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Inboxing/outboxing clarification

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Can somebody point me on a document explaining this feature?
I use a construction:

It works perfectly when map value 'attendet' is ther, however gives me null ptr exception if there is no value. Ok, probably inboxing happens in this way:

Now I change order:

I'm getting again null ptr exception. Is now outboxing happens, like:

How to figure out a real expression after out/in boxing happen?
Is there a solution without checking for null or using explicit inboxing like:
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First, none of the code that you have posted is valid Java code. The single quote is used for characters, and 'attendet' is not a valid character. Anyway, assuming that you meant to use "attendet" instead...

Whenever a primative value is compared with a wrapper object, the wrapper object is *always* unboxed. Regardless, of whether the Boolean object is the first or the second operand in the comparison, the booleanValue() method will be used.

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