为什么非空列表会引发空指针异常？

Question

I have an List array 'details'.

List<HistoryDetails> details

where HistoryDetails is an object containing strings.

When I check the size it gives me a positive value.

details.size()

But when I try to access an element it throws a null pointer exception.

details.get(0).getFirstElement();


Attempt to invoke virtual method 'java.lang.String com.HistoryDetails.getFirstElement()' on a null object reference

I am invoking the element at the very next line to where I check the size. Hence, nothing should be reset. What could be wrong?

Answer 1

It looks like details is a List , but the first element of the List is null. That is, it looks like details.get(0) == null .

Answer 2

A List details having many items can have first item a null object. before using that item check null.

if(details.get(0)!=null){
details.get(0).getFirstElement();
}

Answer 3

因为它包含null。

Log.d("TEST", details.get(0)); //should return null

Answer 4

This will avoid the null pointer exception:

if (!details.isEmpty()) {
    details.get(0).getFirstElement();
}