Printing ArrayList to an output file [duplicate]

I am trying to print the content of a PriorityQueue to an output file by converting it to an ArrayList.

(NOTE: I'm not printing to the console!)

Besides I declared a toString method, I am still getting the outputs like this: I@7c3df479

Converting the queue to ArrayList:

private PriorityQueue<GameState> unexpanded = new PriorityQueue<>(Comparator.comparing(GameState::getF_n));

...

public ArrayList<GameState> getUnexpanded()
    {
        ArrayList<GameState> unExpanded = new ArrayList<>(unexpanded);
        return unExpanded;
    }

Getting the ArrayList and trying it to print:

private void printSolution() throws IOException
    {
        FileWriter outFile = new FileWriter("output.txt");
        PrintWriter output = new PrintWriter(outFile);
        ArrayList<GameState> unexpanded = game.getUnexpanded();

        for (int i = 0; i < unexpanded.size(); i++)
        {
            output.printf(unexpanded.get(i).toString() + "\n");
        }

        output.close();
    }

toString method:

public class GameState
{
private int[][] grid;

...

@Override
    public String toString()
    {
        return "{" + grid + "}";
    }
}

Everything is working fine but the program print the contents like: I@7c3df479

Can anybody please help me with this?

Many thanks for the answers and comments in advance.