问题
If I declare Arraylist like this-
private ArrayList<Integer[]> nodeList;
then, while adding array into it, getting NullPointerException
But, if I change it to-
private ArrayList<Integer[]> nodeList= new ArrayList<Integer[]>();
-it works fine.
Why the first one fails!
回答1:
The first only declares a variable, but does not create the actual object. only when you use new, you actually create the object.
In java unlike C++, declaring a variable does not allocate a local variable of it. To actually create the object, you need to explicitly create it [in your example: by using the new keyword].
(*)Note that this is only true to reference types objects, and java primitives are created with declaration.
来源：https://stackoverflow.com/questions/8067382/android-issue-during-arraylist-declaration