Java test

// file Node.java
public class Node {
    protected final int value;

    public Node(int value) {
        this.value = value;
    }
}

// file InnerNode.java
public class InnerNode extends Node {
    private final List<Node> children;

    public InnerNode() {
    }

    public void add(Node node) {
        children.add(node);
    }

    public void print() {
        print(0);
    }

    private void print(int padding) {
        for (int i = 0; i < children.size(); i++) {
            Node child = children.get(i);
            printPadding(padding);
            System.out.println(child.value);

            child.print(padding + 1);
        }
    }

    private static void printPadding(int padding) {
        for (int i = 0; i < padding; i++) {
            System.out.print(" ");
        }
    }

}

// file Main.java
public class Main {
    public static void main(String[] args) {
        Node left = new Node(42);
        Node right = new Node(99);

        InnerNode inner = new InnerNode();
        inner.add(left);
        inner.add(right);

        inner.print();
    }
}