Collections.newSetFromMap使用场景

在看Spring源码的时候，偶然看到了BeanUtils.java类中有

private static final Set<Class<?>> unknownEditorTypes =
            Collections.newSetFromMap(new ConcurrentReferenceHashMap<>(64));

之前没看到过newSetFromMap这种用法，基本上都是直接new HashSet。
进入Collections类查看newSetFromMap方法的注释：

    /*
    *Returns a set backed by the specified map. The resulting set displays the same ordering, concurrency, and performance characteristics as the backing map. In essence, this factory method provides a Set implementation corresponding to any Map implementation. There is no need to use this method on a Map implementation that already has a corresponding Set implementation (such as HashMap or TreeMap).
    *Each method invocation on the set returned by this method results in exactly one method invocation on the backing map or its keySet view, with one exception. The addAll method is implemented as a sequence of put invocations on the backing map.
    *The specified map must be empty at the time this method is invoked, and should not be accessed directly after this method returns. These conditions are ensured if the map is created empty, passed directly to this method, and no reference to the map is retained, as illustrated in the following code fragment:
    *          Set