What happens with LinkedHashMap elements when I convert them to Set?

LinkedHashMap<String,ArrayList<String>> h;

It contains:

key 1 : value 开发者_开发技巧A, B, C, D
2 : E,F
3 : G

I don't quite understand the Set interface. Maybe a little visualization would help. Please help me to visualize how would all these elements inside LinkedHashMap look like once I convert them into Set?

Set set = h.entrySet();

Sorry, maybe my question is a little obscure. I will try to sharpen it as the thread develops.

The entry-set of a Map is the set of entries in the map.

An entry in a map is a mapping from key to value. In your case where the map is of type ...<String, ArrayList<String>>, an single entry is a pair of a String (the key) and an ArrayList<String> (the value).

The following snippet may shed some light on this:

LinkedHashMap<String, List<String>> map =
        new LinkedHashMap<String, List<String>>();

map.put("1", Arrays.asList("A", "B", "C"));
map.put("2", Arrays.asList("E", "F"));
map.put("3", Arrays.asList("G"));

System.out.println(map.entrySet());
System.out.println(map.entrySet().getClass());

Output:

[1=[A, B, C], 2=[E, F], 3=[G]]     # All entries (key/value pairs)
class java.util.HashMap$EntrySet   # This particular entry set class.

In this case for instance, the entry set is actually an inner class of HashMap, named EntrySet.

The entrySet() is just the entries (key-value pairs) in the map. It would help you to look at the type returned by entrySet(), which is Set<Map.Entry<String, ArrayList<String>>> in your example. See the Map.Entry javadoc.

Also note that the map is not converted to an entry set--rather, the entry set is a view of the entries in the map.

As far as what a Set is: it's a set of elements that are each unique according to some definition of unique (equals/hashCode for most Sets, Comparable/Comparator for SortedSets). Since a Map can have at most one value mapped to a single key, each key-value entry is guaranteed to be unique.