The conditional operator ?: uses the boolean value of one expression to decide which of the other two expressions should be evaluated. (see See JLS, Section 15.25, "Conditional Operator ? :".) .
The general form of a Java conditional expression is operand1 ? operand2 : operand3.
- If the value of the first operand (
operand1) istrue, 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-defined rules for determining the type of the result of a conditional expression (tabulated below) are quite complicated; programmers may could be surprised by the type conversions required for expressions they have written.
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Operand 2 | Operand 3 | Resultant type |
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type T | type T | type T |
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other numeric | other numeric | promoted type of the 2nd and 3rd operands |
T1 = boxing conversion (S1) | T2 = boxing conversion(S2) | apply capture conversion to lub(T1,T2) |
See JLS, Section 5.1.7, "Boxing Conversion,"; JLS, Section 5.1.10, "Capture Conversion"; and JLS, Section 15.12.2.7, "Inferring Type Arguments Based on Actual Arguments" for additional information on the final table entry.
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This noncompliant code example prints the value of alpha as A, which is of the char type. The third operand is a constant expression of type int, whose value 0 can be represented as a char; numeric promotion is unnecessary. However, this behavior depends on the particular value of the constant integer expression; changing that value may can lead to different behavior.
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This noncompliant example prints 65 — the {{65}}â”the ASCII equivalent of A — , instead of the expected A, because the second operand (alpha) must be promoted to type int. The numeric promotion occurs because the value of the third operand (the constant expression '12345') is too large to be represented as a char.
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The compliant solution explicitly states the intended result type by casting alpha to type int. Casting 12345 to type char would ensure that both operands of the conditional expression have the same type, and would result resulting in A being printed. However, it would result in data loss when an integer larger than Character.MAX_VALUE is downcast to type char. This compliant solution avoids potential truncation by casting alpha to type int, the wider of the operand types.
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When the second and third operands of a conditional expression have different types, they may can be subject to type conversions that were not anticipated by the programmer.
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