Map

The map enables us to store data in key-value pairs (keys should be immutable). A map in java cannot contain duplicate keys; each key can map to at most one value. The Map interface consists of three collection views, a set of keys, a collection of values, or a set of key-value mappings.
Eg –
Map< String,Integer> map = new HashMap< String,Integer>();
map.put(“a”, new Integer(100));

Types of Map –

• HashMap –
  • It is the implementation of Map.
  • It doesn’t maintain any order.
  • It allows null keys and values.
  • Eg –
    • HashMap cities= new HashMap();
    • cities.put(“England”, “London”);
• HashTable –
  • Hashtable implements the Map interface.
  • It is the same as HashMap but is synchronized.
  • Similar to HashMap, Hashtable stores key/value pairs in a hash table.
  • The key in hashtable is then hashed, which results in a hash code. This code is then used as the index at which the value is stored within the table.
  • Eg –
    • Hashtable table= new Hashtable();
    • table.put(“One”, new Double(3434.34));
• LinkedHashMap –
  • LinkedHashMap is the implementation of Map.
  • It inherits the HashMap class.
  • It maintains insertion order.
  • It allows null keys and values.
  • Eg –
    • LinkedHashMap lhm = new LinkedHashMap();
    • lhm.put(“one”, “two”);
• TreeMap –
  • TreeMap is the implementation of Map and SortedMap.
  • It maintains ascending order.
  • It doesn’t allow null keys and values.
  • E.g. –
    • TreeMap treemap = new TreeMap();
    • treemap.put(10, “One”);