Tree generator algorithm in Java

I am trying to generate an n-ary or k-ary tree using the GraphStream API, in Java.

The algorithm may or may not generate a full tree. This code is pretty far off from being able to generate such a tree:

public class Demo {

public static void main(String args[]) {

    for (int i = 0; i <= 30; i ++) {
        graph.addNode("a" + i);
        graph.addNode("b" + i);
    }
    TreeGenerator(graph, 5, 3);
    graph.display(); 
}

   static Graph graph = new SingleGraph("graph");
   static Graph TreeGenerator(Graph g, int n, int tiers) {

    while (n > 0) {
        for (int i = n; i > 0; i--)
        g.addEdge("ab" + n,"a" + (n-n),"b" + n);
        //g.addEdge("ad" + n,"a" + n,"b" + n*Math.round(Math.log(n)));
        n--;
    }

    if (tiers > 0) {
        tiers--;
        return TreeGenerator(g, n, tiers);
    } else return g;
}  
}