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  1. Practice
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  4. Covariant Return Types
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Covariant Return Types

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votes

  • kugelblitz + 3 comments

    What a bad and lazy example. 1) I spent more time spelling indian states than coding. 2) The approach with "hidden" code is good only to train monkeys. Humans learn better when they understand what happens and why.

    • anchit_h + 1 comment

      Seconded. As it is none of the question's wordings are clear. You need to read and re-read each question several times before you can finally start understanding what exactly they want. Sometimes even re-reading multiple times is not enough because the questions are worded badly. And then you need to refer to the existing code to figure out what exactly is expected of you.

      • abhishek_pandey1 + 0 comments

        agreed, its really hard to understand the questions

    • al_zoraw + 0 comments

      plx enable copy paste from diagram.

    • sunny_su + 0 comments

      Such a bad training!

  • kimanx + 2 comments

    IMHO this problem would have been more interesting if State and flower would have been interfaces.

    And why so much JAVA problems have method/variable names with underscores? This bothers me so much.

    • daku_daddy + 0 comments

      Underscores are really annoying.

    • Shafaet + 0 comments

      We fixed the variable and method names for this problem.

  • RodneyShag + 3 comments

    Java solution - passes 100% of test cases

    From my HackerRank solutions.

    class Flower {
    String whatsYourName() {
        return "I have many names and types.";
    }
}

class Jasmine extends Flower {
    @Override
    String whatsYourName() {
        return "Jasmine";
    }
}

class Lily extends Flower {
    @Override
    String whatsYourName() {
        return "Lily";
    }
}

class Lotus extends Flower {
    @Override
    String whatsYourName() {
        return "Lotus";
    }
}

class Region {
    Flower yourNationalFlower() {
        return new Flower();
    }
}

class WestBengal extends Region {
    @Override
    Jasmine yourNationalFlower() {
        return new Jasmine();
    }
}

class Karnataka extends Region {
    @Override
    Lotus yourNationalFlower() {
        return new Lotus();
    }
}

class AndhraPradesh extends Region {
    @Override
    Lily yourNationalFlower() {
        return new Lily();
    }
}

    8/24/18 Edit: The HackerRank problem is now flawed and always fails to compile. Hopefully they fix it soon.

    • abhijitcse40 + 0 comments

      cool...:)

    • rohanraz91 + 1 comment

      @Override is not required when using the case of covariant return types. Am i correct? So @Override is not actually requred for subclass of states.

      • RodneyShag + 1 comment

        @Override is always optional. It just lets the compiler know what you are doing but it never affects the code. The first 2 paragraphs of this link were useful. It seems that you should still write @Override for the classes extending State.

        HackerRank solutions.

        • ian_lua + 1 comment

          there's a problem however, how about the region region = null; I can't edit that part of the code as it is locked when i put in ur code Rodney

          • RodneyShag + 1 comment

            Correct, you should only be editing the part of the code I pasted above. region starts off as null but is then assigned a value.

            HackerRank solutions.

            • ian_lua + 1 comment

              thanks for the help, but i'm quite new to coding so can you pls explain more in depth. Sorry i'm really bad

              • RodneyShag + 1 comment

                It is fine that region starts off as null. You don't need to edit that part of the code.

                HackerRank solutions.

                • ian_lua + 1 comment

                  This is specifically what I get when i put in your code:

                  Your code: class Flower { String whatsYourName() { return "I have many names and types."; } }

                  class Jasmine extends Flower { @Override String whatsYourName() { return "Jasmine"; } }

                  class Lily extends Flower { @Override String whatsYourName() { return "Lily"; } }

                  class Lotus extends Flower { @Override String whatsYourName() { return "Lotus"; } }

                  class State { Flower yourNationalFlower() { return new Flower(); } }

                  class WestBengal extends State { @Override Jasmine yourNationalFlower() { return new Jasmine(); } }

                  class Karnataka extends State { @Override Lotus yourNationalFlower() { return new Lotus(); } }

                  class AndhraPradesh extends State { @Override Lily yourNationalFlower() { return new Lily(); } }

                  System Response:

                  Compile Message

                  Solution.java:62: error: cannot find symbol Region region = null; ^ symbol: class Region location: class Solution 1 error Exit Status

                  255

                  • ian_lua + 2 comments

                    pls help me understand the compile message

                    • RodneyShag + 0 comments
                      [deleted]
                    • RodneyShag + 0 comments

                      The code is now updated and will work again. (Had to change "State" to "Region" as reeta310867 suggested).

                      HackerRank solutions.

    • reeta310867 + 0 comments

      Replace 'State' with 'Region' in your code then it is successfully compiled

  • pravin3 + 1 comment

    This is very poor understanding of inheritance, The classes WestBengal, Karnataka and AndhraPradesh being shown as subclasses of State. They are instances of State class and not subclasses as given in the problem. Such poor examples create wrong concepts for students. A better example for sub class could have been Region as super class and Country, State, City as subclasses.

    • jewel25m + 0 comments

      agreed

  • cekinmezyucel + 1 comment

    The "hidden" main method is looks like this;

    public static void main(String[] args)
			throws InstantiationException, IllegalAccessException, ClassNotFoundException {

		Scanner sc = new Scanner(System.in);
		String className = sc.next();

		StringBuilder sb = new StringBuilder();
		sb.append("hackerrank.java.advanced.");
		sb.append(className);

		State state = (State) Class.forName(sb.toString()).newInstance();
		Flower flower = state.your_National_Flower();
		System.out.println(flower.whats_Your_Name());

		sc.close();

	}
    • a_anand_IT110 + 2 comments

      Can you explain 

      State state = (State) Class.forName(sb.toString()).newInstance();

      How is it working afterall?

      I am not able to understand the problem properly, can you please explian it?

      • cekinmezyucel + 0 comments
        State state = (State) Class.forName(sb.toString()).newInstance();

        This line create a new Instance of a State class.

        For example; If stdin class name is AndhraPradesh, Class AndhraPradesh's yourNationalFlower() method returns an instance of Lily class.

        Class Lily's whatsYourName() method returns "Lily".

      • utalipoglu + 0 comments

        http://stackoverflow.com/questions/4202252/how-does-class-forname-work

  • amcodehard + 0 comments

    Can you make the "hidden" main method code available?

  • HugoBaes + 0 comments

    I managed to solve without using covariant return. I suggest adding another test-case using covariant attribution.

    class Flower {
    String whatsYourName() {
        String clazz = getClass().getSimpleName();
        if (clazz.equals("Flower"))
            return "I have many names and types.";
        return clazz;
    }
}

class Jasmine extends Flower {} //@Override String whatsYourName() {return "Jasmine";}
class Lily    extends Flower {} //@Override String whatsYourName() {return "Lily";   }
class Lotus   extends Flower {} //@Override String whatsYourName() {return "Lotus";  }

class State {
    Flower yourNationalFlower() {
        String clazz = getClass().getSimpleName();
        switch(clazz)  {
            case "WestBengal":    return new Jasmine();
            case "Karnataka":     return new Lotus();
            case "AndhraPradesh": return new Lily();
        }
        return new Flower();
    }
}
  
class WestBengal    extends State {} //@Override Jasmine yourNationalFlower() {return new Jasmine();}
class Karnataka     extends State {} //@Override Lotus   yourNationalFlower() {return new Lotus();  }
class AndhraPradesh extends State {} //@Override Lily    yourNationalFlower() {return new Lily();   }

// Checking for co-variant return...
/**/
class CheckCovariant 
{{
    Flower  flower  = new State()        .yourNationalFlower();
    Jasmine jasmine = new WestBengal()   .yourNationalFlower();
    Lotus   lotus   = new Karnataka()    .yourNationalFlower();
    Lily    lily    = new AndhraPradesh().yourNationalFlower();
}}
/**/

  • tao_zhang + 1 comment
    class Flower{
    String whats_Your_Name(){
        return "I have many names and types";
    }
}

class Jasmine extends Flower{
    @Override
    String whats_Your_Name(){
        return "Jasmine";
    }
}
class Lily extends Flower{
    @Override
    String whats_Your_Name(){
        return "Lily";
    }
}
class Lotus extends Flower{
    @Override
    String whats_Your_Name(){
        return "Lotus";
    }
}

class State{
    Flower your_National_Flower(){
        return new Flower();
    }
}

class WestBengal extends State{
    @Override
    Jasmine your_National_Flower(){
        return new Jasmine();
    }
}
class Karnataka extends State{
    @Override
    Lotus your_National_Flower(){
        return new Lotus();
    }
}
class AndhraPradesh extends State{
    @Override
    Lily your_National_Flower(){
        return new Lily();
    }
}
    • wlsc1 + 2 comments

      Fixed code.

      class Flower{
    String whatsYourName(){
        return "I have many names and types";
    }
}

class Jasmine extends Flower{
    @Override
    String whatsYourName(){
        return "Jasmine";
    }
}
class Lily extends Flower{
    @Override
    String whatsYourName(){
        return "Lily";
    }
}
class Lotus extends Flower{
    @Override
    String whatsYourName(){
        return "Lotus";
    }
}

class State{
    Flower yourNationalFlower(){
        return new Flower();
    }
}

class WestBengal extends State{
    @Override
    Jasmine yourNationalFlower(){
        return new Jasmine();
    }
}
class Karnataka extends State{
    @Override
    Lotus yourNationalFlower(){
        return new Lotus();
    }
}
class AndhraPradesh extends State{
    @Override
    Lily yourNationalFlower(){
        return new Lily();
    }
}
      • samplenhold + 0 comments

        Underscores make the code easier to read. It was better the other way.

      • sirekanyan + 1 comment

        You may write whatsYourName() implementation only once:

        class Flower {
    String whatsYourName() {
        return getClass().getSimpleName();
    }
}

class Jasmine extends Flower {
}

class Lily extends Flower {
}

class Lotus extends Flower {
}

...
        • kencluff + 0 comments

          Ha! good hack.

  • ajaykrc + 0 comments

    Usage of Abstract class and Abstract method : 

    //Complete the classes below
abstract class Flower {
     abstract String whatsYourName();
}

class Jasmine extends Flower {
    String whatsYourName(){
        return "Jasmine";
    }
}

class Lily extends Flower {
        String whatsYourName(){
        return "Lily";
    }
}

abstract class Region {
  abstract Flower yourNationalFlower();
}

class WestBengal extends Region{
        Jasmine yourNationalFlower(){
        return new Jasmine();
    }
}

class AndhraPradesh extends Region {
        Lily yourNationalFlower(){
        return new Lily();
    }
}

  • 10DARSHANSINGH + 0 comments
    class Flower {
    String whatsYourName(){
    return "I have many names and types.";
    }
}

class Jasmine extends Flower{
    String whatsYourName(){
    return "Jasmine";
    }
}

class Lily extends Flower{
       String whatsYourName(){
    return "Lily";
       }
}

class Region {
    Flower yourNationalFlower(){
    return(new Flower());
    }
}

class WestBengal extends Region{
    Jasmine yourNationalFlower(){
    return (new Jasmine());
    }
}

class AndhraPradesh extends Region{
    Lily yourNationalFlower(){
    return (new Lily());
    }
}
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