Making a little library

Newbie & Debugging Questions
#1

Hello everybody :smiley:
I’m trying to make myself a little collection of classes that are custom to my needs. I started making a couple like Vector and Point. The problem is I want to be able to use any data type that I want, double, float, int etc. but I want to make one class that can do any of them. the problem would be the getters and setters as I would have to define what type I will return. Also I don’t want to allocate memory for things that I won’t be using in the instance of the class as it’s pretty much useless. So my question is is there a dynamic way to get around this problem without something like the code below ?


class Point{
  int nx,ny;
  double dx,dy;

  Point(int x, int y){
    this.nx = x;
    this.ny = y
  }

  Point(double x, double y){
    this.dx = x;
    this.dy = y
  }
}

Thank you in advance :slight_smile:

#2

If your class is immutable then you could parameterise it using a generic type along these lines:

public class Point<T extends Number> {
	private T x, y;
	
	public Point( T x, T y ) {
		this.x = x;
		this.y = y;
	}
}

However this will quickly break down if you need your class to do anything vaguely useful such as addition (e.g. try using a += operator in that class!)

Check out the Point2D classes in the java.awt.geom package which does something similar.

#3

Haha, not in java :smiley:

Here is a scala example of how to create a generic Vec2:


And don’t worry, It’s the same performance as if you’d write a Vec2 for each numeric type (that is: byte, char, short, int, float and double, or only float and double if you decide to use Fractional).

Yeah, it’s not trivial, but it’s possible in scala :slight_smile:

In java: No… not really. Try to find a code precompiler for java maybe, but in the end that’s what scala is.

#4

Man that’s a shame. I will have to make int and double version of everything :frowning:
Thank you for your replies though. You illuminated me :smiley:

#5

I think multiple vector types are overrated. I never needed anything else than floats and ints for vectors.

#6

It shouldn’t be too tough to make a method for all of them. Just type one up and then CnP and adjust for each. =P

#7

It’s not tough as coding goes. But it certainly is more elegant if I could do what I originally wanted to do. I’ll have to live with it thought :emo:

#8

Other alternative is to write the class in c(++) where this is trivial then jni it it. But if you’re going to go to this much trouble then you might as well use scala.

#9

With Java, it’s essentially cut and paste code, or using code generation with templates the way most primitive collection libraries do it.


public class Point<T extends Number> {
   private T x, y;
   
   public Point( T x, T y ) {
      this.x = x;
      this.y = y;
   }
}

Isn’t another problem with the generics approach is that now you are dealing with Float, Integer, and Double rather than their primitive counterparts?

Not to side track the discussion, but I have toyed with the idea of separate semantically meaningful vector classes (such as units of measure). Ideally you would get a compile time error if you tried to add a Velocity to an Acceleration or something like that.

I haven’t thought of a way to do it in Java that reads nicely and has good performance, and isn’t a hassle. Phantom types get you a part of the way there but type erasure is a pain. Scala’s value classes looks to be a nice approach.

#10

If you write the class in scala, you may want to add the @specialized annotation to the type parameter, which basically creates instances for primitive types.


class Vec2[@specialized T](val x: T, val y: T)(implicit num: Fractional[T]) {
  // ... etc
}

Truth is, I don’t know how compatible it is with typeclasses though, and I don’t have scala on this box, so try it out?

#11

Just write a little more code and save yourself the hassle of unexpected bugs and incompatibilities.

#12

Thanks! I had almost given up on what those carrots meant in java! Then I just randomly find this!