f(f(n)) == -n

FFN.java

import org.junit.Assert;

public class FFN {

    public static int f(int n) {
        if (n > 0) {
            if (n % 2 == 0) {
                return n - 1;
            } else {
                return -n - 1;
            }
        } else if (n < 0) {
            if (n % 2 == 0) {
                return n + 1;
            } else {
                return -n + 1;
            }
        } else {
            return 0;
        }

    }

    public static void check(int i) {
        Assert.assertEquals(-i, f(f(i)));
    }

    @org.junit.Test
    public void testF() throws Exception {
        for (int i = Integer.MIN_VALUE + 2; i < Integer.MAX_VALUE; i++) {
            if (i % 100_000_000 == 0) {
                System.out.println(i);
            }
            check(i);
        }
        check(Integer.MAX_VALUE);
    }

}