SEI CERT Oracle Coding Standard for Java

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The conditional operator ? : uses the boolean value of one expression to decide which of the two other expressions should be evaluated.

The conditional operator is syntactically right-associative. For instance a?b:c?d:e?f:g is equivalent to a?b:(c?d:(e?f:g)).

Format:

ConditionalExpression:
ConditionalOrExpression
ConditionalOrExpression ? Expression : ConditionalExpression EXP00-A. Use same type for second and third operands in conditional expressions

  • If the value of the first operand is true, then the second operand expression is chosen
  • If the value of the first operand is false, then the third operand expression is chosen

The rules that define the resultant type are given below, where * refers to constant expressions of type int (such as '0' or variables declared as final):

Operand 2

Operand 3

Resultant type

type T

type T

type T

boolean

Boolean

boolean

Boolean

boolean

boolean

null

reference

reference

reference

null

reference

byte or Byte

short or Short

short

short or Short

byte or Byte

short

byte,short,char

const int*

byte,short,char if value of int representable

const int*

byte,short,char

byte,short,char if value of int representable

Byte

const int*

byte if int is representable as byte

const int*

Byte

byte if int is representable as byte

Short

const int*

short if int is representable as short

const int*

Short

short if int is representable as short

Character

const int*

char if int is representable as char

const int*

Character

char if int is representable as char

other

other

promoted type of the 2nd and 3rd operands

T1 = boxing conversion (S1)

T2 = boxing conversion(S2)

apply capture conversion to lub(T1,T2)

Non-Compliant Code Example

This non-compliant example prints A65 instead of AA. The first print statement prints the value of alpha as type char, that is as A since the third operand is a constant expression of type int (0). The second statement, however, prints 65, the integer equivalent of A. This is because of numeric promotion between the second operand (int) and the third (char) resulting from the use of variable i. 

public class expr {
  public static void main(String[] args) {
    char alpha = 'A';
    int i = 0;
    System.out.print(true  ? alpha  : 0);
    System.out.print(false ? i : alpha);
  }
}

Compliant Solution

This compliant solution recommends the use of same types for second and third operands. This helps avoid confusion due to clearer semantics.

public class expr {
  public static void main(String[] args) {
    char alpha = 'A';
    char i = 0;        //declare as char
    System.out.print(true  ? alpha  : 0);
    System.out.print(false ? i : alpha);
  }
}

Another solution is to declare the offending type as final. As a result, it turns into a constant expression and numeric promotion does not occur.

public class expr {
  public static void main(String[] args) {
    char alpha = 'A';
    int i = 0;
    System.out.print(true  ? alpha  : 0);
    System.out.print(false ? i : alpha);
  }
}

References

JLS 15.25 Conditional Operator ? :
Java Puzzlers, Traps, Pitfalls, Corner Cases 2.8

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