Poziom 6

Zielona Gora

15.01.2020
1703widoki
9komentarze

stuck

Pytania dotyczące zadania

Rozwiązane

im stuck, can someone tell me how am i supposed to write it?

Zaimplementuj metodę boolean walka(Kot innyKot):
zaimplementuj mechanizm, zgodnie z którym koty będą walczyć w oparciu o takie cechy, jak waga, wiek i siła.
Możesz sam/a zadecydować, jaka będzie rola danej kociej cechy w walce.
Metoda powinna określać, czy walka jest wygrana bądź nie, np. zwracać true, jeśli wygrywamy, a jeśli nie – false.

Musi zostać spełniony poniższy warunek:
jeśli kot1.walka(kot2) zwraca true,
to kot2.walka(kot1) musi zwrócić false

Wymagania:

Klasa Kot musi mieć konstruktor bez parametrów.
Klasa Kot musi zawierać pola publiczne o nazwach wiek, waga i sila.
Klasa Kot musi mieć metodę walka.
W metodzie walka zaimplementuj mechanizm, który sprawi, że o wynikach walk między kotami będą decydowały wartości ich wagi, wieku i siły.
Za każdym razem, gdy walczymy z tym samym kotem, metoda powinna zwracać tę samą wartość.
Jeśli kot1 pokonuje kot2, to kot2 musi przegrywać z kot1.

package pl.codegym.task.task05.task0502; /* Zaimplementuj metodę walka */ public class Kot { public int wiek; public int waga; public int sila; public Kot() { } public boolean walka(Kot innyKot) { if(Kot.wiek > innyKot.wiek || Kot.sila > innyKot.sila || Kot.waga > innyKot.waga) { return true; } else return false; } public static void main(String[] args) { Kot kot1 = new Kot(); Kot kot2 = new Kot(); kot1.wiek = 1; kot2.wiek = 1; kot1.waga = 2; kot2.waga = 3; kot1.sila = 8; kot2.sila = 13; kot1.walka(kot2); kot2.walka(kot1); } }

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horst

Poziom 26 , Not in list

15 stycznia 2020, 23:53

I guess that the conditions that are not satisfied are the ones that say "The method should return the same value every time we fight the same cat." and "If cat1 beats cat2, then cat2 must lose to cat1." Look at your if-conditions again. First: Make sure that your syntax is correct. At the moment you are comparing Classes and not objects. (Kot.wiek ...). Try using "this" instead of Kot. Second: While the requirements might be met for the two concrete cats you just created, your conditions will fail, for example, in the following corner case: What happens if two cats have absolutely identical wiek, waga and sila? Then your fight-method will always return false. So,

kot1.walka(kot2);   // kot1 loses
kot2.walka(kot1);  // kot2 loses

because none of the if-conditions are met. In short: Since the rest of your code looks decent, just play around with the winning conditions a little and you will work it out. I am sure you will eventually find a nice pattern that always creates reproducible results.

Tomasz

Poziom 6 , Zielona Gora, Poland

16 stycznia 2020, 15:17

non-static variable wiek cannot be referenced from a static context. file pl/codegym/task/task05/task0502/Kot.java, line 19, position 5637 well rn i have such error

horst

Poziom 26 , Not in list

16 stycznia 2020, 19:16

rozwiązanie

I am not too great at interpreting error-messages, but as I mentioned above, in line 19 ( = your if-conditions) you try to compare the whole Class of Cats with a single instance of it:

if(Kot.wiek > innyKot.wiek || Kot.sila > innyKot.sila || Kot.waga > innyKot.waga)

Admittedly, I did not make myself very clear, but what I meant is that you should try this instead:

if(this.wiek > innyKot.wiek || this.sila > innyKot.sila || this.waga > innyKot.waga)

If it still won't pass the tests, work on your conditions like I suggested. Merry Coding!

Tomasz

Poziom 6 , Zielona Gora, Poland

16 stycznia 2020, 21:08

yes, replacing "Kot" with "this" solved problem i was fighting with for around 2 weeks, dude thanks a lot!

horst

Poziom 26 , Not in list

17 stycznia 2020, 00:45

My pleasure! I'm glad I could help out. Wszystkiego dobrego!

Daniel

Poziom 7 , Debowiec, Poland

30 marca 2020, 17:51

Why do we use "this" not "Kot"? How does the system know that we want to use "Kot" as "this"?

horst

Poziom 26 , Not in list

31 marca 2020, 18:44

I know this can be confusing sometimes. In general, "this" always refers to the current object, i.e. the class you are currently coding in. So, in the class "Kot" the keyword "this" refers to a Kot-object. If you create another class called "Boris" then "this" will refer to a Boris object. But what does this do? Why do we need it? Two fairly common uses of it are: 1. Purely for clarification when two variables have similar or identical names, especially inside constructors:

public class HotDog {
   int length;
   double price;
   boolean withOnions;

   public HotDog(int length, double price, boolean onions){
     this.length = length; // neccessary, because weight = weight would confuse the compiler
     this.price = price; // neccessary
     this.withOnions = onions; // here it is optional, because the variables have different names
// you could also write:
     withOnions = onions;
/* But even if "this" is optional here, it is still considered good practice to use it,
   because it helps clarifying what is the name of the instance-variable (withOnions)
   and what is the variable name of the parameter (onions) */
   }
}

2. If you want to use your whole object as an argument inside a method:

// Method that returns the current HotDog-object as a datatype:
public HotDog meWantsHotdog(){
  return this;
}
// Note that the return-type of the method is "HotDog", so the method expects you to return an object of the HotDog-type.

You'll find many more examples with much more precise terminology here.

horst

Poziom 26 , Not in list

31 marca 2020, 18:51

But I almost forgot: back to our concrete Kot-problem you mentioned. In the faulty code, we had:

if (Kot.wiek > innyKot.wiek)

"innyKot.wiek" works perfectly fine, because the method gets the innyKot-object ( = a concrete cat with a concrete weight) and can access its wiek-variable. However, in "Kot.wiek" the "Kot"-part would stand for the whole Kot-class and not for a concrete single object like this concrete cat I am currently calling the walka-method for. Here, "this" is a great thing to refer to the actual object before it even exists. You can think of it as a sort of placeholder for the variable name of thefuture Cat object you are about to create. In the code above, the walka-method will be called several times for different cats. (kot1, kot2). If we use "this", the keyword will be replaced by the "real object" every time we call the method:

if (this.wiek > innyKot.wiek)
// can be seen as kot1.wiek > innyKot.wiek when we call the method for kot1
// can be seen as kot2.wiek > innyKot.wiek when we call the method for kot2

Daniel

Poziom 7 , Debowiec, Poland

1 kwietnia 2020, 16:05

Thank you horst! Very good explained. :)