Java Legacy Classes ~ NIIT POST

Legacy Classes

Early version of java did not include the Collection framework. It only defined several classes and interface that provide method for storing objects. When Collection framework were added in J2SE 1.2, the original classes were reengineered to support the collection interface. These classes are also known as Legacy classes. All legacy claases and interface were redesign by JDK 5 to support Generics.

The following are the legacy classes defined by java.util package

1. Dictionary
2. HashTable
3. Properties
4. Stack
5. Vector

There is only one legacy interface called Enumeration

NOTE: All the legacy classes are syncronized

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Enumeration interface

1. Enumeration interface defines method to enumerate through collection of object.
2. This interface is suspended by Iterator interface.
3. However some legacy classes such as Vector and Properties defines several method in which Enumeration interface is used.
4. It specifies the following two methods

boolean hasMoreElements()

Object nextElement()

---

Vector class

1. Vector is similar to ArrayList which represents a dynamic array.
2. The only difference between Vector and ArrayList is that Vector is synchronised while Array is not.
3. Vector class has following four constructor

   Vector()
   
   Vector(int size)
   
   Vector(int size, int incr)
   
   Vector(Collection< ? extends E> c)
   

Vector defines several legacy method. Lets see some important legacy method define by Vector class.

Method	Description
addElement()	add element to the Vector
elementAt()	return the element at specified index
elements	return an enumeration of element in vector
firstElement()	return first element in the Vector
lastElement()	return last element in the Vector
removeAllElement()	remove all element of the Vector

---

Example of Vector

import java.util.*;
public class Test 
{
  public static void main(String[] args) 
   {
      Vector ve = new Vector();
       ve.add(10);
       ve.add(20);
       ve.add(30);
       ve.add(40);
       ve.add(50);
       ve.add(60);
       
       Enumeration en = ve.elements();
       
       while(en.hasMoreElements())
       {
           System.out.println(en.nextElement());
       }
   }
    
}

Output :

10
20
30
40
50
60

---

Hashtable class

1. Like HashMap, Hashtable also stores key/value pair in hashtable. However neither keys nor values can be null.
2. There is one more difference between HashMap and Hashtable that is Hashtable is synchronized while HashMap is not.
3. Hashtable has following four constructor

   Hashtable()
   Hashtable(int size)
   Hashtable(int size, float fillratio)
   Hashtable(Map< ? extends K, ? extends V> m)
   

---

Example of Hashtable

import java.util.*;
class HashTableDemo
{
 public static void main(String args[])
 {
  Hashtable< String,Integer> ht = new Hashtable< String,Integer>();
  ht.put("a",new Integer(100));
  ht.put("b",new Integer(200));
  ht.put("c",new Integer(300));
  ht.put("d",new Integer(400));

  Set st = ht.entrySet();
  Iterator itr=st.iterator();
  while(itr.hasNext())
  {
   Map.Entry m=(Map.Entry)itr.next();
   System.out.println(itr.getKey()+" "+itr.getValue());
  }
 }
}

Output:

a 100
b 200
c 300
d 400

---

Difference between HashMap and Hashtable

Hashtable	HashMap
Hashtable class is synchronized.	HastMap is not synchronize.
Because of Thread-safe, Hashtable is slower than HashMap	HashMap works faster.
Neither key nor values can be null	Both key and values can be null
Order of table remain constant over time.	does not guarantee that order of map remain constant over time.

---

Properties class

1. Properties class extends Hashtable class.
2. It is used to maintain list of value in which both key and value are String
3. Properties class define two constructor

   Properties()
   Properties(Properties default)
   

4. One advantage of Properties over Hashtable is that we can specify a default property that will be useful when no value is associated with a certain key.

---

Example of Properties class

import java.util.*;

public class Test 
{
    
   public static void main(String[] args) 
   {
     Properties pr = new Properties();
     pr.put("Java", "James Ghosling");
     pr.put("C++", "Bjarne Stroustrup");
     pr.put("C", "Dennis Ritchie");
     pr.put("C#", "Microsoft Inc.");
     Set< ?> creator = pr.keySet();
     
     for(Object ob: creator)
     {
         System.out.println(ob+" was created by "+ pr.getProperty((String)ob) );
     }
     
   }
    
}

Output :

Java was created by James Ghosling
C++ was created by Bjarne Stroustrup
C was created by Dennis Ritchie
C# was created by Microsoft Inc