Содержание
Map Interface in Java
In Java, Map Interface is present in java.util package represents a mapping between a key and a value. Java Map interface is not a subtype of the Collection interface. Therefore it behaves a bit differently from the rest of the collection types. A map contains unique keys.
Geeks, the brainstormer should have been why and when to use Maps.
Maps are perfect to use for key-value association mapping such as dictionaries. The maps are used to perform lookups by keys or when someone wants to retrieve and update elements by keys. Some common scenarios are as follows:
- A map of error codes and their descriptions.
- A map of zip codes and cities.
- A map of managers and employees. Each manager (key) is associated with a list of employees (value) he manages.
- A map of classes and students. Each class (key) is associated with a list of students (value).
Creating Map Objects
Since Map is an interface, objects cannot be created of the type map. We always need a class that extends this map in order to create an object. And also, after the introduction of Generics in Java 1.5, it is possible to restrict the type of object that can be stored in the Map.
Syntax: Defining Type-safe Map
Map hm = new HashMap(); // Obj is the type of the object to be stored in Map
Characteristics of a Map Interface
- A Map cannot contain duplicate keys and each key can map to at most one value. Some implementations allow null key and null values like the HashMap and LinkedHashMap, but some do not like the TreeMap.
- The order of a map depends on the specific implementations. For example, TreeMap and LinkedHashMap have predictable orders, while HashMap does not.
- There are two interfaces for implementing Map in Java. They are Map and SortedMap, and three classes: HashMap, TreeMap, and LinkedHashMap.
Methods in Java Map Interface
| Method | Action Performed |
|---|---|
| clear() | This method is used in Java Map Interface to clear and remove all of the elements or mappings from a specified Map collection. |
| containsKey(Object) | This method is used in Map Interface in Java to check whether a particular key is being mapped into the Map or not. It takes the key element as a parameter and returns True if that element is mapped in the map. |
| containsValue(Object) | This method is used in Map Interface to check whether a particular value is being mapped by a single or more than one key in the Map. It takes the value as a parameter and returns True if that value is mapped by any of the keys in the map. |
| entrySet() | This method is used in Map Interface in Java to create a set out of the same elements contained in the map. It basically returns a set view of the map or we can create a new set and store the map elements into them. |
| equals(Object) | This method is used in Java Map Interface to check for equality between two maps. It verifies whether the elements of one map passed as a parameter is equal to the elements of this map or not. |
| get(Object) | This method is used to retrieve or fetch the value mapped by a particular key mentioned in the parameter. It returns NULL when the map contains no such mapping for the key. |
| hashCode() | This method is used in Map Interface to generate a hashCode for the given map containing keys and values. |
| isEmpty() | This method is used to check if a map is having any entry for key and value pairs. If no mapping exists, then this returns true. |
| keySet() | This method is used in Map Interface to return a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. |
| put(Object, Object) | This method is used in Java Map Interface to associate the specified value with the specified key in this map. |
| putAll(Map) | This method is used in Map Interface in Java to copy all of the mappings from the specified map to this map. |
| remove(Object) | This method is used in Map Interface to remove the mapping for a key from this map if it is present in the map. |
| size() | This method is used to return the number of key/value pairs available in the map. |
| values() | This method is used in Java Map Interface to create a collection out of the values of the map. It basically returns a Collection view of the values in the HashMap. |
| getOrDefault(Object key, V defaultValue) | Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key. |
| merge(K key, V value, BiFunction remappingFunction) | If the specified key is not already associated with a value or is associated with null, associate it with the given non-null value. |
| putIfAbsent(K key, V value) | If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current associate value. |