Triangle is so easy that you don't need to use either
iteration or recursion for it.
You might recognize
Triangle(N) as the sum of the series
1 + 2 + 3 + ... + (N-1) + N = (N(N+1)) / 2
1 + 2 + 3 + 4 = (4(5)) / 2 = 10
Of course a Java method that implements this formula is simple. And that is how you would do it in a practical situation. But the goal of this chapter was to show recursion with Java and and triangle numbers work well for that.