Implementing a Fifteen Puzzle AStar pseudo code into a java code

Question

We have been given a task to do the "TODO" part of the following java file:

1 To do part

https://courses.cs.duke.edu/spring09/cps170/AStar.java

https://courses.cs.duke.edu/spring09/cps170/Node.java

The 3 To do part here is the isGoal(), evaluateHeuristic(), and generateChildren() methods.

https://courses.cs.duke.edu/spring09/cps170/FifteensNode.java

I am having a trouble converting the pseudocode inside the AStar.java's run() function. Below is the pseudocode:

* create a TreeSet and provide it with the node's comparator like this:
* TreeSet<Node> fringe = new TreeSet<Node>(rootNode.getComparator());
* General algorithm:
* 1. Create an empty fringe, as above
* 2. Put the root search node to the fringe
* 3. While fringe is not empty:
* 4.     Get the first search node from the fringe
* 5.     Remove that node from the fringe
* 6.     If that's a goal node, print the solution using node.printSolution()
* 7.     Otherwise, generate the successor nodes using node.generateChildren
* 8.     Add all the generated nodes to the fringe
* 9. If no solution was found, print "No solution found."

This is how I implemented the code initially:

public static void run(Node rootNode) {
        // TODO: add your code here
        Node node;
        TreeSet<Node> fringe = new TreeSet<Node>(rootNode.getComparator());
        fringe.add(rootNode);
        while(!fringe.isEmpty()){
            node = fringe.first();
            fringe.remove(node);
            if(node.isGoal()){
                node.printSolution();
                return;
            }
            node.generateChildren();
            fringe.add(node); 
        }
        System.out.print("No solution found.");
}

But it does not make any sense since the node.generateChildren() inside the FifteenNodes.java should return a List<FifteenNode> so I tried to change this part

node.generateChildren();
fringe.add(node);

into this

List<Node> nodes; //Created before the while loop.
nodes = node.generateChildren();
fringe.add(nodes);

But it caused an conversion error so I red the TreeSet class and it provided a fringe.addAll() function so yet again I changed the previous part of the code into this:

fringe.addAll(node.generateChildren());

However it loops forever without stopping I don't know the reason why it is happening so I thought maybe my implementation of the pseudocode is wrong and wanted to know on how should I Implement the pseudo code.