Working with Maps in Java

//Map Interface has three implementations ie. HashMap, LinkedHashMap and TreeMap.

//Map stores elements in key value pair. Each key and value pair is known as entry. Keys are always unique.

//Retrieving of elements in HashMap is in random order of keys.

//Retrieving of elements in LinlkedHashMap is in insertion order of keys.

// Retrieving of elements in TreeMap is in ascending order of keys.  

//Following is a sample of implementation of java HashMap

public class MapExample {

public static void main(String[] args) {
Map mapObj = new HashMap();
mapObj.put(100, "amit");
mapObj.put(101,"Vijay");  
mapObj.put(102,"Rahul");  
mapObj.put(500,"Vijay"); 

//Here value is replaced for already existing key. 
mapObj.put(99,"sumit");
mapObj.put(99,"rahul");
mapObj.put(99,"luke");

//one null key is allowed in HashMap and LinkedHashMap but not in TreeMap.
mapObj.put(null,"Rahul2");
mapObj.put(null,"Rahul1");

//duplicate values are allowed in map.
mapObj.put(933,null);
mapObj.put(993,null);
// iterate map object with keySet() method of Map interface. This returns set of keys.
Set ks = mapObj.keySet();
for(Integer object : ks){
System.out.println(object+" "+mapObj.get(object));
}
System.out.println();

// iterate map object with entrySet() method of Map interface. This returns set of entries. Each entry is object of Map.Entry interface.
Set> entrySet = mapObj.entrySet();
for(Map.Entry sampleEntry: entrySet){
System.out.println(sampleEntry.getKey()+" "+sampleEntry.getValue());
}
System.out.println();
}
}
// Output will be:

null Rahul1
993 null
99 luke
100 amit
500 Vijay
101 Vijay
933 null
102 Rahul

null Rahul1
993 null
99 luke
100 amit
500 Vijay
101 Vijay
933 null
102 Rahul

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