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. |
Interface Map
An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
This interface takes the place of the Dictionary class, which was a totally abstract class rather than an interface.
The Map interface provides three collection views, which allow a map’s contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order of a map is defined as the order in which the iterators on the map’s collection views return their elements. Some map implementations, like the TreeMap class, make specific guarantees as to their order; others, like the HashMap class, do not.
Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a map.
All general-purpose map implementation classes should provide two «standard» constructors: a void (no arguments) constructor which creates an empty map, and a constructor with a single argument of type Map , which creates a new map with the same key-value mappings as its argument. In effect, the latter constructor allows the user to copy any map, producing an equivalent map of the desired class. There is no way to enforce this recommendation (as interfaces cannot contain constructors) but all of the general-purpose map implementations in the JDK comply.
The «destructive» methods contained in this interface, that is, the methods that modify the map on which they operate, are specified to throw UnsupportedOperationException if this map does not support the operation. If this is the case, these methods may, but are not required to, throw an UnsupportedOperationException if the invocation would have no effect on the map. For example, invoking the putAll(Map) method on an unmodifiable map may, but is not required to, throw the exception if the map whose mappings are to be «superimposed» is empty.
Some map implementations have restrictions on the keys and values they may contain. For example, some implementations prohibit null keys and values, and some have restrictions on the types of their keys. Attempting to insert an ineligible key or value throws an unchecked exception, typically NullPointerException or ClassCastException . Attempting to query the presence of an ineligible key or value may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible key or value whose completion would not result in the insertion of an ineligible element into the map may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as «optional» in the specification for this interface.
Many methods in Collections Framework interfaces are defined in terms of the equals method. For example, the specification for the containsKey(Object key) method says: «returns true if and only if this map contains a mapping for a key k such that (key==null ? k==null : key.equals(k)) .» This specification should not be construed to imply that invoking Map.containsKey with a non-null argument key will cause key.equals(k) to be invoked for any key k . Implementations are free to implement optimizations whereby the equals invocation is avoided, for example, by first comparing the hash codes of the two keys. (The Object.hashCode() specification guarantees that two objects with unequal hash codes cannot be equal.) More generally, implementations of the various Collections Framework interfaces are free to take advantage of the specified behavior of underlying Object methods wherever the implementor deems it appropriate.
Some map operations which perform recursive traversal of the map may fail with an exception for self-referential instances where the map directly or indirectly contains itself. This includes the clone() , equals() , hashCode() and toString() methods. Implementations may optionally handle the self-referential scenario, however most current implementations do not do so.
Unmodifiable Maps
- They are unmodifiable. Keys and values cannot be added, removed, or updated. Calling any mutator method on the Map will always cause UnsupportedOperationException to be thrown. However, if the contained keys or values are themselves mutable, this may cause the Map to behave inconsistently or its contents to appear to change.
- They disallow null keys and values. Attempts to create them with null keys or values result in NullPointerException .
- They are serializable if all keys and values are serializable.
- They reject duplicate keys at creation time. Duplicate keys passed to a static factory method result in IllegalArgumentException .
- The iteration order of mappings is unspecified and is subject to change.
- They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
- They are serialized as specified on the Serialized Form page.
This interface is a member of the Java Collections Framework.