I know how hash map works. We can't use duplicate keys in Hashmap. I want to know the logic behind this used by Sun people. How it has been coded that we can't store duplicate key in HashMap.
It's part of the contract of the Map interface:
An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
What should Map.get(key) return if a key has multiple values?
An HashMap is a data structure where is possible to save couples of key values. This data structure implements Map.
From javadoc:
A map cannot contain duplicate keys; each key can map to at most one value.
It is possible to retrieve directly a value using its key, so is not possible to have more values associated to the same key.
Instead is possible to have multiple keys pointing to the same value.
If you need a different data structure where multiple values are associated to the same key you can use libraries like Guava that has the concept of Multimap
Note: Using standard Map is possible to create a Map<MyKey, List> where you associate a list to a key. So you can also add multiple values to that key storing them in the associated list.
Related
I have a
Stack<MyThread> rts;
each Thread contains a HashMap with a Custom Object Key.
HashMap<CustomObject> myMap = rts.pop().mapOfThread;
and I have the HashMap of these CustomObjectKeys
MyThread.CustomObjectMap
I need the fastest way to check if any of the HashMaps in the Threads in the Stack contains the same key as my CustomObject HashMap.
So, if I understand correctly, you have a number of disjoint HashMaps that may share keys, and you want to see if any keys are the same across any or all?
If so, I don't really see a solution other than iterating through the keySet() and checking each HashMap to see if the key is present, probably via containsKey().
In my code I am using a set of interleaved LinkedHashMaps inside each other as below. The code is fine and gives me the result I want except it automatically removes the duplicates. I couldnt find out how I can use TreeMap or Set in order to keep the duplicates.
LinkedHashMap<String, LinkedHashMap<Integer, LinkedHashMap<String, Vector<String>>>>
dataAll =new LinkedHashMap<String, LinkedHashMap<Integer, LinkedHashMap<String,
Vector<String>>>>();
LinkedHashMap is still a Map data structure. It maps a unique key to a value. If you assign two different values to a key the second value will simply replace the first value assigned to that key.
Also imagine why do you need a Map of duplicated key? The sole purpose of Map is to provide a one to one relationship between key/value pair. It does not handle one to many relationship.
If you have to map a key with a list of values, use something like:
LinkedHashMap<String, List<..>>
This allows you to have one key maps to a list of values.
I am trying to create a trigram model using LinkedHashMap>
where Entry is the entry of last inputed bigram (whose structure is:
LinkedHashMap
Now the problem is, being a map it does not store multiple keys (overwrites the existing key-value pair with new key-value pair for existing key).
I am not sure whether a multimap will be useful for this or not? If so, how?
Every collections that implements Map interface keeps it's keys in Set implementation. Set can not keep multiple values.
Multimap is just a Map<Key, Collection<Value>>. It allows to keep multiple values under one key.
Read more about Maps.
Is there a built in implementation in java for hash map whose values are linked lists?
like, if I put:
map.put(1, "A");
map.put(1, "B");
then it automatically add A and B to the linked list. When I retrieve from the map, as:
map.get(1)
I get back a list containing both of them?
Java does not have it but you can use MultiMap from Google Guava.
A collection similar to a Map, but which may associate multiple values with a single key. If you call put(K, V) twice, with the same key but different values, the multimap contains mappings from the key to both values.
The methods get(K), keySet(), keys(), values(), entries(), and asMap() return collections that are views of the multimap
This article Multimaps - Google Guava gives you complete idea about how to use it and also how to do it with HashMap using List as value.
Second put will overwrite first put. You will get B as response.
As per javadoc
If the map previously contained a mapping for the key, the old value is replaced
If you want to keep both entries, you need to use thrid party library google guava MultiMap
Nope, just build your own.
First you take a HashMap, if the key does not exist you put the linkedList in...
Simple...
I have two questions on Java HashMap:
1) Is it possible in any implementation of Java HashMap to get corresponding Key from the value ? I am using HashMultiMap (key -) Multiple values).
2) Is it possible in any implementation of Java HashMap to get Key position in the HashMap ? If so, then If I add new key, is it possible that the key position is changed ? I am using HashMultiMap (key -) Multiple values).
If both of them answer is NOT, how is it possible to implement manually (any idea ?) ?
1) Yes, but not in an efficient way, and there are no methods in interface Map to do this with one method call. You'd have to iterate over the entries of the map until you find one with the value you're looking for; then you have the key of the corresponding entry. There are implementation such as Google Guava's BiMap that do let you do reverse lookups efficiently.
2) No, because a map is not an ordered collection: keys do not have a defined position in a map. If you need this, you could use for example LinkedHashMap, which keeps key-value pairs in the order they are inserted in the map.