这个 NullPointerException 是否指 getFoos 返回 null 或者从 Collection 到 ArrayList 的转换存在问题？
为什么此行会导致 NullPointerException：
List<Foo> foos = new ArrayList<Foo>(userDetailsService.getFoos(currentUser));
getFoos 方法仅返回 Foo 的 Collection：
public Collection<Foo> getFoos(final String username)
我可以'判断 NullPointerException 是否引用 getFoos 返回 null 或从 Collection 到 Collection 的转换出现问题代码>ArrayList。
Why does this line cause a NullPointerException:
List<Foo> foos = new ArrayList<Foo>(userDetailsService.getFoos(currentUser));
The getFoos method simply returns a Collection of Foos:
public Collection<Foo> getFoos(final String username)
I can't tell if the NullPointerException refers to getFoos returning null or some problem with the cast from a Collection to an ArrayList.
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取决于堆栈跟踪。前往第一行。如果它指向
java.util.ArrayList.，则getFoos()返回null。或者，如果它指向问题中的特定行，则userDetailsService为null。Depends on the stacktrace. Head to the first line. If it points to
java.util.ArrayList.<init>, thengetFoos()has returnednull. Or if it points to the particular line in your question, thenuserDetailsServiceisnull.这就是为什么我们有德墨忒尔法则。如果将连接在一起的两个语句分开，解决问题的运气会更好。
但是，很明显，返回的 null 集合（不是空集合）是导致该问题的原因。毕竟，使用空集合调用 ArrayList 构造函数不会导致空指针异常。
编辑：
我的错。 ArrayList 构造函数将尝试复制集合。所以空集合会导致空指针异常。我坚持最初的建议，将这两个语句脱钩。按以下方式编码将有助于更快地识别问题：
And that's why we have the Law of Demeter. If you separate the two statements that have been conjoined together, you would have better luck in resolving the issue.
However, it should be apparent that a null collection (not an empty collection) is returned which is responsible for the issue. After all, invoking the ArrayList constructor with a null collection will not result in a null pointer exception.
EDIT:
My bad. The ArrayList constructor will attempt to copy the collection. So a null collection can cause a null pointer exception. I stick to my original recommendation of decoupling the two statements. Coding in the following manner will help identify the issue faster:
失败的类型转换不会给您一个 NullPointerException 异常。
对
null进行类型转换根本不会引发异常。因此，
getFoo返回null的理论是合理的。但也可能是userDetailsService为null或 NPE 被抛出到getFoo中。您应该能够通过仔细阅读堆栈跟踪来区分这些情况...您没有将其包含在问题中（tsk，tsk）。
A failed typecast cannot give you a
NullPointerException.Typecasting a
nullwill not throw an exception at all.Therefore the theory that
getFoois returning anullis plausible. But it could also be thatuserDetailsServiceisnullor that the NPE is thrown ingetFoo.You should be able to distinguish these cases by carefully reading the stacktrace ... which you didn't include in your question (tsk, tsk).