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The 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))}}. 

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 rules (tabulated below) used by a Java compiler to determine the type of the result of a conditional expression are 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 of the table, is usedapplied. In the table, * 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:

Due to Because of the complicated nature of the rules used to determine the result type of a conditional expression and the potential for possibility of unintended type casting, it is recommended that the second and third operands of the conditional expression should always explicitly have the same type. This also applies to boxed primitives.

Noncompliant Code Example

The print statement This noncompliant code example prints the value of alpha as A, which is of the char type. The third operand '0', is a constant expression of type int whose value can be represented as a char and hence does not cause any numeric promotion. However, this behavior depends on the value of the constant integer expression. Changing the value of the constant integer expression may lead to different behavior, as will be demonstrated in the second noncompliant code example.

...

This noncompliant example prints 65 instead of the expected A. The print statement prints 65, is the integer ASCII equivalent of A. This is happens because of the numeric promotion of the second operand alpha to an int, which happens . The numeric promotion occurs because the third operand , (the constant expression '12345', ) is an int that cannot be of type int and consequently, inappropriate for being represented as a char.

public class Expr {
  public static void main(String[] args) {
    char alpha = 'A';
    System.out.print(true  ? alpha  : 12345);
  }
}

...

The compliant solution casts alpha to int to for explicitly state stating the result type (int) of the conditional expression. Note that while While casting 12345 to type char would ensure ensures that both operands in the second nonconforming conditional expression have the same type (and result in A being printed), it would result results in data loss when 12345 is converted an integer larger than Character.MAX_VAUE is downsized to a char. Therefore the conforming example This compliant solution casts alpha to int, the wider of the operand types, to avoid this issue.

public class Expr {
  public static void main(String[] args) {
    char alpha = 'A';
    // Cast alpha as an int to explicitly state that the type of the 
    // conditional expression should be int.
    System.out.print(true  ? ((int) alpha)  : 12345);
  }
}

Noncompliant Code Example

This noncompliant code example prints 65 instead of A. The print statement prints 65. This is because of numeric promotion of the second operand alpha to an int, which happens because the third operand, variable i, is an int.

...

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

...

public class Expr {
  public static void main(String[] args) {
    Integer i = Integer.MAX_VALUE;
    Integer f = 0;        //declare as Integer
    System.out.print(true ? i : f);
  }
}

...