Hashmap的get方法中的NullPointerException

Question

I have a NullPointerException on this line:

 int qty = mSelectedBottles.get(bottleID);

I've checked that mSelectedBottles and bottleID are both NOT null.

mSelectedBottles is of type Hashmap<Integer, Integer> and bottleID is of type int

Answer 1

Because, Unboxing of a null value to primitive datatype. Here's enough code to recreate that NullPointerException yourself

int x;
HashMap<String,Integer> map = new HashMap<String,Integer>();
x = map.get("hello");

The auto-unboxing of the non-existent value was the issue.

We know autoboxing was introduced to make coding easier, but it's a definite performance anti-pattern and can lead to annoying bugs like this which are non-intuitive.

Personally, I always try to remove autoboxing by making any such calls explicit, so that although the code is a little uglier, it's also clearer what's happening.

You have to check mSelectedBottles.get(bottleID) is null or not before unboxing it to primitive int

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