unboxing, object reference refer to heap vs "class reference"

[wei liu]

Can someone help me to understand this paragraph from SCJP book

SCJP sun certified programmer for java 6 exam 310-065 book page 606 in the exam watch

"Remember that unboxing can't convert a plain old object to a primitive, even if that object reference refers to an integer(or some other wrapped primitive) on the heap.
unboxing converts onoly from a wrapper class reference (like an Integer or Long) to primitive."

I am quite confused about the concept between object reference refers to an integer on the heap vs unboxing converts onoly from a wrapper class reference (like an Integer or Long) to primitive., Can someone provide some examples to help me understand the difference between them.


thanks in advance

[Harpreet Singh janda]

Means unboxing only works if you are having an object of some wrapper class like :

  Integer i = new Integer(12)  
int j =i;

Here i is a wrapper class and we can directly assign i to and integer (primitive) variable.

But this will not work if you have an object of Object class which is referring to an object of Wrapper class like:

   		Object i = new Integer(12);
		int j = i; // Compile time error - Can not convert from an Object to int