Java collection Map review and application

Understand map features

Unordered LinkedHashMap: unique key, not unique value (order of addition) TreeMap: unique key, not unique value (natural order)Copy the code

Find value by key Return NULL if key does not exist

Map common methods



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Put (Object obj2,Object obj1) Add an element GET (key) Get the element key is not subscript size() The size of the map set keySet() All the keys of the map set Values () All the values of the map set EntrySet () All keys and values of the map collectionCopy the code

Use map to store country names

Public class TestMap {public static void main(String[] args) {// Create map set // map <String,String> map = new HashMap<String,String>(); //Map<String,String> map = new LinkedHashMap<String,String>(); Map<String,String> map = new TreeMap<String,String>(); Put ("cn", "Chinese"); // Add keys and values to the map set. map.put("us", "America"); map.put("en", "England"); map.put("jp", "Japan"); map.put("us", "the united stas"); map.put("uk", "Japan"); map.put("cs", "cs"); String value = map.get("cs"); System.out.println(value); System.out.println(map.size()); System.out.println(map.keyset ()); System.out.println(map.values()); System.out.println(map); // Print all values system.out.println (map); // Print all key-value pairs. System.out.println(map.entrySet()); // Print all key-value pairs. }}Copy the code

Print the result