Method does not correctly show expected result

Question

I have this interface

public interface Promocional {

    boolean aplicaDesconto(double porcentagem);
    
    default boolean aplicaDescontoDe10Porcento() {
        return aplicaDesconto(0.1);
    }
}

The class LivroFisico that extends the superclass Livro and implements the interface Promocional

public class LivroFisico  extends Livro implements Promocional{
    public LivroFisico(Autor autor) {
        super(autor);
    }

    @Override
    public boolean aplicaDesconto(double porcentagem) {
        // TODO Auto-generated method stub
        return true;
    }

}

And here in the method main it does not display properly.

main()

LivroFisico fisico = new LivroFisico(autor);
fisico.setValor(59.90);
if(fisico.aplicaDescontoDe10Porcento()) {
        System.out.println("Valor agora e: " + fisico.getValor());
    }

It displays 59.90 and not 53.91 which is the discounted amount.

I believe that in class LivroFisico which is mandatory the use of the method, is giving error.

Answer 1

You have two problems.

Your problem is recurring these days. Most people learn to program by methods I don’t understand. It seems to be a thing of following recipes and not learning the concepts, the functioning of what you are using. Even being full of information about it in books, tutorials, blogs, and even here in various questions. I myself I answered a few hours ago about C#, but the problem applies to all languages. Just follow all the links to learn about the subject.

The most specific answer in Java is here.

In short: the processor for better performance treats data types float and double binary form and therefore cannot represent all numbers with decimal places, there is the so-called rounding problem and there is nothing you can do about it other than to use the right type of data when dealing with monetary values or representing something else that needs accuracy. This guy is the Bigdecimal.

In addition your discount method (aplicaDesconto()) is not doing any operation. It does not calculate any discount.

Behold What is the correct way to use the float, double and decimal types?.

Answer 2

A quick and simple way to solve this is to change your method of applicationso:

public boolean aplicaDesconto(double porcentagem) {
        setValor(getValor() - (getValor()*porcentagem))
        return true;
    }

This will cause him to actually multiply the value. Remember to create the gets and value sets.

public Double getValor() {
    return valor;
}

public void setValor(Double valor) {
    this.valor = valor;
}