Factorial with tail-call optimization in Java 8

BigFactorial.java

// Code from “Functional Programming in Java, Chapter 7”.

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


@FunctionalInterface
interface TailCall<T> {
  
    abstract TailCall<T> apply();
  
    default boolean isComplete() { return false; }

    default T result() { throw new Error("not implemented"); }
  
    default T invoke() {
        return Stream.iterate(this, TailCall::apply)
                     .filter(TailCall::isComplete)
                     .findFirst()
                     .get()
                     .result();
    }

}


class TailCalls {
  
    public static <T> TailCall<T> done(final T value) {
        return new TailCall<T>() {      
            @Override public boolean isComplete() { 
                return true; 
            }
            @Override public T result() { 
                return value; 
            }
            @Override public TailCall<T> apply() { 
                throw new Error("not implemented"); 
            }
        };
    } 
   
}


public class BigFactorial {
    
    public static BigInteger decrement(final BigInteger number) {
        return number.subtract(BigInteger.ONE);
    }
      
    public static BigInteger multiply(final BigInteger first, final BigInteger second) {      
        return first.multiply(second);
    }

    final static BigInteger ONE = BigInteger.ONE;
    final static BigInteger TEN = BigInteger.TEN;
    final static BigInteger TWENTYK = new BigInteger("20000");  

    private static TailCall<BigInteger> factorialTailRec(final BigInteger factorial, final BigInteger number) {
        if(number.equals(BigInteger.ONE)){
            return TailCalls.done(factorial);
        } else {
            return () -> factorialTailRec(multiply(factorial, number), decrement(number));
        }
    }

    public static BigInteger factorial(final BigInteger number) {
        return factorialTailRec(BigInteger.ONE, number).invoke();
    }
      
    public static void main(final String[] args) {
        System.out.println(factorial(TEN));
        System.out.println(String.format("%.10s...(first ten digits)", factorial(TWENTYK)));      
        // System.out.println(factorial(TWENTYK)); // prints all 77339 digits
    }

}