Question

I’m writing a program to solve an 8 tile sliding puzzle for an AI class. in theory this is pretty easy, but the number of node states generated is pretty large (estimated 180,000 or so). We’re comparing different heuristic functions in class, so my code has to be able to handle even some very inefficient functions. I’m getting “OutOfMemoryError: Java heap space” when using java’s PriorityQueue class. Heres the relevant code withing my solver function: (the error is on the openList.add(temp); line)

public void solve(char[] init,int searchOrder)
{
    State initial = new State(init,searchOrder); //create initial state
    openList = new PriorityQueue<State>();       //create open list
    closedList = new LinkedList<State>();        // create closed list
    generated = new HashSet();                   //Keeps track of all nodes generated to cut down search time

    openList.add(initial);                       //add initial state to the open list
    State expanded,temp = null,solution = null;  //State currently being expanded
    int nodesStored = 0, nodesExpanded = 0;
    boolean same;                                //used for checking for state redundancy
    TreeGeneration:
    while(openList.size() > 0)
    {
        expanded = openList.poll();
        closedList.addLast(expanded);
        for (int k = 0; k < 4; k++)
        {
            if (k == 0)
            {
                temp = expanded.moveLeft();
            }
            else if (k == 1)
            {
                temp = expanded.moveRight();

            }
            else if (k == 2)
            {
                temp = expanded.moveAbove();
            }
            else 
            {
                temp = expanded.moveBelow();
            }                          

            if(temp.isSolution())
            {
                solution = temp;
                nodesStored = openList.size() + closedList.size();
                nodesExpanded = closedList.size();
                break TreeGeneration;
            }
            if(!generated.contains(temp))
            {
            //    System.out.println(temp.toString());
                openList.add(temp); // error here
                generated.add(temp);
            }
           // System.out.println(openList.toString());
        }
    }

Am I doing something wrong here, or should I be using something else to handle this quantity of data? Thanks.

Answer 1

By default, JVM starts with 64 MB heap space, you can increase this amount by passing a parameter like below;

java -Xmx1024m YOUR_CLASS 

this gives 1024 MB heap space in memory, you can change the amount of memory as you need.

If you are using NetBeans, Netbeans doesn’t scale heap space automatically, you can achieve this by following below steps;

1- Right click on your project

2- Navigate to Set Configuration -> Customize

3-Add -Xmx256m into VM Options then click Ok

Now, you can run your project with custom heap space.