Comparable And Comparator Interface

Comparable Interface

• It is in java.lang package
• It have only one method
• public int compareTo(Object o);
• How to call compareTo method
• obj1.compareTo(obj2);
• There are three options of comparision
• Return +1 when obj1 > obj2
• Return -1 when obj1 < obj2
• Return 0 when obj1 == obj2

Lets take a simple example

class comapareToExample{

 public static void main(){

 System.out.prinlt("A".compareTO("B")); //-1 negative value

 System.out.prinlt("Z".compareTO("B")); //+1 positive value

 System.out.prinlt("A".compareTO("A")); //0

 System.out.prinlt("A".compareTO(null));// NullPointerException

 }
}

How comparable related to TreeSet ?
Lets take simple example

class comapareToExample{

 public static void main(){

 TreeSet ts = new TreeSet();

 ts.add("A");

 ts.add("Z");

 ts.add("C");

 ts.add("B");

 System.out.prinlt(ts);

 }
}

So here in this example whenever we try to add new element a compareTo(Object o); method is called  by JVM for comparison
call to a method will e like this
obj1.compareTo(obj2);
obj1 is the element which is passed in add and obj2 is element which is already inserted

class comapareToExample{

 public static void main(){

 TreeSet ts = new TreeSet();

 ts.add("A"); 

 ts.add("Z"); //"Z".compareTo("A") -ive

 ts.add("C"); //"C".compareTo("A") +ive then "C".compareTo("Z") +ive

 ts.add("B"); //"B".compareTo("A") -ive then "B".compareTo("Z")

 System.out.prinlt(ts); //{A,B,C,Z}

 }
}

This is the case when we want default natural sorting order
Note:

• If we are not satisfied by default natural sorting order then we can define our own customized sorting order by sing Comparator
• Comparable meant for default natural sorting order whereas Comparator for customized sorting order.

Comparator Interface

• Used for customizing sorting order
• It is in java.util package
• It contain two method
• public int compare(Object o,object o1);
• public boolean equals(Object o);
• obj1.compare(obj2) this will return:
• -ve when obj1 has to come before obj2
• +ve when obj1 has to come after obj2
• 0 when obj1 is equal to obj2

There is compulsion that for an interface a class should provide implementation for all the method of interface but incase of Comparator there is a loop hole that we only give definition of compare() method not for equals() method it is optional why?

class myClass implements Comparator{

 compare(Object o1,Object o2){

 }

}

Note :

• Whenever we implement Comparator interface then we must give implementation of compare() method
• Implementation of equals method is optional because it is already present in every java class from Object class through inheritance

Implementation of Comparator example
Here we are trying to insert Integer object in descending order
class  MyComparatorClass implements Comparator{
  public int compare(Object o1,Object o2){
  Integer i1 = (Integer) o1;
  Integer i2 = (Integer) o2;
    if(i1 > i2){
     return -1;
    } 
    else if(i1 < i2){
     return  1;
    } 

    else{

     return 0;
    }
  }
}

so how to use it

class comapareExample{

 public static void main(){

 TreeSet ts = new TreeSet(new MyComparatorClass());

 ts.add(1); 

 ts.add(22); // compare(22,1) -ive

 ts.add(12); //compare(12,1) -ive then compare(12,22) +ive

 ts.add(0);//compare(0,1) +ive 
 System.out.prinlt(ts); //{22,12,1,0}
 }

}

 Balanced Tree which is generated is 

class  MyComparatorClass implements Comparator{

  public int compare(Object o1,Object o2){

  Integer i1 = (Integer) o1;

  Integer i2 = (Integer) o2;

   return i2.compareTo(i1); // code can be replace by compareTo   method

  }

}

then the output will be in descending order

Various other implementation of compare method if be return

• return -i1.compareTo(i2);// Descending order [22,12,1,0]
• return  i1.compareTo(i2);// Ascending order  [0,1,12,22]
• return  i2.compareTo(i1);// Descending order [22,12,1,0]
• return -i2.compareTo(i1);// Ascending order  [0,1,12,22] 
• return +1;//insertion order [1,22,12,0]
• return -1;//reverse of insertion order [0,12,22,1]
• return 0;//only first element will be inserted [1]and all other elements are considered as duplicates

Comparision between Comparator and Comparable interface

S.no	Comparable	Comparator
1	It is in java.lang package	It is in java.util package
2	It is used for Default Natural sorting order	It is used for Customized sorting order
3	It has only one method public int compareTo(Object o);	It has  two methods public int compareTo(Object o); public boolean equals(Object o);
4	All wrapper class and String class aready implement compareable	There is no implements class that are implementing Comparator interface