Differences in auto-unboxing between Java 6 vs Java 7

Question

I have noted a difference in auto unboxing behavior between Java SE 6 and Java SE 7. I\'m wondering why that is, because I can\'t find any documentation of changes in this b

Answer 1

You are right; to put it more simply:

Object o = new Integer(1234);
int x = (int) o;

This works in Java 7, but gives a compilation error in Java 6 and below. Strangely, this feature is not prominently documented; for example, it's not mentioned here. It's debatable if it's a new feature or a bug fix (or a new bug?), see some related info and discussion. The consensus seems to point to an ambiguity in the original spec, which led to a slightly incorrect/inconsistent implementation on Java 5/6, which was fixed in 7, because it was critical for implementation of JSR 292 (Dynamically Typed Languages).

Java autoboxing has now some more traps and surprises. For example

Object obj = new Integer(1234);
long x = (long)obj;

will compile, but fail (with ClassCastException) at runtime. This, instead, will work:

long x = (long)(int)obj;

Answer 2

It looks like the language in section 5.5 Casting Conversion of Java 7 JLS was updated in comparison to the same section in the Java 5/6 JLS, probably to clarify the allowed conversions.

Java 7 JLS says

An expression of a reference type may undergo casting conversion to a primitive type without error, by unboxing conversion.

Java 5/6:

A value of a reference type can be cast to a primitive type by unboxing conversion (§5.1.8).

The Java 7 JLS also contains a table (table 5.1) of allowed conversions (this table is not included in the Java 5/6 JLS) from reference types to primitives. This explicitly lists casts from Object to primitives as a narrowing reference conversion with unboxing.

The reason is explained in this email:

Bottom line: If the spec. allows (Object)(int) it must also be allowing (int)(Object).