# [plt-scheme] Functional Abstraction in Java

Andrei Estioco wrote:
>* Hello. My apologies if I am asking this in the wrong mailing list (this
*>* being a Java question).
*
This is *the* list for this sort of question.
You can think of functions A -> B as objects satisfying the interface B
apply(A). So all of your favorite functional idioms can be expressed in
Java, just with considerable notational overhead. It's fun to try -- so
don't just take my word for it.
Here is a simple program that shows how to develop filter (which
abstracts the predicate to apply):
http://www.ccs.neu.edu/home/dvanhorn/tmp/213-notes/02-18-2009-2.java
Here is a higher-order example, the derivative function:
class Calculus {
interface IFun1<X,Y> { Y apply(X x); }
IFun1<IFun1<Double,Double>,IFun1<Double,Double>> deriv =
new IFun1<IFun1<Double,Double>,IFun1<Double,Double>>() {
double d = 0.001;
public IFun1<Double,Double> apply(final IFun1<Double,Double> f) {
return new IFun1<Double,Double>() {
public Double apply(Double x) {
return (f.apply(x + d) - f.apply(x - d)) / (d * 2);
}
};
}
};
IFun1<Double,Double> sin = new IFun1<Double,Double>() {
public Double apply(Double x) { return Math.sin(x); }
};
IFun1<Double,Double> cos = deriv.apply(sin);
}
Finally, you can find more of this in the Little Java book (and in the
forthcoming HtDC book).
David