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Java subclass does not recognize its common superclass

I need help with a problem that I am facing generics in Java. I write this computer algebra system, where the user enters a mathematical expression, and the system works with it in different ways (expand it, simplify it, etc.). It worked great for expressions containing natural numbers, and I wanted to expand it to work with mathematical sets. Instead of + you will have an intersection operator, etc.

At first I started recording everything for kits, but then I realized that this was probably not very good, and started using generics.

Instead of having one parsing tree, for example MathExpr, one, like SetExpr, I thought I could just create a common one Expression<T>and build a base class Numberand a base class Set.

To try to clarify, I want a mathematical expression like (2 * a) + (3 + 2) to be an instance of the class Expression<Number>, and an expression like (A ∪ B) ∩ C to be an instance Expression<Set>. Then I can perform various operations, for example, calculate the depth, etc.

Operation + is implemented as one class, * as one class, etc. Both of these classes are subclasses of the abstract class with a name TwoExpr, which, in turn, is a subclass of the abstract class Expr. This is how I did it now, and everything is working fine.

When I wanted to change my code, I made a general class Expr. This is Expr<T>. I also changed TwoExpr to TwoExpr<T>and created a base class Number.

The problem is that now I can not imagine type objects Sum<Number>.

"Type mismatch: cannot convert from Sum to Expr<Number>". Sum TwoExpr<Number>, , , Expr<Number>. , Sum generic Sum<Number>, .

,

Expr zero= new Leaf(0);
Variable a = new Variable("a");
Expr aPlusZero = new Sum(a, zero);

generics, :

Expr<Number> zero= new Leaf<Number>(new Number(0)); //works fine
Variable<Number> a = new Variable<Number>("a");     //works fine
Expr<Number> APlusZero=new Sum(a,zero); //gives a "Type mismatch:
//cannot convert from Sum to Expr<Number>" error

, Sum(a,zero) Expr<Number>, Sum

public class Sum extends TwoExpr<Number> {

    public Sum(Expr<Number> a, Expr<Number> b) {
        super(a, b);
    }
...
}

TwoExpr

public abstract class TwoExpr<T> extends Expr<T> {

    protected Expr<T> a;
    protected Expr<T> b;

    public TwoExpr(Expr<T> a, Expr<T> b) {
        this.a=a;
        this.b=b;
    }
    ...
}

, Lizkows . Number ( Object) . , , , , . - , ? , , - .

.

+3
3

, , , :

Expr<Number> zero= new Expr<Number>();
Expr<Number> a= new Expr<Number>(); 
Expr<Number> APlusZero=new Sum(a,zero);

, Variable Expr?

UPDATE:

, Variable Leaf, , :

public class Number {

    public Number(int i){}
}


public class Variable<T> extends Expr<T> {

    public Variable(String s){}
}


public class Leaf<T> extends Expr<T> {

    public Leaf(T t) {
        super();
    }
}


public class Expr<T> {

}


public class TwoExpr<T> extends Expr<T> {

    public TwoExpr(Expr<T> a, Expr<T> b) {
    }
}


public class Sum extends TwoExpr<Number> {

    public Sum(Expr<Number> a, Expr<Number> b) {
        super(a, b);
    }
}


public class AllTogether {

    public static void main(String[] args) {

        Expr<Number> zero= new Leaf<Number>(new Number(0));
        Variable<Number> a = new Variable<Number>("a");
        Expr<Number> APlusZero=new Sum(a,zero);
    }
}

Expr Variable, , , , ?

+2

, :

Object = new Sum(a,zero);
System.out.println(o.getClass().getGenericSuperclass());

, , .

0

Sumdoes not import Types.Number. So this is not Expr<Types.Number>, but Expr<java.lang.Number>. I would suggest that this would give a compilation error not only for assignment, but also for construction new Sum(vA, zero), but maybe the compiler will see another error first.

0
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