Hashtable,HashMap,TreeSet and Hashset

Hashtable:
Hashtable is basically a datastructure to retain values of key-value pair.
  • It didn’t allow null for both key and value. You will get NullPointerException if you add null value.
  • It is synchronized. So it comes with its cost. Only one thread can access in one time 

Hashtable<Integer,String>; cityTable = new Hashtable<Integer,String>();
cityTable.put(1, "Lahore");
cityTable.put(2, "Karachi");
cityTable.put(3, null); /* NullPointerEcxeption at runtime*/
System.out.println(cityTable.get(1));
System.out.println(cityTable.get(2));
System.out.println(cityTable.get(3));
Like Hashtable it also accepts key value pair.
  • It allows null for both key and value
  • It is unsynchronized. So come up with better performance
 

HashMap<Integer,String> productMap = new HashMap<Integer,String>();
productMap.put(1, "Keys");
productMap.put(2, null);
HashSet
 HashSet does not allow duplicate values. It provides add method rather put method. You also use its contain method to check whether the object is already available in HashSet. HashSet can be used where you want to maintain a unique list.

HashSet<String> stateSet = new HashSet<String>();
stateSet.add ("CA");
stateSet.add ("WI");
stateSet.add ("NY");
if (stateSet.contains("PB")) /* if CA, it will not add but shows following message*/
     System.out.println("Already found");
else
    stateSet.add("PB");
TreeSet:
Doesn't allow duplicate values.
It is an Ordered set. 
import java.util.*;
class TreeSetDemo {
public static void main(String args[]) {
// Create a tree set
TreeSet ts = new TreeSet();
// Add elements to the tree set
ts.add("C");
ts.add("A");
ts.add("B");
ts.add("E");
ts.add("F");
ts.add("D");
System.out.println(ts);
}
} The output from this program is shown here:
[A, B, C, D, E, F]

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