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Level 41

12/3/20
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Why do I also have to create the abstract constructor to be able to extend an abstract class?

Question about the task

Currencies

Java Core, Level 4, Lesson 8

Under discussion

In USD, Euro and Ruble class.

Expand the program's ability to manipulate currencies.

1. Create a private double field amount in the abstract Money class.
2. Create a public getter for the field amount (public double getAmount()) so you can access it outside the Money class.
3. In the separate files, create Euro, Ruble, and USD classes, which will be descendants of the Money class.
4. In the Euro, Ruble, and USD classes, implement the getCurrencyName method, which will return the abbreviated name of the currency (String) (EUR, RUB, USD).
5. In the Euro, Ruble, and USD classes, implement a public constructor that takes one argument and calls the constructor of the base class (super class) with this argument.
6. Populate the allMoney list with all possible currency objects in accordance with the task conditions and the functionality of the program.

Requirements:

The Money class must have a private double field amount.
The Money class must implement a public constructor with one double parameter, which sets the variable amount to the passed parameter.
The Money class must implement a public getAmount method that returns the value of the field amount.
The Euro, Ruble, and USD classes must be descendants of the Money class and be located in separate files.
The Euro, Ruble and USD classes must implement the getCurrencyName method.
The getCurrencyName method must return the abbreviated name of the corresponding currency (String).
The Euro, Ruble, and USD classes must implement a public constructor with one double parameter. This constructor must set the value of the Money class's field amount by calling the parent class's constructor with the same argument.

package com.codegym.task.task14.task1417; public class USD extends Money { public USD(double amount) { super(amount); } @Override public String getCurrencyName(){return "USD";} }

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Nouser

Level 36 , Germany

3 December 2020, 12:27

useful

I'm not really sure that I understand what you exactly mean. But think of an abstract class as a fully functional regular class. You're just not supposed to create instances of it. So if Money is a functional class, then it may have fields an of course one or more constructors. Child classes may use that constructor as well. Just take this task as an example. You have a Money class with a field amount (you haven't coded that yet). That field you set in the constructor. Child classes won't need that field anymore as it already exists in the parent class. Same for the logic inside the constructor. Here it is really simple logic, just an assignment. But of course that could be more complex. So in the child classes inside the constructor you grab the passed amount and pass it to the super class. This now is able to store the passed value inside its amount field.

public abstract class Money {
    private double amount;

    public Money(double amount) {
        this.amount = amount;
    }
}

public class Euro extends Money {
    public Euro(double amount) {
        super(amount);
    }
}

You probably see it now in the classes without methods. Just construcor and field here. The child class receives the argument and passes it to the parent. The parent again is doing the work.

Andrei

Level 41

3 December 2020, 14:04

Hi Nouser, sorry, reading back my question I realise it is not clear as to what my lack of understanding is. What I mean is, there is abstract class Money (like you exemplified) and this abstract class has the Money constructor which takes a double variable as an argument. My question is, when I create a new class, that extends Money, like Euro class that you showed, why to I have to write the constructor as well. Shouldn't it work like in methods? Meaning if the method in the abstract class is not abstract, you can skip implementing it in the new class. Does this make more sense?

Nouser

Level 36 , Germany

3 December 2020, 16:56

useful

Then let's stay with this exmple ;) The Money abstract class requires the amount to be set. Means the child classes need to set that amount as well. That's cause Euro is Money as well and as child it shares the fields... and cause the coder though it is a good idea to use the constructor to do that. He could have used some setter, too. But here it is the constructor. Of course you can extend the child class constructor with other parameters and, if you know, that eg. amount always will be 100, then you might not ask that in the child constructor but just give these 100 directly to the base class using super. So the child class coder can decide to do totally different things in the constructor and has every freedom to make the class fitting best to its new purpose. It's just a way to do things. Edit: And if you want to allow the way you just suggested, you are of course free to do that. Just add a default constructor to your base class and make the fields protected (or use setters). Then you can initialize them later, if you don't like the way using the constructor.

Andrei

Level 41

4 December 2020, 09:08

"So if Money is a functional class, then it may have fields an of course one or more constructors. Child classes may use that constructor as well." The problem in this exercise was that the program gave an error in Euro/USD/Ruble classes when I did not implement the Money constructor. If you say I may (or may not) use the constructor from the parent class, then why am I forced to implement it in the Euro/USD/Ruble classes?

Nouser

Level 36 , Germany

4 December 2020, 10:27

useful

It's coded that way. If you want to it to work different, then you need to layout that class different. Eg. give it a default constructor. A child class is always (invisible except you do anything else) calling as first action inside of the constructor the parent constructor. If there is no (default) constructor cause you added an own, it will throw an error. In this example the base class has no default constructor cause you have written an own implementation. So the default super call of the child class will fail and you have to add the call to the overloaded constructor. That probably is what you should learn in this task. I admit this is extremly confusing and in my first two attempts to get this into my brain I gave up. So if you cmpare your success to mine, I'd say you're far more ambitious ;)

Andrei

Level 41

4 December 2020, 11:26

Haha, thank you for your words! I just keep pounding information in the brain in hopes that some of it sticks, lol. Although sometimes I just want to punch a hole through the monitor.

Amo programmare

Level 32 , Italy

Expert

13 February 2023, 12:54

Instead of opening another thread, I figured I would ask the question here as it is related. It wasn't super clear to me why I kept getting the error: constructor Money in class com.codegym.task.task14.task1417.Money cannot be applied to given types; required: double found: no arguments reason: actual and formal argument lists differ in length. When I tried to create a constructor

public class Euro extends Money {
    private double amount;

    public String getCurrencyName() {
        return "EUR";
    }
    public Euro(double amount) {
       this.amount = amount
    }
}

And why it was necessary to use the super keyword.

Thomas

Level 41 , Bayreuth, Germany

13 February 2023, 18:58

When an object is created, the first thing that is implicitly called in its constructor (no-args or not) is the no-args parent constructor. If the parent no-args constructor does not exist, then there is an error. It is the same here. A constructor with arguments has been created in the parent, so there is no longer a no-args constructor. The contract to call the super constructor first is not invalidated. You just have to take care of it yourself and call the parameterized constructor. The code you posted here is an example of how not to do it. a) duplicate code, amount already exists in the parent. So you should use this b) Regarding polymorphism, your example quickly leads to hard to find errors. The reason is that methods are polymorphic, whereas attributes are not. If you add variable shadowing (like you did) the confusion is perfect and your colleagues will hate you.

Misiu

Level 41 , Gdansk, Poland

3 December 2020, 12:26

Constructors are for initializing data. All classes have constructors, concrete or default. Objects are created by new constructor() instruction.

Andrei

Level 41

3 December 2020, 14:05

Thank you, but I did not formulate my question clearly enough, sorry. I have given more details above.

Misiu

Level 41 , Gdansk, Poland

3 December 2020, 14:47

Constructor in abstract class has parameter. If you will not create a constructor with parameter in Euro class then a constructor in Money class will not be invoked. While creating Euro object there will be invoked a default constructor of Money class, a constructor without parameters.

Andrei

Level 41

3 December 2020, 14:58

Yes, and what is wrong with a constructor without parameters? Or maybe I want another type of constructor and the one in Money is no good for me. Am I missing something? Am I too tired? 😂

Misiu

Level 41 , Gdansk, Poland

3 December 2020, 15:39

You may to have other constructors. Constructors may be overloaded. What is your idea to initialize "private double amount" field by Euro object?

Andrei

Level 41

3 December 2020, 15:43

I'm sorry, I don't really understand your question. If you mean if it was my idea to have

private double amount;

in the MOney class then my answer is no, it was a requirement from Codegym.

Misiu

Level 41 , Gdansk, Poland

3 December 2020, 16:13

Yes, yes, I know... Generally, it is our (programmers) task to write all the code and initialize "amount" field. But the way to do that initializing is shown in the task - constructors in base and child classes. Is there an another way? What type of other constructor(s) do you have in mind?

Andrei

Level 41

3 December 2020, 16:51

I don't know yet if there is another way. I also did not have any other type of constructors in mind, my confusion came from the fact that in abstract classes you don't have to 'override' non abstract methods yet I have to 'override' non abstract constructors, even though there probably isn't such a thing as an abstract constructor. I hope it makes sense. Thank you for your help!

Misiu

Level 41 , Gdansk, Poland

3 December 2020, 18:04

useful

You don't have to override. You can use default or any other constructor in Euro class. Constructor of child class invokes constructor of base class (programmer use super() or with default constructors does this compiler). Objects are created not by constructors (they initialize object) but by new constructor() instruction. If you create a constructor with parameters then you may not use default constructor unless you yourself write such a default constructor into the class. So if you want to use default constructor of Euro class then create such a constructor in Money class (or delete other constructors, default constructor of Money class will be invoked by compiler). If you want to use other constructor of Euro class then create such a constructor in Money class.

Andrei

Level 41

4 December 2020, 09:02

You say I don't have to override but the program was giving an error in Euro class when I did not implement the constructor from the Money class.

public Money(double amount) {
    }

Why is that?

Misiu

Level 41 , Gdansk, Poland

4 December 2020, 11:49

useful

I was saying about using constructors. Generally. There is no necessity to override. All it depends on programmer willing, how he/she wants his/her code looks like. As all other classes abstract class has a constructor. Concrete or default. In this task there is concrete constructor in Money, so Euro has to have such a constructor too.

Andrei

Level 41

4 December 2020, 12:00

Thank you!