Map Classes

Hash map class

• A Hash Map contains values based on the key. It implements the Map interface and extends Abstract Map class.
• It contains only unique elements.
• It may have one null key and multiple null values.
• It maintains no order.

 HashMap<Integer,String> colorMap =new HashMap<Integer,String>();  
 colorMap .put(1,"Red"); 
 colorMap .put(1,"Green"); 
 colorMap .put(1,"Blue"); 

 for(Map.Entry m:hm.entrySet()){  
     System.out.println(m.getKey()+" "+m.getValue());  
 }  

Output:
1 Red
2 Green
3 Blue

Note :
HashSet contains only values whereas HashMap contains entry(key and value).

Linked Hash Map,

• A LinkedHashMap contains values based on the key. It implements the Map interface and extends HashMap class.
• It contains only unique elements.
• It may have one null key and multiple null values.
• It is same as HashMap instead maintains insertion order.

Example,

 LinkedHashMap<Integer,String> colorMap =new LinkedHashMap<Integer,String>();  

Tree Map

• A Tree Map contains values based on the key. It implements the NavigableMap interface and extends Abstract Map class.
• It contains only unique elements.
• It cannot have null key but can have multiple null values.
• It is same as Hash Map instead maintains ascending order.

Example,

 TreeMap<Integer,String> colorMap =new TreeMap<Integer,String>();