![]() ![]() This data structure is based on the hashing principle. What is HashMapWhat is HashMap? It is the most popular implementation of Map interface. public Set entrySet() returns a Set view with all the keys and values.public Set keySet() returns a Set view that contains all the keys.public boolean containsKey(Object key) searches the specified key from this map.public Object get(Object key) returns the value for the specified key.public Object remove(Object key) deletes an entry according to the specified key.public void putAll(Map map) inserts the specified map inside the map.public Object put(Object key, Object value) inserts an element into the map.Map methodsThe main operations of any Map are insertion, remove, and search of elements. Say, TreeMap and LinkedHashMap have predictable order of the elements, while HashMap doesn’t. By the way, the order of map elements depends on specific implementations. Here we are interested in the most popular implementations of Map Interface: HashMap, TreeMap and LinkedHashMap. IdentityHashMap uses referential equivalence checking when comparing documents, mapping with keys compared using =operation instead of equals() method ![]() EnumMap extends the AbstractMap class for use with enum keys.LinkedHashMap is a map with the order of adding elements, allows iteration in the insertion order.WeakHashMap to use a hash table with weak keys, display with values that can be deleted by the garbage collector if they are no longer used.Java TreeMap is a data structure to use a tree, i.e.Java HashMap is a data structure for storing key-value pairs using a hash table.Java AbstractMap is an abstract class that implements most of the Map interface.For example in Map String is an ID of some Item from the online shop.Īccording to documentations Map has the next Subinterfaces: The key is a unique identifier of the value in a Map. You may think of Map in Java such as a kind of dictionary or online-shop catalog, where you can find any item using its unique index. All keys in a particular map are unique, while values are not, so they can be duplicated. So it behaves in a different way compared to, say, Lists, or other collection Objects.Įach element of Map represents a key-value pair. Import static .Assertions.What is Java Map InterfaceThe Java Map interface is a part of Java Collection framework, but it is not a subtype of the Collection interface. assertThat(put2).isEqualTo("pt1") // pt1 was prev value for key: Point(1, 2)įull code example: import.assertThat(put1).isNull() // there was no previous value.String put2 = map.put(new Point(1, 2), "pt1-override") // override value for key: Point(1, 2).String put1 = map.put(new Point(1, 2), "pt1").See below example, we implemented Comparator interface for Point object. or we need to pass Comparator implementations by constructor (example below).implement Comparable, see this question for more details: Java TreeMap Comparable vs Comparator.Object that we use as a Key in TreeMap or HashSet needs to: The behavior of a sorted map is well-defined even if its ordering is inconsistent with equals it just fails to obey the general contract of the Map interface. (See Comparable or Comparator for a precise definition of consistent with equals.) This is so because the Map interface is defined in terms of the equals operation, but a sorted map performs all key comparisons using its compareTo (or compare) method, so two keys that are deemed equal by this method are, from the standpoint of the sorted map, equal. Note that the ordering maintained by a tree map, like any sorted map, and whether or not an explicit comparator is provided, must be consistent with equals if this sorted map is to correctly implement the Map interface. We can read in TreeMap JDK docs about equals implementation: TreeMap is backed by Red-black tree algorithm (self-balancing binary search tree) - implementations based on algorithm described in book: Cormen, Leiserson, and Rivest's Introduction to Algorithms. HashMap works on hashCode and equals method (so called contract). It is good to undetstand the difference between HashMap and TreeMap. If the key is the same than the value is overrided by new value. TreeMap and HashSet in java works by comparing objets (that's why we have sorted order). In case we use the key with hash map, we need to implement equals and hash code (without implementing it the hash map / hash set won't work correctly), list of hash maps implemntations in java: It is good practice to implement equals and hashCode methods. We don't need to implement equals and hashCode in key object used in TreeMap or HashSet. ![]()
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