CodeIQ_Collatz.java

import java.util.function.IntFunction;
import java.util.stream.IntStream;

public class CodeIQ_Collatz {

    static IntFunction<Integer> d = n -> n / 2;
    static IntFunction<Integer> m = n -> n * 3 + 1;
    static IntFunction<Integer> ev = n -> {
        if (n % 2 == 0)
            return d.apply(n);
        else
            return m.apply(n);
    };

    static IntFunction<Boolean> e = n -> {
        int r = m.apply(n);
        while (r != 1) {
            if (r == n)
                return true;
            r = ev.apply(r);
        }
        return false;
    };

    public static void main(String[] args) {
        System.out.println(IntStream.range(2, 10000).filter(p -> p % 2 == 0).filter(e::apply).count());
    }
}