I have a Map<String, List<List<Double>>> with only one key, e.g.
{onlyKey=[[1.0, 2.3, 20.1], [6.5, -9.3, 4.5]]}
But I don't actually know what the name of the key is.
How can I get the name of the only key stored within the map?
Answered here.
I know that keySet exists, but I already tried map.keySet().get(0) and Set doesn't have a get(int) method. Pretty sure sets are generally used when you know (or have some idea of) what they contain.
The Map<First, Second> has the method keySet() that returns a Set<First> of the key.
Also: There's entrySet() that returns a Set<Entry<First, Second>>.
Note that return is a Set and not a List, the order of insertion is not preserved nor you have access to get(int index), you need to iterate over a set to get a first value.
From the Map Documentation you can see you can use keySet() to recover the Set of all keys in your Map, which, in your case, will contain your only key.
As an alternative for using iterator you might use stream() and findFirst():
map.keySet().stream().findFirst().get()
I used map.keySet().iterator().next() to get the only key within the map.
Related
Regarding the Map<K,V> interface:
Why does keySet() return a Set that supports the remove operation but doesn't support add() and addAll() operations?
The Set returned by keySet is backed by the Map, so changes to the map are
reflected in the set, and vice-versa. This means that calling remove on that Set removes the matching Entry from the Map.
It would make no sense to call add or addAll on that Set, since you can't add key[s] without corresponding value[s] to the Map.
Think about what you are asking for:
you want to retrieve all KEYS of a map (and that set is not a "copy" of the keys; it represents the keys of the map).
And then you ask to add elements to those KEYS. In other words: the "data set" you are looking at has the semantic meaning of keys coming from a map. And you want to increase that "data set" - but without providing the corresponding entries for that map.
Deletion on the other hand is straight forward; deleting a key will also delete the corresponding entry from the map.
It's because each key in the set is linked to a value in the map. Removing a key will remove the associated value, but to add you'll need a value and not just a key.
I have a ConcurrentSkipListMap of keys and values. It is very important to hold the order of the keys.
The problem appears when I try to insert a new value in a particular position. The only one method to insert a value is the put() that put this value in the last position.
With the replace method it can only edit the value, not the key.
Is it possible? What can I do? Can you tell me another class to do it?
The ConcurrentSkipListMap holds the order of the keys on its own as they are sorted.
So either you were not aware of it or you actually do not want to hold order of the keys but manipulate them yourself.
If you meant that you want the keys returned in the order in which they were put into the map than use the separet List (ConcurrentQueue if you need concurrency) and place the keys there manually.
I'd like to check if there are a certain number of values for a given key within a Multimap. How would I go about doing this?
Either multimap.get(key).size() or multimap.keys().count(key) will work. If it matters, the first will probably "waste" an object instance and the second will probably not.
you can perform a get(key) that will return a collection, over that collection you invoke size() to find out how many elements are on the Multimap associated with the given key.
From the documentation:
Collection get(K key) Returns a collection view of all values
associated with a key.
HashMap model1 = wordobject.getMap();
Set sample = model1.keySet();
Iterator it = sample.iterator();
==
Can you please explain me the above 3 lines?
I see that we are trying to get the hash table from the object and get it assigned to the HashMapmodel1.
1) what is keyset?
2) what does .iterator do?
You are declaring a typical Java HashMap in the first line (a little obvious). You usually construct a HashMap using generics for key value pairs: HashMap<K,V>
The Java API HashMap class allows you to get a set of the keys used for the HashMap. The keySet() method returns a Set<K>.
An iterator allows you to iterate through the set calling methods like next() and hasNext(). It is a way to traverse the set sequentially.
A Hashtable != HashMap
The documentation for Map#keyset() says Returns a Set view of the keys contained in this map
The documentation for Iterator in Java - a more general discussion can be found on Wikipedia
the ketSet() is clearly going to return a Set object (hence when we are instantiating the Set object, sample, with it's result). This Set contains all of the Key Values from the HashMap. Its type will be whatever type the Keys in the hasMap were. The iterator provides a way to step through the elements of the set.
oh and as someone pointed out, we are getting a HashMap from wordobject.
An iterator allows you to loop through the Set. A Set is like an ArrayList but does not allow you to index it. Quick google of set or iterator will give you some more info about them both. Here's a link that explains iterators: http://www.java-samples.com/showtutorial.php?tutorialid=235
1) keySet() is a method on map that returns all the keys for the map. Just to make it clearer, map is like a collection of pairs. i.e. each item in a map has a key and a value associated with it. Like a english dictionary, where each item in the dictionary is a word (the key) and a corresponding meaning (the value). So, keySet() will return a set of all the keys, i.e. the words in the dictionary.
2).iterator() returns an Iterator for the set. You can use the iterator "it" to iterate through the items in the set by using its methods like "next()", "hasNext()", "remove()".. etc..
Read up a little more Java docs to learn more.
Why doesn't Java provide functions to get the key/value pairs in a HashSet like exists in Hashtable? It seems like a real pain to have to iterate over it every time you need to get at something. Or is there an easier way to do this?
HashSet doesn't have key/value pairs. It is a Set of objects and you would use an implementer of Set to ensure that a collection of objects contained no duplicates.
Implementers of Map like HashMap have key/value pairs and provide a get(Object key) method to get the value associated with a key.
Since a Set doesn't contain keys and values, there is no way such a view could be provided.
What would you consider to be the key and what would be the value in a Set?
A Set don't have any key/value pairs, just (unique) values. As you already said you get these values via the Iterator or by returning an array with these values with the toArray() method.
Maybe you are looking for a List instead.