So everybody thinks this is normal?

[JosAH]

I bumped my reply from another thread just because I don't believe that everybody
thinks this is normal. Have a close read:

I don't like autoboxing very much; below you'll see an example derived from an
original example by someone else posted in Sun's Java forum quite a while ago:

import java.util.*;
 
public class Autoboxing {
 
    public static void main(String[] args) {
 
        Object foo = new Long(0xcafebabedeadbeefL);
        List<Object> bars = new ArrayList<Object>();
 
        bars.add(bool() ? (Long)foo : (Number)foo);
        bars.add(bool() ? (Long)foo : (Long)foo);
        bars.add(bool() ? (Long)foo : (Double)foo);
        bars.add(bool() ? (Long)foo : ((Long)foo).longValue());
 
        System.out.print("==    :");
        for (Object bar : bars)
            System.out.print(" "+(foo == bar));
        System.out.println();
 
        System.out.print("equals:");
        for (Object bar : bars)
            System.out.print(" "+foo.equals(bar));
        System.out.println();
    }
 
    private static boolean bool() {
        return true;
    }
}

Output:

==    : true true false false
equals: true true false true
 

That's why I don't like autoboxing very much ...

kind regards,

Jos

[r035198x]

Is that what they were talking about in

Ideally, boxing a given primitive value p, would always yield an identical reference. In practice, this may not be feasible using existing implementation techniques. The rules above are a pragmatic compromise. The final clause above requires that certain common values always be boxed into indistinguishable objects. The implementation may cache these, lazily or eagerly.

For other values, this formulation disallows any assumptions about the identity of the boxed values on the programmer's part. This would allow (but not require) sharing of some or all of these references.

[JosAH]

Is that what they were talking about in

Not really; it's the ternary operator that spoils all that nice hulla baloo:

The type of a conditional expression is determined as follows:


If the second and third operands have the same type (which may be the null type), then that is the type of the conditional expression.
If one of the second and third operands is of type boolean and the type of the other is of type Boolean, then the type of the conditional expression is boolean.
If one of the second and third operands is of the null type and the type of the other is a reference type, then the type of the conditional expression is that reference type.
Otherwise, if the second and third operands have types that are convertible (§5.1.8) to numeric types, then there are several cases:
If one of the operands is of type byte or Byte and the other is of type short or Short, then the type of the conditional expression is short.
If one of the operands is of type T where T is byte, short, or char, and the other operand is a constant expression of type int whose value is representable in type T, then the type of the conditional expression is T.
If one of the operands is of type Byte and the other operand is a constant expression of type int whose value is representable in type byte, then the type of the conditional expression is byte.
If one of the operands is of type Short and the other operand is a constant expression of type int whose value is representable in type short, then the type of the conditional expression is short.
If one of the operands is of type; Character and the other operand is a constant expression of type int whose value is representable in type char, then the type of the conditional expression is char.
Otherwise, binary numeric promotion (§5.6.2) is applied to the operand types, and the type of the conditional expression is the promoted type of the second and third operands. Note that binary numeric promotion performs unboxing conversion (§5.1.8) and value set conversion (§5.1.13).
Otherwise, the second and third operands are of types S1 and S2 respectively. Let T1 be the type that results from applying boxing conversion to S1, and let T2 be the type that results from applying boxing conversion to S2. The type of the conditional expression is the result of applying capture conversion (§5.1.10) to lub(T1, T2) (§15.12.2.7).
At run time, the first operand expression of the conditional expression is evaluated first; if necessary, unboxing conversion is performed on the result; the resulting boolean value is then used to choose either the second or the third operand expression:

If the value of the first operand is true, then the second operand expression is chosen.
If the value of the first operand is false, then the third operand expression is chosen.
The chosen operand expression is then evaluated and the resulting value is converted to the type of the conditional expression as determined by the rules stated above. This conversion may include boxing (§5.1.7) or unboxing conversion. The operand expression not chosen is not evaluated for that particular evaluation of the conditional expression

A Long isn't a Long when one of the other operands happens to be a Double etc.

funny eh?

Jos

[r035198x]

Not really; it's the ternary operator that spoils all that nice hulla baloo:



A Long isn't a Long when one of the other operands happens to be a Double etc.

funny eh?

Jos

At least they wrote it down.
Determining the type of an expression was always going to involve some voodoo somewhere.

[JosAH]

At least they wrote it down.
Determining the type of an expression was always going to involve some voodoo somewhere.

Not in C or C++ and not in Java before that darn autoboxing that is. But I guess
most of the people take it all for granted until something goes wrong ;-)

kind regards,

Jos

[r035198x]

Not in C or C++ and not in Java before that darn autoboxing that is. But I guess
most of the people take it all for granted until something goes wrong ;-)

kind regards,

Jos

What? You expect them all to read (whatever that word means) that JLS?

[JosAH]

What? You expect them all to read (whatever that word means) that JLS?

Mwah, not me; you're the optimist of the group here ;-)

kind regards,

Jos