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.
I'd would like to use a collection of "key - value" pairs:
Which is the base for a JTable data model (TableModel implementation), so I need to access elements by their index/position (e.g. to implement Object getValueAt(int rowIndex, int columnIndex)). The collection must preserve the order of the elements. An ArrayList would be good for that.
Which allows to retrieve elements by the value of the key. A HashMap would be good for that.
The closest usable collection I've found is LinkedHashMap. It preserves the order and allows retrieval by key. However the access to an item is not possible by index/position, I must iterate over the items until the good one is found. This is not be time-effective.
Is there a better one than this one?
Thanks.
(Question is similar to this one, but the solution provided uses the conversion toArray() which is not time-effective. If the set of pairs changes, the conversion needs to be done again.)
There is no such collection implementation in the JRE.
But you can easily overcome this issue by using a Map<K,V> for storing the key-value pairs and an additional List<K> to store the keys in sequential order. You can then access a value either by key or by index using: keyValueMap.get(keys.get(index)).
You must make sure that modifications are always synchronized on both collections:
Add/Change an entry: if (keyValueMap.put(key, value) == null) keys.add(key)
Remove an entry: if (keyValueMap.remove(key) != null) keys.remove(key)
Note that this implementation assumes that values are never null. In case null values are required too, the code gets slightly more complex as we have to check for existence of an entry using keyValueMap.contains(key).
Without implementing your own collection, it might be easiest to just have two collections, with the key-value reversed in the second one but otherwise ordered the same.
(wanted this to be a comment, but not enough rep yet)
Apache Commons Collection has a ListOrderedMap class that makes what you want.
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.
Apart from the fact that HashSet does not allow duplicate values, what is the difference between HashMap and HashSet?
I mean implementation wise? It's a little bit vague because both use hash tables to store values.
HashSet is a set, e.g. {1,2,3,4,5}
HashMap is a key -> value (key to value) map, e.g. {a -> 1, b -> 2, c -> 2, d -> 1}
Notice in my example above that in the HashMap there must not be duplicate keys, but it may have duplicate values.
In the HashSet, there must be no duplicate elements.
They are entirely different constructs. A HashMap is an implementation of Map. A Map maps keys to values. The key look up occurs using the hash.
On the other hand, a HashSet is an implementation of Set. A Set is designed to match the mathematical model of a set. A HashSet does use a HashMap to back its implementation, as you noted. However, it implements an entirely different interface.
When you are looking for what will be the best Collection for your purposes, this Tutorial is a good starting place. If you truly want to know what's going on, there's a book for that, too.
HashSet
HashSet class implements the Set interface
In HashSet, we store objects(elements or values)
e.g. If we have a HashSet of string elements then it could depict a
set of HashSet elements: {“Hello”, “Hi”, “Bye”, “Run”}
HashSet does not allow duplicate elements that mean you
can not store duplicate values in HashSet.
HashSet permits to have a single null value.
HashSet is not synchronized which means they are not suitable for thread-safe operations until unless synchronized explicitly.[similarity]
add contains next notes
HashSet O(1) O(1) O(h/n) h is the table
HashMap
HashMap class implements the Map interface
HashMap is
used for storing key & value pairs. In short, it maintains the
mapping of key & value (The HashMap class is roughly equivalent to
Hashtable, except that it is unsynchronized and permits nulls.) This
is how you could represent HashMap elements if it has integer key
and value of String type: e.g. {1->”Hello”, 2->”Hi”, 3->”Bye”,
4->”Run”}
HashMap does not allow duplicate keys however it allows having duplicate values.
HashMap permits single null key and any number of null values.
HashMap is not synchronized which means they are not suitable for thread-safe operations until unless synchronized explicitly.[similarity]
get containsKey next Notes
HashMap O(1) O(1) O(h/n) h is the table
Please refer this article to find more information.
It's really a shame that both their names start with Hash. That's the least important part of them. The important parts come after the Hash - the Set and Map, as others have pointed out. What they are, respectively, are a Set - an unordered collection - and a Map - a collection with keyed access. They happen to be implemented with hashes - that's where the names come from - but their essence is hidden behind that part of their names.
Don't be confused by their names; they are deeply different things.
The Hashset Internally implements HashMap. If you see the internal implementation the values inserted in HashSet are stored as keys in the HashMap and the value is a Dummy object of Object class.
Difference between HashMap vs HashSet is:-
HashMap contains key value pairs and each value can be accessed by key where as HashSet needs to be iterated everytime as there is no get method.
HashMap implements Map interface and allows one null value as a key and multiple null values as values, whereas HashSet implements Set interface, allows only one null value and no duplicated values.(Remeber one null key is allowed in HashMap key hence one null value in HashSet as HashSet implemements HashMap internally).
HashSet and HashMap do not maintain the order of insertion while iterating.
HashSet allows us to store objects in the set where as HashMap allows us to store objects on the basis of key and value. Every object or stored object will be having key.
As the names imply, a HashMap is an associative Map (mapping from a key to a value), a HashSet is just a Set.
Differences between HashSet and HashMap in Java
1) First and most significant difference between HashMap and HashSet is that HashMap is an implementation of Map interface while HashSet is an implementation of Set interface, which means HashMap is a key value based data-structure and HashSet guarantees uniqueness by not allowing duplicates.In reality HashSet is a wrapper around HashMap in Java, if you look at the code of add(E e) method of HashSet.java you will see following code :
public boolean add(E e)
{
return map.put(e, PRESENT)==null;
}
where its putting Object into map as key and value is an final object PRESENT which is dummy.
2) Second difference between HashMap and HashSet is that , we use add() method to put elements into Set but we use put() method to insert key and value into HashMap in Java.
3) HashSet allows only one null key, but HashMap can allow one null key + multiple null values.
That's all on difference between HashSet and HashMap in Java. In summary HashSet and HashMap are two different type of Collection one being Set and other being Map.
Differences between HashSet and HashMap in Java
HashSet internally uses HashMap to store objects.when add(String) method called it calls HahsMap put(key,value) method where key=String object & value=new Object(Dummy).so it maintain no duplicates because keys are nothing but Value Object.
the Objects which are stored as key in Hashset/HashMap should override hashcode & equals contract.
Keys which are used to access/store value objects in HashMap should declared as Final because when it is modified Value object can't be located & returns null.
A HashMap is to add, get, remove, ... objects indexed by a custom key of any type.
A HashSet is to add elements, remove elements and check if elements are present by comparing their hashes.
So a HashMap contains the elements and a HashSet remembers their hashes.
A HashSet uses a HashMap internally to store its entries. Each entry in the internal HashMap is keyed by a single Object, so all entries hash into the same bucket. I don't recall what the internal HashMap uses to store its values, but it doesn't really matter since that internal container will never contain duplicate values.
EDIT: To address Matthew's comment, he's right; I had it backwards. The internal HashMap is keyed with the Objects that make up the Set elements. The values of the HashMap are an Object that's just simply stored in the HashMap buckets.
Differences:
with respect to heirarchy:
HashSet implements Set.
HashMap implements Map and stores a mapping of keys and values.
A use of HashSet and HashMap with respect to database would help you understand the significance of each.
HashSet: is generally used for storing unique collection objects.
E.g: It might be used as implementation class for storing many-to-one relation ship between
class Item and Class Bid where (Item has many Bids)
HashMap: is used to map a key to value.the value may be null or any Object /list of Object (which is object in itself).
A HashSet is implemented in terms of a HashMap. It's a mapping between the key and a PRESENT object.
HashMap is a Map implementation, allowing duplicate values but not duplicate keys.. For adding an object a Key/Value pair is required. Null Keys and Null values are allowed. eg:
{The->3,world->5,is->2,nice->4}
HashSet is a Set implementation,which does not allow duplicates.If you tried to add a duplicate object, a call to public boolean add(Object o) method, then the set remains unchanged and returns false. eg:
[The,world,is,nice]
Basically in HashMap, user has to provide both Key and Value, whereas in HashSet you provide only Value, the Key is derived automatically from Value by using hash function. So after having both Key and Value, HashSet can be stored as HashMap internally.
HashSet and HashMap both store pairs , the difference lies that in HashMap you can specify a key while in HashSet the key comes from object's hash code
HashMaps allow one null key and null values. They are not synchronized, which increases efficiency. If it is required, you can make them synchronized using Collections.SynchronizedMap()
Hashtables don't allow null keys and are synchronized.
The main difference between them you can find as follows:
HashSet
It does not allow duplicate keys.
Even it is not synchronized, so this will have better performance.
It allows a null key.
HashSet can be used when you want to maintain a unique list.
HashSet implements Set interface and it is backed by the hash table(actually HashMap instance).
HashSet stores objects.
HashSet doesn’t allow duplicate elements but null values are allowed.
This interface doesn’t guarantee that order will remain constant over time.
HashMap
It allows duplicate keys.
It is not synchronized, so this will have better performance.
HashMap does not maintain insertion order.
The order is defined by the Hash function.
It is not Thread Safe
It allows null for both key and value.
It allows one null key and as many null values as you like.
HashMap is a Hash table-based implementation of the Map interface.
HashMap store object as key and value pair.
HashMap does not allow duplicate keys but null keys and values are allowed.
Ordering of the element is not guaranteed overtime.
EDIT - this answer isn't correct. I'm leaving it here in case other people have a similar idea. b.roth and justkt have the correct answers above.
--- original ---
you pretty much answered your own question - hashset doesn't allow duplicate values. it would be trivial to build a hashset using a backing hashmap (and just a check to see if the value already exists). i guess the various java implementations either do that, or implement some custom code to do it more efficiently.
HashMap is a implementation of Map interface
HashSet is an implementation of Set Interface
HashMap Stores data in form of key value pair
HashSet Store only objects
Put method is used to add element in map
Add method is used to add element is Set
In hash map hashcode value is calculated using key object
Here member object is used for calculating hashcode value which can be same for two objects so equal () method is used to check for equality if it returns false that means two objects are different.
HashMap is faster than hashset because unique key is used to access object
HashSet is slower than Hashmap
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.