Using generics in Java and numeric types

Question

Hey there I have just started Java for a course I will be taking soon (yeah I'm a nerd, I study before I have start the class). I wanted to cram a few concepts together to learn how everything works together in Java.

Now, what I wanted to try to do is a simple Shape (Forme in french) class which has a least 1-2 classes inheriting from it and make a few instances inside a collections object. I also wanted to make those classes generics to try that out as well. That is the part I'm having trouble with.

I'm very used to C++ so that may be where the confusion lies, but it seems that I can't use generics so that, for example, I can specify that dimensions will be in, say, Integer or Double or Float or whatever. Is it possible ?

The GetArea() method below can't compile because of base and hauteur.

package testconcepts;

import java.util.*;
import java.lang.*;

abstract class Forme <T extends Number> {
    protected String nom_forme;

    String NomForme() { return nom_forme; }

    abstract Double GetArea();
}

class Triangle <T extends Number> extends Forme {
    protected T base, hauteur;

    Triangle() { 
        this.nom_forme = "TRIANGLE"; 
    }

    @Override
    Double GetArea() { return base * hauteur / 2; }

    T GetBase() { return base; }
    T GetHauteur() { return hauteur; }
    void SetBase(T base) { this.base = base; }
    void SetHauteur(T hauteur) { this.hauteur = hauteur; }
}

public class TestConceptsJava {
    public static void main(String[] args) {
        ArrayList list = new ArrayList();
        list.add(new Triangle<Double>());
    }
}

Notice that I also cannot make assignments to 0 in the constructor because of type issues.

Also, I know there are very similar topics, but I haven't found a solution to my problem yet in those other questions.

TLDR How can I make work generics with primitive type wrappers (Double, Integer..) ?

"yeah I'm a nerd, I study before I have start the class" You don't have to defend yourself in front of other nerds =p — Code-Apprentice
– Code-Apprentice, Commented Oct 5, 2012 at 21:07
Sorry about the error message, I can't seem to copy-paste it. It complains that T is a bad operand type on that line. — ApplePie
– ApplePie, Commented Oct 5, 2012 at 21:16

arshajii · Accepted Answer · 2012-10-05 21:11:29Z

6

So your problem is this line I guess:

Double GetArea() { return base * hauteur / 2; }

The reason you receive an error is because the * operator is undefined for objects of type Number. Since you know that you will be dealing with ints and doubles (I assume), you can try this:

Double GetArea() { return base.doubleValue() * hauteur.doubleValue() / 2; }

As a note, usually we start method names with a lower case letter, i.e. getArea().

answered Oct 5, 2012 at 21:11

arshajii

130k26 gold badges246 silver badges293 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Jordan Kaye · Accepted Answer · 2012-10-05 21:11:36Z

3

In GetArea(), you're calling the * operator on two classes. How much sense would it make to say something like:

Object o1 = //whatever;
Object o2 = //whatever;

return o1 * o2;

Not much right? You can't multiply two classes. What you actually want to do is multiply the underlying value of those wrapper classes (which is not done implicitly for you in Java). Rewriting GetArea() like this should solve your problem:

Double GetArea() { return base.doubleValue() * hauteur.doubleValue() / 2; }

Note that you'll also have to use the underlying value everywhere else that you care about it in that class.

answered Oct 5, 2012 at 21:11

Jordan Kaye

2,88717 silver badges15 bronze badges

4 Comments

ApplePie Over a year ago

Is Java really that limited in terms of working with primitive types ? What if I really want to work with whatever type I pass it to ? Would I have to use instanceof and list every type I can think of ?

Jordan Kaye Over a year ago

If you wanted the method to work with absolutely anything, you wouldn't really be able to call the * operator on the generic.. Your best bet would probably be to write a method to handle a multiplication of two unknown types.

ApplePie Over a year ago

It's ok really. I don't have a need for it. I'm just looking at the possibilities and limitations of the language and my point of comparison is C++ so that's why I was trying to test that.

Louis Wasserman Over a year ago

Generics and primitive types really don't get along in this way -- just being a Number subclass doesn't let you do arithmetic. That said, I've almost never felt the need for this -- just pick a number type and stick to it.

Victor Sorokin · Accepted Answer · 2012-10-05 21:10:57Z

1

Changing to Double GetArea() { return base.doubleValue() * hauteur.doubleValue() / 2; } will make code compile, but this is bad idea in Java, since generics in java aren't for this. They just, basically, to ensure static type safety when using collections of objects.

Java has no operator overloading, so you can't expect * to work for anything except primitive types.

I'd suggest reading basic Java generics tutorial to get an idea what are they for in Java: http://docs.oracle.com/javase/1.5.0/docs/guide/language/generics.html

answered Oct 5, 2012 at 21:10

Victor Sorokin

12k2 gold badges38 silver badges53 bronze badges

Comments

Code-Apprentice · Accepted Answer · 2012-10-05 21:11:28Z

1

Look at this part of your calculation:

base * hauteur

Since both base and hauteur are declared using a wrapper class, you cannot use the * operator like you can with primitive types. Instead, you need to get the value explicitly:

base.doubleValue() * hauteur.doubleValue()

Note that this wouldn't be a problem if either base or hauteur were declared with a primitive type. Then the Java compiler would be able to use auto-unboxing to convert the other one to the appropriate type and do any implicit conversions needed.

answered Oct 5, 2012 at 21:11

Code-Apprentice

84k26 gold badges162 silver badges289 bronze badges

Comments

jpm · Accepted Answer · 2012-10-05 21:11:56Z

1

What you're looking for is called "auto-(un)boxing". Unfortunately, operations on T must be valid for all subtypes of Number, and "auto-(un)boxing" is not such an operation (e.g., it will not work with BigInteger).

answered Oct 5, 2012 at 21:11

jpm

3,15519 silver badges24 bronze badges

Collectives™ on Stack Overflow

Using generics in Java and numeric types

5 Answers 5

Comments

4 Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

Comments

4 Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related