Generic type in Java

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I have 3 classes, Expression, Operation, Scalar

public abstract class Expression<R extends Expression> {
    public abstract R calcular();
}

public abstract class Operation<T extends Expression, R extends Expression> extends Expression {
    protected T arg;
    public Operation(T arg) {
        this.arg = arg;
    }
    public T getArgumento() {
        return arg;
    }
}
public class Scalar extends Expression {
    private double valor;

    public double getValor() { }
    public void setValor(double valor) { }
    public Scalar(double valor) {
        this.valor = valor;
    }
    public Scalar calcular() {
        return this;
    }
}

What I want is to create the class of inverting a scalar number, for example:

 public class InversaoEscalar extends Operation{
     public InversaoEscalar(Expression<Scalar> arg){
        super(arg);
     }
     public Scalar calcular(){
         return new Scalar(Parametro);
     }
 }

Whereas in Parametro I’d like to pass 1/arg.calcular().getValor().

But I don’t understand why arg.calcular() does not return me an object of type Scalar, and yes of the type Expression, since in my understanding, if I am passing a argument of the type Expression<Scalar>, the method calcular He should bring me a guy Scalar, no? How to solve this problem?

java generic polymorphism
  • 1

    se eu estou passando um arg do tipo Expression, o metodo calcular dele deveria me trazer um tipo Scalar Why do you think that? If Arg is Expression Arg is Expression.

  • My method of calculating() in Expression<R> returns R, so I believe that calculating in an Expression<Scalar> would return Scalar, not?

  • You in the case just have 1/arg.calcular().getValor() at the place where it is written Parametro?

  • That, 1/arg.calcular().getValor() in place of parameter, but as arg.calcular() is Expression, it has no method getValor(). And the return is even Scalar, I edited there in the question.

1 answer

5


You are inheriting without specifying generic types:

Operation extends Expression -> R sem tipo
Scalar extends Expression -> R sem tipo
InversaoEscalar extends Operation -> T e R sem tipo

When you do not declare a type, the generic implementation uses a base type (in this case R extends Expression, thus the basic type of R will be Expression, if there were no upper bound explicit the type would be Object).

Point solution:

Do Scalar be a Expression<Scalar> (in that case calcular will return a Scalar).

class Scalar extends Expression<Scalar>

Do InversãoScalar define at least the type Expression<Scalar> for T (arg will be that kind T), and some kind of Expression for R that isn’t even being used:

class InversaoEscalar extends Operation<Expression<Scalar>, Expression<?>> {

    public InversaoEscalar(Expression<Scalar> arg) {
        super(arg);
    }

    public Scalar calcular() {
        return new Scalar(1/arg.calcular().getValor());
    }
}

P.S.: your solution is a bit loaded with generics and inheritance (the possibilities of combinations there make the use of the API complicated; even if you want to go this way you will have to rethink your Bounds and what should be parameterizable). I recommend reading this question: "It is wrong to use class inheritance to group common behaviors and attributes?", as well as in the code of other libraries dealing with mathematical expressions in Java.

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