What is MultiKeyMap?
Hi everyone , recently I came accross a usecase where I had to map multiple keys against one value. So Juggling with this I came to know about MultiKeyMap of apache. MultikeyMap is used to map multiple keys against one value . The purpose of this class is to avoid the need to write code to handle maps of maps.
How do I store duplicate keys on a map?
You can use a TreeMap with a custom Comparator in order to treat each key as unequal to the others. It would also preserve the insertion order in your map, just like a LinkedHashMap. So, the net result would be like a LinkedHashMap which allows duplicate keys!
How do you make a map with multiple keys?
- import com. google. common.
- { public static void main(String[] args) {
- // Create a map. Map, String> listKeyMap = Maps. newHashMap();
- // [key1, key2] -> value1. listKeyMap.
- // [key3, key4] -> value2. listKeyMap.
- // print multikey map. System.
- // print value corresponding to key1 and key2. System.
Can a map have duplicate keys?
Duplicate keys are not allowed in a Map. Basically, Map Interface has two implementation classes HashMap and TreeMap the main difference is TreeMap maintains an order of the objects but HashMap will not.
Can HashMap have two keys?
You can’t have an hash map with multiple keys, but you can have an object that takes multiple parameters as the key.
What is a MultiMap Java?
A Multimap is a new collection type that is found in Google’s Guava library for Java. A Multimap can store more than one value against a key. Both the keys and the values are stored in a collection, and considered to be alternates for Map> or Map> (standard JDK Collections Framework).
Can HashMap store duplicate keys?
HashMap does not allow duplicate keys however it allows to have duplicate values. HashSet permits to have a single null value.
What is MultiValueMap in Java?
A MultiValueMap decorates another map, allowing it to have more than one value for a key. A MultiMap is a Map with slightly different semantics. Putting a value into the map will add the value to a Collection at that key. Getting a value will return a Collection, holding all the values put to that key.
Can HashMap have same HashCode?
It’s perfectly legal for two unequal objects to have the same hash code. It’s used by HashMap as a “first pass filter” so that the map can quickly find possible entries with the specified key. The keys with the same hash code are then tested for equality with the specified key.
Which is faster HashMap or HashSet?
HashMap is faster than HashSet because the values are associated to a unique key. In HashSet , member object is used for calculating hashcode value which can be same for two objects so equals() method is used to check for equality.
Is MultiKeyMap thread-safe?
Note that MultiKeyMap is not synchronized and is not thread-safe. If you wish to use this map from multiple threads concurrently, you must use appropriate synchronization.
What is the difference between Map and MultiValueMap?
A map cannot contain duplicate keys; each key can map to at most one value. So in a MultivaluedMap you can insert 0, 1, 2, 3 or more objects related to the same key. In a Map you can insert exactly 1 object related to a key.
What is bucket in HashMap?
A bucket is one element of the HashMap array. It is used to store nodes. Two or more nodes can have the same bucket. In that case, a link list structure is used to connect the nodes. Buckets are different in capacity.
Are HashMaps indexed?
Arrays store items in an ordered collection and are accessed using an index number (which is an integer). HashMap stores items as key/value pairs. Values can be accessed by indexes, known as keys, of a user-defined type.
How do I use multikeymap?
MultiKeyMap is about using tuples as keys, not about matching one value to more than one key. Use a normal map and just put your value twice, with different keys. Some more caution is needed when removing values. When you remove a value for the first key, do you want to automatically remove other keys with the same value?
How to use multiple keys to map the value in Java?
Basically, we need a map implementation that uses multiple keys to map the value in Java. 1. Using Plain Java The idea is to construct a custom class Key, which consists of all keys, and use an instance of the Keyclass in our map as a key to map the value.
What is multiple keymap in Apache Commons-Collections?
Here we have learned about Multiple keymap using apache commons-collections. It also provides other collections like OrderedMap, BidiMap – get value from key and get key from value, Bag – Add multiple copy of the same object in Map. Was this post helpful?
Is the multikeymap class Thread-safe?
Note that MultiKeyMap is not synchronized and is not thread-safe. If you wish to use this map from multiple threads concurrently, you must use appropriate synchronization. This class may throw exceptions when accessed by concurrent threads without synchronization. Constructs a new MultiKeyMap that decorates a HashedMap.