Comparable vs



Classes should implement the Comparable interface to control their natural ordering.
Objects that implement Comparable can be sorted by Collections.sort() and Arrays.sort() and can be used as keys in a sorted map or elements in a sorted set without the need to specify a Comparator.
« Interface »
Comparable
+ compareTo(Object) : int
compareTo() compares this object with another object and returns a negative integer, zero, or a positiveinteger as this object is less than, equal to, or greater than the other object.Use Comparator to sort objects in an order other than their natural ordering.
« Interface »
Comparator
+ compare(Object, Object) : int
+ equals(Object) : boolean
compare() compares its two arguments for order, and returns a negative integer, zero, or a positiveinteger as the first argument is less than, equal to, or greater than the second.

Examples:

1) Comparable 


public class Book implements Comparable {
    String title;
    int    isbn;

    Book(String title, int isbn) {
        this.title = title;
        this.isbn  = isbn;
    }

    public String toString() {
        return title + " " + String.valueOf(isbn);
    }

    public boolean equals(Object object) {
        if (object instanceof Book) {
            Book other = (Book) object;

            return this.title.equals(other.title) && (this.isbn == other.isbn);
        } else {
            return false;
        }
    }

    public int compareTo(Object object) {
        Book other = (Book) object;

        if (this.title.equals(other.title)) {
            return this.isbn - other.isbn;
        }

        return this.title.compareTo(other.title);
    }
}


2)
import java.util.*;

public class TestCollections {
    static List list = new LinkedList();

    public static void main(String[] args) {
        list.add(new Book("Patterns", 12345));
        list.add(new Book("Patterns", 34567));
        list.add(new Book("Examples", 23456));

        testContains();
        testISBNSort();

        try {
            testNaturalSort();
        } catch (Throwable t) {
            System.out.println(t);
        }
    }

    static void testContains() {
        System.out.println("\nTESTING CONTAINS");

        // this is what we're looking for
        Book patterns = new Book("Patterns", 12345);

        // do we have it?
        System.out.println("We have it: " + list.contains(patterns));
    }

    static void testISBNSort() {
        System.out.println("\nTESTING ISBN SORT");

        // sort using a comparator
        Collections.sort(list, new Comparator() {
            public int compare(Object object1, Object object2) {
                Book book1 = (Book) object1;
                Book book2 = (Book) object2;

                return book1.isbn - book2.isbn;
            }
        });

        // print the collection
        Iterator i = list.iterator();
        while (i.hasNext()) {
            System.out.println(i.next());
        }
    }

    static void testNaturalSort() {
        System.out.println("\nTESTING NATURAL SORT");

        // sort if we're comparable
        Collections.sort(list);

        // print the collection
        Iterator i = list.iterator();
        while (i.hasNext()) {
            System.out.println(i.next());
        }
    }
}


==============================================================================



Use of comparable and comparator interface



In java the element in collections can be sorted by using TreeSet or TreeMap. To sort the data elements a class needs to implement Comparator or Comparable interface. Thats why all Wrapper classes like Integer,Double and String class implements Comparable interface.



A class implementing Comparable interface need to override compareTo(Object obj) method and put the logic for sorting.







?





1



2



3



4





 The method returns an int value :-1,0,1



 It will return -1 : If this object is lesser than the passed object



 It will return  0 : If this object is same the passed object



 It will return 1 : If this object is greater than the passed object













Consider a class Person.







?





1



2



3



4



5



6



7



8



9



10



11



12



13



14



15



16



17



18



19



20



21



22



23



24



25



26



27



28



29



30



31





 class Person {



  public String name;



  public String lastName;







  public Person(String name, String lastName){



     this.name=name;



     this.lastName=lastName;



  }



  public String getName(){



    return name;



  }



  public String getLastName(){



    return lastName;



  }







  public static void main(String arg[]){



        List<person> myList = new ArrayList<person>();







    myList.add(new Person("Robert","USA"));



    myList.add(new Person("Andy","UK"));



    myList.add(new Person("Harish","India"));



    for(Person person : myList){



     System.out.println("My name is "+person.getName());



    }



  }



 }



  Output is :



  My name is Robert



  My name is Andy



  My name is Harish



</person></person>













But now i want that the objects to be sorted on name basis should be retrieved in sorted order. 
Consider a class Person.







?





1



2



3



4



5



6



7



8



9



10



11



12



13



14



15



16



17



18



19



20



21



22



23



24



25



26



27



28



29



30



31



32



33



34



35



36





 class Person implements Comparable{



  public String name;



  public String lastName;







  public Person(String name, String lastName){



     this.name=name;



     this.lastName=lastName;



  }



  public String getName(){



    return name;



  }



  public String getLastName(){



    return lastName;



  }







  public int compareTo(Object obj){



    Person p = (Person)obj;



    return this.name.compareTo(p.getName);



  }



  public static void main(String arg[]){



        List<person> myList = new ArrayList<person>();







    myList.add(new Person("Robert","USA"));



    myList.add(new Person("Andy","UK"));



    myList.add(new Person("Harish","India"));



    Collections.sort(myList);



    for(Person person : myList){



     System.out.println("My name is "+person.getName());



    }



  }



 }



  Output is :



  My name is Andy



  My name is Harish



  My name is Robert



</person></person>













Couple of things which needs to be taken in consideration:
1) Collections.sort() will sort only the collection having objects which implements either one of the comparing interface.
2) Collections.sort() will sort the same list.



Comparator interface is used when an extra logic is required to sort the objects. One need to override compare(Object obj1, Object obj2) method.For example you want the list of Person object to be sorted on the basis of complete name i.e "name lastName" but also on the other hand doesnt want to change the Person class default sorting implementation or Person class is a jar so so no code modification in it can be done. First create a Custom Comparator.







?





1



2



3



4



5



6



7



8



9



10



11



12



13



14



15



16



17



18



19



20



21



22



23



24



25



26



27





 public class MyCustomComparator implements Comparator{



  public int compare(Object obj1, Object obj2){



   Person p1 =(Person) obj1;



   Person p2 =(Person) ob2;



   String p1Name = p1.getName()+ " " +p1.getLastName();



   String p2Name = p2.getName()+ " " +p2.getLastName();



   return p1Name.toCompareTo(p2Name);



  }



 }



// Changes made in main method of Person class.



  public static void main(String arg[]){



        List<person> myList = new ArrayList<person>();







    myList.add(new Person("Robert","USA"));



    myList.add(new Person("Robert","UK"));



    myList.add(new Person("Robert","India"));



    Collections.sort(myList new MyCustomComparator());



    for(Person person : myList){



     System.out.println("My name is "+person.getName() + " " + person.getLastName());



    }



  }







  OutPut:



  My name is Robert India



  My name is Robert UK



  My name is Robert USA



</person></person>













Couple of things which needs to be taken in consideration:
1) For Comparator interface you need to override method compare(obj)
2) In collections.sort() the instance of Comparator need to be passed. In this example the list is sorted according to the custom Comparator created






































Comments








Post a Comment













Popular posts from this blog




























     Introduction to Web services technologies      Web Services Introduction   Before understanding why web services are popular or so important, you should first assess ‘What is Web Services, what’s its use and how does it work?’ The nature and functionality of web services have made it very popular. Nowadays, our business systems have matured, transparent and more logical and high tech, and all for these are because of web services.   Definition   Web services are the amalgamation of eXtensible Markup Language (XML) and HyperText Transfer Protocol HTTP that can convert your application into a Web-application, which publish its function or message to the rest of the world.   In other words, we can say, web services are just Internet Application Programming Interfaces (API) that can be accessed over a network, such as Internet and intranet, and executed on a remote system hosting the requested services.   Web-applications are simple applications that run on the web. Web services are b...








Read more










Important Points may ask in Interviews





















 Here I'm posting some important facts may ask in interviews,hope these are helpful for you.      If a Controller Extension extends Standard Controller,the logic from the standard controller does not execute in system mode,instead its execute in user mode.In which the permissions, sharing rules,field level security of the current user apply.  The view State Size of visualforce page must be under 135kb.By reducing view state,page can be load quickly.  Use Transient Keyword in apex controllers to define variables,that aren't essential for state maintaining,aren't necessary during page refreshes.  In BatchApex,by using DATABASE.QUERYLOCATOR object  returns 50 millions of records,if more than 50 millions records are returned,batch job is immediately  terminated and marked a fail.  The Default Interval value for Apex:ActionPoller is 60 sec,its must be 5 sec  or greater .   Use CustomController When you want to run visualforce page...








Read more










Sending Mass Emails From Salesforce




















Image







  Create a Email Template         Then you either go to the Lead Tab or the Contact Tab and scroll down the page until you see the Mass Email link in the Tools section.     On the first step of the mass email wizard is to create your target list. To do this you have the option of leveraging an existing View that is available on the Leads tab, or you can create a new View from within the wizard. In this example we will create our target list from within the wizard by clicking the   Create New View   link.     In my example I’m   creating a quick View   to grab any Lead that is in the State of California. Notice in Step 2 of the View you can use a Campaign to filter (handy if your Campaign has your target list as Campaign Members!). Remember you have governor limits based on your edition (see paragraph two of this blog posts).     If you’re running Professional Edition for example, and your View pulls more than 250 records, you’ll need to add additional filters to...








Read more