当该实例可能具有不同的数据类型时，如何返回该实例的正确数据类型？

Question

I have this code in Modula-2,

PROCEDURE Prune(typeExp: TypeExp): TypeExp;
    BEGIN
        CASE typeExp.^class OF
            | VarType:
                IF typeExp^.instance = NIL THEN
                    RETURN typeExp;
                ELSE
                    typeExp^.instance = Prune(typeExp^.instance);
                    RETURN typeExp^.instance;
                END;
            | OperType: RETURN typeExp;
            END;
END Prune;

I have several problems when I try to convert this code into java. I can create an instance and judge if its instance is null and then choose what to return. But I don't really know what to do with the case 2, which is the instance might be a new Opentype(); because only one value can be returned in this case.

public TypeExp Prune(TypeExp typeExp){
    TypeExp r = new VarType();
    if (r.instance == null) {
        return r;
    }
    else {
        r.instance = Prune(r.instance);
        return r.instance;
    }
}

The second issue is I don't think I can call the function Prune() inside itself, so what can I do? Thanks in advance.

Answer 1

I dont really know Modula-2, but it might be something like this:

public TypeExp Prune(TypeExp typeExp) {
    if (typeExp instanceof VarType) {
        if (typeExp.instance == null) {
            return typeExp;
        }
        else {
            typeExp.instance = Prune(typeExp.instance);
            return typeExp.instance;
        }
    } else if (typeExp instanceof OperType) {
        return typeExp;
    }
    //if typeExp is not an instance of VarType or OperType
    return null;
}

The Modula code does not return in all code paths. Thats not possible in Java. I inserted return null in those cases. Thats probably wrong for your application though.

Answer 2

Below example not same as your func, but I think you can modify to your needs. It hides your return types behind Type class => you can return objects of two classes.

Main

package com.type;

public class Main {
    public static void main(String[] args) {
        Type first = new FirstType();
        Type second = new SecondType();

        System.out.println(func(first).getTypeName());
        System.out.println(func(first).getTypeName());
        System.out.println(func(second).getTypeName());
    }

    public static Type func(Type type) {
        if(type instanceof FirstType) {
            type.setTypeName("First");
        } else {
            type.setTypeName("Second");
            // something here
        }
        return type;
    }
}

Type

package com.type;

public class Type {
    private String typeName;

    public Type() {}

    public String getTypeName() {
        return typeName;
    }

    public void setTypeName(String typeName) {
        this.typeName = typeName;
    }
}

FirstType

package com.type;

public class FirstType extends Type {
}

SecondType

package com.type;

public class SecondType extends Type {
}