Is this really widening vs autoboxing?

Question

I saw this in an answer to another question, in reference to shortcomings of the Java spec:

There are more shortcomings and this is a subtle topic. Check

Answer 1

Yes it is, try it out in a test. You will see "long" printed. It is widening because Java will choose to widen the int into a long before it chooses to autobox it to an Integer, so the hello(long) method is chosen to be called.

Edit: the original post being referenced.

Further Edit: The reason the second option would print Integer is because there is no "widening" into a larger primitive as an option, so it MUST box it up, thus Integer is the only option. Furthermore, java will only autobox to the original type, so it would give a compiler error if you leave the hello(Long) and removed hello(Integer).

Answer 2

In the first case, you have a widening conversion happening. This can be see when runinng the "javap" utility program (included w/ the JDK), on the compiled class:

public static void main(java.lang.String[]);
  Code:
   0:   iconst_ 5
   1:   istore_ 1
   2:   iload_ 1
   3:   i2l
   4:   invokestatic    #6; //Method hello:(J)V
   7:   return

}

Clearly, you see the I2L, which is the mnemonic for the widening Integer-To-Long bytecode instruction. See reference here.

And in the other case, replacing the "long x" with the object "Long x" signature, you'll have this code in the main method:

public static void main(java.lang.String[]);
  Code:
   0:   iconst_ 5
   1:   istore_ 1
   2:   iload_ 1
   3:   invokestatic    #6; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
   6:   invokestatic    #7; //Method hello:(Ljava/lang/Integer;)V
   9:   return

}

So you see the compiler has created the instruction Integer.valueOf(int), to box the primitive inside the wrapper.

Answer 3

Another interesting thing with this example is the method overloading. The combination of type widening and method overloading only working because the compiler has to make a decision of which method to choose. Consider the following example:

public static void hello(Collection x){
   System.out.println("Collection");
}

public static void hello(List x){
   System.out.println("List");
}

public static void main(String[] args){
   Collection col = new ArrayList();
   hello(col);
}

It doesn't use the run-time type which is List, it uses the compile-time type which is Collection and thus prints "Collection".

I encourage your to read Effective Java, which opened my eyes to some corner cases of the JLS.