nul的用法

Because the WordNet documents don't contain any relative URIs, this parameter can be given as null.

因为WordNet文档不包含任何相对URI，所以此参数可以指定为null。

If the weak reference has been cleared ( either because the referent has already been garbage collected or because someone called WeakReference. clear()), get() returns null.

如果弱引用被清除了（不管是referent已经被垃圾收集了，还是有人调用了WeakReference.clear（）），get（）会返回null。

Dead mappings could be eliminated by periodically scanning the Map, calling get() on each weak reference, and removing the mapping if get() returns null.

可以通过周期性地扫描Map，对每一个弱引用调用get（），并在get（）返回null时删除那个映射而消除死映射。