Java Language Maps Creating and Initializing Maps
Example
Introduction
Maps stores key/value pairs, where each key has an associated value. Given a particular key, the map can look up the associated value very quickly.
Maps, also known as associate array, is an object that stores the data in form of keys and values. In Java, maps are represented using Map interface which is not an extension of the collection interface.
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Way 1 :-
/*J2SE < 5.0*/ Map map = new HashMap(); map.put("name", "A"); map.put("address", "Malviya-Nagar"); map.put("city", "Jaipur"); System.out.println(map); -
Way 2 :-
/*J2SE 5.0+ style (use of generics):*/ Map<String, Object> map = new HashMap<>(); map.put("name", "A"); map.put("address", "Malviya-Nagar"); map.put("city", "Jaipur"); System.out.println(map); -
Way 3 :-
Map<String, Object> map = new HashMap<String, Object>(){{ put("name", "A"); put("address", "Malviya-Nagar"); put("city", "Jaipur"); }}; System.out.println(map); -
Way 4 :-
Map<String, Object> map = new TreeMap<String, Object>(); map.put("name", "A"); map.put("address", "Malviya-Nagar"); map.put("city", "Jaipur"); System.out.println(map); -
Way 5 :-
//Java 8 final Map<String, String> map = Arrays.stream(new String[][] { { "name", "A" }, { "address", "Malviya-Nagar" }, { "city", "jaipur" }, }).collect(Collectors.toMap(m -> m[0], m -> m[1])); System.out.println(map); -
Way 6 :-
//This way for initial a map in outside the function final static Map<String, String> map; static { map = new HashMap<String, String>(); map.put("a", "b"); map.put("c", "d"); } -
Way 7 :- Creating an immutable single key-value map.
//Immutable single key-value map Map<String, String> singletonMap = Collections.singletonMap("key", "value");Please note, that it is impossible to modify such map.
Any attemts to modify the map will result in throwing the UnsupportedOperationException.
//Immutable single key-value pair Map<String, String> singletonMap = Collections.singletonMap("key", "value"); singletonMap.put("newKey", "newValue"); //will throw UnsupportedOperationException singletonMap.putAll(new HashMap<>()); //will throw UnsupportedOperationException singletonMap.remove("key"); //will throw UnsupportedOperationException singletonMap.replace("key", "value", "newValue"); //will throw UnsupportedOperationException //and etc
