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java map example

Map Interface
A Map stores data in key and value association. Both key and values are objects. The key must be unique but the values can be duplicate. Although Maps are a part of Collection Framework, they cannot actually be called as collections because of some properties that they posses.
Interface
Description
Map
Maps unique key to value.
Map.Entry
Describe an element in key and value pair in a map. Entry is sub interface of Map.
NavigableMap
Extends SortedMap to handle the retrienal of entries based on closest match searches
SortedMap
Extends Map so that key is maintained in an ascending order.

 Commonly used Methods defined by Map
·         boolean containsKey(Object k): returns true if map contain k as key. Otherwise false.
·         Object get(Object k) : returns values associated with the key k.
·         Object put(Object k, Object v) : stores an entry in map.
·         Object putAll(Map m) : put all entries from m in this map.
·         Set keySet() : returns Set that contains the key in a map.
·         Set entrySet() : returns Set that contains the entries in a map.

HashMap class
1.     HashMap class extends AbstractMap and implements Map interface.

2.     It uses a hashtable to store the map data. 

Example1:- TestMap.java
import java.util.*;
class TestMap
{
public static void main(String[]args)
{
Map m=new HashMap();
m.put(101,"Java");
m.put(102,"Cpp");
m.put(103,"C#");
m.put(104,"C");
m.put("Amit",101);
m.put("Neha",102);
System.out.println(m);
}
}
Output:-
{Neha=102, 101=Java, 102=Cpp, 103=C#, 104=C, Amit=101}
Example2:-
import java.util.*;
class TestMap
{
public static void main(String[]args)
{
Map<Object,Object> m=new HashMap<Object,Object>();
m.put(101,"Java");
m.put(102,"Cpp");
m.put(103,"C#");
m.put(104,"C");
m.put("Amit",101);
m.put("Neha",102);
Set s=m.entrySet();
Iterator itr=s.iterator();
System.out.println("Key   :::::::::Value");
while(itr.hasNext())
{
Map.Entry me=(Map.Entry)itr.next();
System.out.println(me.getKey()+"    :::: "+me.getValue());
}
}
}
Output:-
Key :::::::::Value
Neha  :::: 102
101    :::: Java
102    :::: Cpp
103    :::: C#
104    :::: C
Amit  :::: 101

Example3:- TestMap.java
import java.util.*;
class TestMap
{
public static void main(String[]args)
{
Map<Object,Object> m=new HashMap<Object,Object>();
m.put(101,"Java");
m.put(102,"java");
m.put(102,"C#");
m.put(104,null);
m.put("Amit",101);
m.put("Neha",102);
Set s=m.entrySet();
Iterator itr=s.iterator();
System.out.println("Key   :::::::::Value");
while(itr.hasNext())
{
Map.Entry me=(Map.Entry)itr.next();
System.out.println(me.getKey()+"    :::: "+me.getValue());
}
}
}
Output:-
Key   :::::::::Value
Neha    :::: 102
101    :::: Java
102    :::: C#
104    :::: null
Amit    :::: 101

Example4:- TestMap.java
import java.util.*;
class TestMap
{
public static void main(String[]args)
{
Map<Object,Object> m=new HashMap<Object,Object>();
m.put(101,null);
m.put(102,"java");
m.put(102,"C#");
m.put(104,null);
m.put(null,101);
m.put(null,102);
Set s=m.entrySet();
Iterator itr=s.iterator();
System.out.println("Key   :::::::::Value");
while(itr.hasNext())
{
Map.Entry me=(Map.Entry)itr.next();
System.out.println(me.getKey()+"    :::: "+me.getValue());
}
}
}
Output:-
Key   :::::::::Value
null    :::: 102
101    :::: null
102    :::: C#
104    :::: null
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