Matthew Alesi

Ranch Hand

Posts: 38

posted 10 years ago

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?

Thanks!

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?

Thanks!

-Matt

Current CS undergrad

SCJP 5.0

Keith Lynn

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Posts: 2409

Satish Kota

Ranch Hand

Posts: 88

posted 10 years ago

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 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)

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:

*

**Then:***If any of the operands is of a reference type, unboxing conversion (�5.1.8) is performed.** 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)

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