberate narrowing cast of i; possible truncation OK
System.out.print(trueExp ? alpha : ((char) i)); // prints A
}
}
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This noncompliant code example prints 100 as the size of the HashSet rather than the expected result (some value between 0 and 50). :
| Code Block | ||
|---|---|---|
| ||
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());
}
}
|
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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. :
| Code Block | ||
|---|---|---|
| ||
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
// Cast of i-1 is safe, because value is always representable
s.remove(((i % 2) == 1) ? Short.valueOf((short) (i-1)) : workingVal);
}
System.out.println(s.size());
}
}
|
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Automated detection of condition expressions whose second and third operands are of different types is straightforward.
Bibliography
Puzzle 8, "Dos Equis" | |
"Bx: Primitive Value Is Unboxed and Coerced for Ternary Operator" | |
[JLS 2011] | §15.25, "Conditional Operator ? :" |
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