Java Map的非支持键集

Question

Given the following Java HashMap :

HashMap<String, Integer> map = new HashMap<String, Integer>();

The following statement gives me a 'backed' set of the maps keys:

Set<Integer> keys = map.keySet();

but suppose I'd like a copy of the key set which I can manipulate without affecting the map. Is there a better/more correct way than

Set<Integer> keys = new HashSet<Integer>();
for( Integer key : map.keySet() )
    keys.add( key );

?

Answer 1

There's a slightly simpler way:

Set<Integer> keys = new HashSet<Integer>(map.keySet());

...which just makes a copy in a single line.

Answer 2

Set<Integer> keys = new HashSet<Integer>(map.keySet());

这将初始化一个Set，该Set包含与传递给HashSet构造函数的地图键集中的元素完全相同的元素。