Y Combinator based factorial in Java 8

YCombinatorFactorial.java

package test;

import java.math.BigInteger;
import java.util.function.Function;

public class YCombinatorFactorial<T> {
    private interface Improver<T> {
        Function<T,T> apply( Improver<T> f ) ;
    }

    private Function<T,T> Y( final Function<Function<T,T>,Function<T,T>> r ) {
        return ((Improver<T>)f -> f.apply( f )).apply( f -> r.apply( x -> f.apply( f ).apply( x ) ) ) ;
    }

    public static void main( String[] args ) {
        YCombinatorFactorial<BigInteger> yf = new YCombinatorFactorial<BigInteger>() ;
        BigInteger result = yf.Y(
                f -> n -> n.equals( BigInteger.ZERO ) ?
                        BigInteger.ONE :
                        n.multiply( f.apply(n.subtract( BigInteger.ONE ) ) ) ).apply( BigInteger.valueOf( 100 ) ) ;
        System.out.println( result );
    }
}