overloading + and += operators for “Number Classes”

Question

I want to create extension functions for classes that encapsulate simple Numbers. For example DoubleProperty. I encountered the problem, that I can\'t

Answer 1

Your operator override implementation has two problems:

1. inconsistent type after plus

operator fun ObservableDoubleValue.plus(number: Number): DoubleProperty 
    = SimpleDoubleProperty(get() + number.toDouble())

Any ObservableDoubleValue instance plus a Number, got a DoubleProperty instance(or say a SimpleDoubleProperty instance). Let's say I have a type ComplexDoubleProperty implements ObservableDoubleValue, you will see:

var a = getComplexDoubleProperty()
a = a + 0.1    //compile error, WTF?

//or even
var b = SimpleDoubleProperty(0.1)
b = b + 0.1    //compile error, because b+0.1 is DoubleProperty

You can see this behavior makes no sense.

2. a=a+b and a+=b should be identical

If your implementation compiles, you will have

var a: DoubleProperty = SimpleDoubleProperty(0.1)  //add DoubleProperty to make it compile
var b = a
a += 0.1
println(b == a)

prints true because += sets the value to the original instance. If you replace a+=0.1 with a=a+0.1 you will get false because a new instance is returned. Generally speaking, a=a+b and a+=b are not identical in this implementation.

To fix the two problems above, my suggestion is

operator fun SimpleDoubleProperty.plus(number: Number): SimpleDoubleProperty
        = SimpleDoubleProperty(get() + number.toDouble())

so you don't need to override plusAssign. The solution is not as general as yours, but it's correct if you only have SimpleDoubleProperty calculations, and I believe you do, because in your implementation, plus always returns a SimpleDoubleProperty instance.