We can traverse the map just in the same way in which we traverse the other collections. Iterate Over Map In Java And Print The Map We collect a stream and then use Collectors.toMap () method to construct a map. Some of the methods to construct maps are: We can obtain a stream of data from Java 8 stream API methods and construct maps using Collectors. Map singleton_map = Collections.singletonMap("CHN", "Beijing") #2) Using Java 8 Map myMap = Collections.EMPTY_MAP Map map_unmodifiable = Collections.unmodifiableMap (myMap) Ĭollections class also provides a factory method ‘singletonMap()’ that creates an immutable singleton map having only one entry. The method unModifiableMap () takes another map as an argument and creates an unmodifiable view of the original map. The above method may throw ‘unchecked assignment warning’ and thus we can also use the type-safe form as follows. For example, the following line of code, Map myMap = Collections.EMPTY_MAP The Collections.EmptyMap () returns a serializable and immutable map that is empty. Some methods used to initialize map are as follows: Java Collections class has factory methods that can be used to initialize collections including maps. It can be initialized using the following methods: #1) Using Collections The above definition will have keys of type string and objects as values. We can also create generic maps specifying the types for both key and value. The above statements will create maps with default specifications. Map hash_map = new HashMap() Map tree_map = new TreeMap() The following statements create a map in Java. We need to instantiate a concrete implementation of the map as it is an interface. import java.util.* import import import We can use one of the following statements in the program to import the map functionality. To create a map in Java, first, we have to include the interface in our program. Hence for traversing, it needs to be converted to set using keyset () or entrySet () method. Map implementations like HashMap and LinkedHashMap allow null key and null values. Also, there cannot be duplicate keys in maps. In maps, each key can map to the at most one value.Implements both map and sortedMap interface. HashMap, TreeMap, and LinkedHashMap.Įxtends from HashMap class. The general hierarchy of the map interface is shown below.Īs shown above there are two interfaces to implement map i.e. The map interface is not a part of the collection interface and that is the reason for which maps are different from the other collections. The map in Java is a part of the interface. Iterate Over Map In Java And Print The Map.#3) Using the put method of the Map Interface.
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