I thought I understood the ins and outs of auto(un)boxing, until I thought of this simple situation:

When this code runs it outputs "true." I guessed that it would output "false," since I assumed that i would be autoboxed into a new Integer, and that the two Integer objects (their values being >127) would be separate. Why does this example seem to prefer the unboxing over the boxing? What is the rule?

For reference, section 5.6.2 of the Java Language Specification defines binary numeric promotion.

5.6.2 Binary Numeric Promotion When an operator applies binary numeric promotion to a pair of operands, each of which must denote a value that is convertible to a numeric type, the following rules apply, in order, using widening conversion (�5.1.2) to convert operands as necessary:

* If any of the operands is of a reference type, unboxing conversion (�5.1.8) is performed. Then: * If either operand is of type double, the other is converted to double. * Otherwise, if either operand is of type float, the other is converted to float. * Otherwise, if either operand is of type long, the other is converted to long. * Otherwise, both operands are converted to type int.

After the type conversion, if any, value set conversion (�5.1.13) is applied to each operand.

Binary numeric promotion is performed on the operands of certain operators:

* The multiplicative operators *, / and % (�15.17) * The addition and subtraction operators for numeric types + and - (�15.18.2) * The numerical comparison operators <, <=, >, and >= (�15.20.1) * The numerical equality operators == and != (�15.21.1) * The integer bitwise operators &, ^, and | (�15.22.1) * In certain cases, the conditional operator ? : (�15.25)

SCJP 5.0 77%

Matthew Alesi
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Joined: Sep 13, 2006
Posts: 38

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Thanks a ton, this clears a lot up!!

Don't get me started about those stupid light bulbs.