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Wiki MarkupThe conditional operator {{?:}} uses the {{boolean}} value of one expression to decide which of the other two expressions should be evaluated \[[JLS 05|AA. Java References#JLS 05]\].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)).of its first operand to decide which of the other two expressions will be evaluated. (See §15.25, "Conditional Operator ? :" of the Java Language Specification (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 rules used by a Java compiler to determine the type of the result of a conditional expression (see following table) are complicated; programmers could be surprised by the type conversions required for expressions they have written.

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 quite complicated and may result in unexpected type conversions. The rules used to determine the result type of a conditional expression are given in the following table, where the first matching rule, starting from the top, is used. In the table, * constant int refers to constant expressions of type int (such as '0' or variables declared final), Operand 2 refers to operand2 in the general form of a Java conditional given above, and Operand 3 refers to operand3:.

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 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].

Determining the Result Type of a Conditional Expression

Noncompliant Code Example

This noncompliant example prints A6565 instead of AAA.

...

The complexity of the rules that determine the result type of a conditional expression can lead to 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 System.out.print(true  ? alpha  : 0);code. Value of i may change 
    boolean trueExp = true; // Some expression that evaluates to true
    System.out.print(true trueExp ? alpha  : 123450); // prints A
    System.out.print(falsetrueExp ? ialpha : alphai); // 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 recommends the use of the same uses identical types for the second and third operands of the conditional expressions. The clearer semantics help avoid confusion.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';
    charint i = 0;
    boolean trueExp = true; //declare as char Expression that evaluates to true
    System.out.print(truetrueExp  ? alpha : :((char) 0)); // Prints A
    // CastDeliberate alphanarrowing ascast anof inti; topossible explicitlytruncation stateOK
 that the type of the 
    // conditional expression should be int.
    System.out.print(true  ? ((int) alpha)  : 12345);
    System.out.print(false ? i : alpha 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-J-EX0 of NUM12-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):

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());
  }
}

Note that while casting 12345 to type char would ensure that both operands The combination of values of types short and int in the second nonconforming argument of the conditional expression have the same type, it would result in data loss when 12345 is converted to a char. Therefore the conforming example casts alpha to int, the wider of the operand types.

Risk Assessment

If the types of the second and third operands in a conditional expression are not the same then the result of the conditional expression may be unexpected.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[JLS 05|AA. Java References#JLS 05]\] [Section 15.25, Conditional Operator ? :|http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.25]
\[[Bloch 05|AA. Java References#Bloch 05]\] Puzzle 8: Dos Equis

(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 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.

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.

Automated Detection

Bibliography

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Image Added Image Added Image Added03. Expressions (EXP)      03. Expressions (EXP)      EXP01-J. Ensure a null pointer is not dereferenced