Legacy Classes - Java Collections

Early version of java did not include the Collections framework. It only defined several classes and interfaces that provide methods for storing objects. When Collections 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 classes and interface were redesign by JDK 5 to support Generics. In general, the legacy classes are supported because there is still some code that uses them.

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 synchronized

Enumeration interface

1. Enumeration interface defines method to enumerate(obtain one at a time) through collection of objects.
2. This interface is superseded(replaced) by Iterator interface.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() //It returns true while there are still more elements to extract,
and returns false when all the elements have been enumerated.

Object nextElement() //It returns the next object in the enumeration i.e. each call to nextElement() method
obtains the next object in the enumeration. It throws NoSuchElementException when the
enumeration is complete.

Vector class

1. Vector is similar to ArrayList which represents a dynamic array.
2. There are two differences between Vector and ArrayList. First, Vector is synchronized while ArrayList is not, and Second, it contains many legacy methods that are not part of the Collections Framework.
3. With the release of JDK 5, Vector also implements Iterable. This means that Vector is fully compatible with collections, and a Vector can have its contents iterated by the for-each loop.
4. Vector class has following four constructor

   Vector() //This creates a default vector, which has an initial size of 10.
   
   Vector(int size) //This creates a vector whose initial capacity is specified by size.
   
   Vector(int size, int incr) //This creates a vector whose initial capacity is specified by size and whose
   increment is specified by incr. The increment specifies the number of elements to allocate each time
   when a vector is resized for addition of objects.
   
   Vector(Collection c) //This creates a vector that contains the elements of collection c.

Vector defines several legacy methods. Lets see some important legacy methods defined by Vector class.

Example of Vector

import java.util.*;
public class Test
{
  public static void main(String[] args)
   {
      Vector<Integer> ve = new Vector<Integer>();
       ve.add(10);
       ve.add(20);
       ve.add(30);
       ve.add(40);
       ve.add(50);
       ve.add(60);

       Enumeration<Integer> en = ve.elements();

       while(en.hasMoreElements())
       {
           System.out.println(en.nextElement());
       }
   }

}

10 20 30 40 50 60

Hashtable class

1. Like HashMap, Hashtable also stores key/value pair. 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() //This is the default constructor. The default size is 11.
   
   Hashtable(int size) //This creates a hash table that has an initial size specified by size.
   
   Hashtable(int size, float fillratio) //This creates a hash table that has an initial size specified by size
   and a fill ratio specified by fillRatio. This ratio must be between 0.0 and 1.0, and it determines how full
   the hash table can be before it is resized upward. Specifically, when the number of elements is greater
   than the capacity of the hash table multiplied by its fill ratio, the hash table is expanded.
   If you do not specify a fill ratio, then 0.75 is used.
   
   Hashtable(Map< ? extends K, ? extends V> m) //This creates a hash table that is initialized with the
   elements in m. The capacity of the hash table is set to twice the number of elements in m.
   The default load factor of 0.75 is used.

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());
    }
  }
}

a 100 b 200 c 300 d 400

Difference between HashMap and Hashtable

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() //This creates a Properties object that has no default values
   
   Properties(Properties propdefault) //This creates an object that uses propdefault for its default values.

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.

Note: In both cases, the property list is empty

5. In Properties class, you can specify a default property that will be returned if 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) );
     }

   }

}

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

Stack class

1. Stack class extends Vector.
2. It follows last-in, first-out principle for the stack elements.
3. It defines only one default constructor

   Stack() //This creates an empty stack

4. If you want to put an object on the top of the stack, call push() method. If you want to remove and return the top element, call pop() method. An EmptyStackException is thrown if you call pop() method when the invoking stack is empty.

You can use peek() method to return, but not remove, the top object. The empty() method returns true if nothing is on the stack. The search() method determines whether an object exists on the stack and returns the number of pops that are required to bring it to the top of the stack.

Example of Stack

import java.util.*;

class StackDemo {
public static void main(String args[]) {
Stack st = new Stack();
st.push(11);
st.push(22);
st.push(33);
st.push(44);
st.push(55);
Enumeration e1 = st.elements();

while(e1.hasMoreElements())
System.out.print(e1.nextElement()+" ");

st.pop();
st.pop();

System.out.println("\nAfter popping out two elements");

Enumeration e2 = st.elements();

while(e2.hasMoreElements())
System.out.print(e2.nextElement()+" ");

}
}

11 22 33 44 55 After popping out two elements 11 22 33

Dictionary class

1. Dictionary is an abstract class.
2. It represents a key/value pair and operates much like Map.
3. Although it is not currently deprecated, Dictionary is classified as obsolete, because it is fully superseded by Map class.