Widening and boxing with Java

Question

In Java programming language widen and boxing doesn't work, but how does it work in following example?

final short myshort = 10;
Integer iRef5 = myshort;

Why does this work? Is this not the same as widen and then box?

But if I write the following code:

final int myint = 10;
Long myLONG = myint;

why it doesn't work?

What compiler are you using to get to your results? It seems to work with the Eclipse compiler, but not with the `javac` compiler — Lukas Eder, Dec 22 '11 at 09:07
I am not able to get either one of them to work (using ideone.com). — Simon Forsberg, Sep 18 '15 at 06:51

score 2 · Answer 1 · answered Dec 22 '11 at 09:06

2

Following what others have said, I can confirm that I can compile your first example with the Eclipse compiler, but not the second. With the javac compiler, both don't compile, as stated by Vlad

This seems to be a bug in either compiler! Let's consult the JLS to find out, which one is right :-)

answered Dec 22 '11 at 09:06

Lukas Eder

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The eclipse compiler has some known and very subtle bugs that are not present on `javac`. I remember another one with the use of generics that left us wondering for a while. – lsoliveira Dec 22 '11 at 11:07
@lsoliveira: You may be right, see also [this issue](http://stackoverflow.com/questions/5361513/reference-is-ambiguous-with-generics). However, it also happened the other way round in the past. – Lukas Eder Dec 22 '11 at 11:22
I guess that is always a possibility (although it never happened to me or anyone I know :-)). – lsoliveira Dec 22 '11 at 11:41

Vlad · Answer 2 · 2011-12-22T08:58:06.457

1

Neither works as it is (using javac 1.6.0_26 from Sun/Oracle, on Linux). See also here.

b.java:4: incompatible types
found   : short
required: java.lang.Integer
        Integer iRef5 = myshort; 
                        ^
b.java:7: incompatible types
found   : int
required: java.lang.Long
        Long myLONG = myint;
                      ^
2 errors

edited Dec 22 '11 at 08:58

answered Dec 22 '11 at 08:51

Vlad

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The first example works for me in Java 6 (Eclipse compiler)...? – Lukas Eder Dec 22 '11 at 08:52
Could it be they behave differently? I'm using Sun's compiler, see [here](http://ideone.com/6W7xi) – Vlad Dec 22 '11 at 08:55
Could be. I've seen differences in the past, but not with such simple cases, see [this question, for instance](http://stackoverflow.com/questions/5361513/reference-is-ambiguous-with-generics) – Lukas Eder Dec 22 '11 at 09:00

score 1 · Answer 3 · answered Dec 22 '11 at 08:51

1

You can either widen or box, but you can't do both.

You can do

final int myint = 10;
Long myLONG = (long) myint;

answered Dec 22 '11 at 08:51

Peter Lawrey

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There has to be a subtlety with implicit casting/promotion of `short` to `int` applied to the first example, before auto-boxing. Do you know by which rule? – Lukas Eder Dec 22 '11 at 08:54
`Object myLONG = 10L;` is perfectly valid – jmj Dec 22 '11 at 08:56
@LukasEder It doesn't compile for me as Vlad also states. ;) – Peter Lawrey Dec 22 '11 at 08:57
@JigarJoshi I don't think widening a reference is what the OP means. `Long l = (Integer) 10;` doesn't compile. – Peter Lawrey Dec 22 '11 at 08:59
`Long` and `Integer` are not in hierarchy to be widen – jmj Dec 22 '11 at 08:59
Jeez, I'm about to go crazy about those little differences between the `javac` and Eclipse compilers... – Lukas Eder Dec 22 '11 at 09:01
@JigarJoshi But `long` and `int` are. – Peter Lawrey Dec 22 '11 at 09:02
1

you can do `int i = 10; long l = i; ` this is primitive widening but, if you do `Long l = 10;` this won't compile it should be long only with `L` at the end – jmj Dec 22 '11 at 09:04

Sumit Singh · Accepted Answer · 2015-09-18T07:04:58.880

1

With java 7 both the examples are not working. you will get below exception:

Type mismatch: cannot convert from short to Integer
Type mismatch: cannot convert from int to Long

Because the problem is not because of boxing but because of conversion.

edited Sep 18 '15 at 07:04

answered Dec 22 '11 at 08:51

Sumit Singh

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score 1 · Answer 5 · answered Dec 22 '11 at 11:02

Let's examine what you're trying to do in detail:

final short myshort = 10;
Integer iRef5 = myshort;

The compiler will try to first box that short into an object, so that it can then perform the assignment (it cannot widen directly, since it is dealing with different types: an object and a primitive).

In short, this is equivalent to:

final short myshort = 10;
final Short box = new Short(myshort);  // boxing: so that objects are assignable.
Integer iRef5 = box;  // widening: this fails as Integer is not a superclass of Short

The same reasoning can be applied to your second example (which also fails), as is visible here. If your compiler does not complain on the first one, then there might be a bug with the compiler, because this is what's defined in the JLS. See the complete set of rules for conversion/promotion in the JLS here.

Bala · Answer 6 · 2011-12-22T09:00:58.277

These are the promotion rules that apply to expressions in Java

all byte and short values are promoted to int

If one operand is long,the whole expression is promoted to long

If one operand is a float ,the whole expression is promoted to float

If one operand is double ,the whole expression is promoted to double

Hence short value is promoted to int. This is not widening. "The conversion of a subtype to one of its supertypes is called widening"

score 0 · Answer 7 · edited May 23 '17 at 11:52

0

You can box then wide but you can't widen and then box

So

final short myshort = 10;
Integer iRef5 = myshort;

is equivalent to

final short myshort = 10;
Integer iRef5 = 10;

which is perfectly valid

but

Long myLONG = 10;//this won't compile ,

But try with 10L it will so it will box then you can have it object too i.e. Object o = 10L;

See also

edited May 23 '17 at 11:52

Community

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answered Dec 22 '11 at 08:55

jmj

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Are you sure the first example is perfectly valid? There seems to be a difference between the Eclipse compiler and the javac compiler. Which one is right? – Lukas Eder Dec 22 '11 at 09:07

Widening and boxing with Java

7 Answers7