Comparator Interface in java

Comparator Interface in java
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Comparator Interface

In Java, Comparator interface is used to order the object in your own way. It gives you the ability to decide how elements are stored within a sorted collection and map.

Comparator Interface defines the methodcompare(). This method compares two objects and returns 0 if two objects are equal. It returns a positive value if object1 is greater than object2. Otherwise, a negative value is return. The method can throw a ClassCastException if the type of object is not compatible for comparison.

---

Example

Student class

class Student
int roll;
  String name;
  Student(int r,String n)
  {
      roll = r;
      name = n;
  }
  public String toString()
  {
      return roll+" "+name;
  }

MyComparator class

This class defines the comparison logic for Student class based on their roll. Student object will be stored in ascending order of their roll.

class MyComparator implements Comparator
{
  public int compare(Student s1,Student s2)
    {
        if(s1.roll == s2.roll) return 0;
        else if(s1.roll > s2.roll) return 1;
        else return -1;
    }  
}

public class Test 
{
    
   public static void main(String[] args) 
   {
       TreeSet< Student> ts = new TreeSet< Student>(new MyComparator());
       ts.add(new Student(45, "Rahul"));
       ts.add(new Student(11, "Adam"));
       ts.add(new Student(19, "Alex"));
       System.out.println(ts);
   }
    
}

Output :

[ 11 Adam, 19 Alex, 45 Rahul ]

As you can see in the output Student object are stored in ascending order of their roll.

Legacy Classes

The early version of Java did not include the Collection framework. It only defined several classes and interface that provide a method for storing objects. When Collection framework was added in J2SE 1.2, the original classes were re-engineered to support the collection interface. These classes are also known as Legacy classes. All legacy classes and interface were redesigns 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 a syncronized

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Enumeration interface

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

boolean hasMoreElements()

Object nextElement()

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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 synchronized 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 methods. Let's 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.	hashmap is not synchronized.
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 remains constant over time.	does not guarantee that order of map remains constant over time.

---

Properties class

1. Properties class extends Hashtable class.
2. It is used to maintain a 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