Java PriorityQueue和NullPointerException

Question

当我尝试添加Node initialState（实现compareTo方法）时，我不确定为什么我会得到一个空指针。以下是我的Node类。

package puzzle;

public class Node implements Comparable<Node> {

int[][] state;
Node parent;
String action;
Node up, down, left, right;
int r,c;
int f;

Node(int[][] s, Node n,int row, int column){  
    state = s;
    parent = n;
    up = null;
    down = null;
    left = null;
    right = null;   
    r = row;
    c = column;
    f = 0;
}

public void setF() {
    int g = 0;  // distance
    int h = 0;  // Manhattan distance 
    int k = 0;
    for (int i = 0; i < state.length; i++){
        for (int j = 0; j < state[i].length; j++){
            if (state[i][j] != k)
                g++;
            k++;    
        }
    }
    // http://stackoverflow.com/questions/12526792/manhattan-distance-in-a
    for (int x = 0; x < state.length; x++)    {        // x-dimension, traversing rows (i)
        for (int y = 0; y < state[x].length; y++) {   // y-dimension, traversing cols (j)
            int value = state[x][y];                  // tiles array contains board elements
            if (value != 0) {                         // we don't compute MD for element 0
                int targetX = (value) / state.length; // expected x-coordinate (row)
                int targetY = (value) % state.length; // expected y-coordinate (col)
                int dx = x - targetX;                 // x-distance to expected coordinate
                int dy = y - targetY;                 // y-distance to expected coordinate
                h += Math.abs(dx) + Math.abs(dy); 
            } 
        }
    }
    for(int i = 0; i < state.length; i++){
        for (int j = 0; j < state.length; j++){
            System.out.print(state[i][j]);
        }
        System.out.println();
    }
    System.out.println(h);
    System.out.println(g);
    f = h + g;
}


    @Override
    public int compareTo(Node node) {
        if (this.f < node.f)
            return -1;
        else if (this.f > node.f)
            return 1;
        else return 0;

    }
}

这里是异常发生的地方

private void Asearch() {
    openListP = new PriorityQueue<Node>(1000);
    closedList = new LinkedList<Node>();
    initialState.setF();
    //print(initialState);
    openList.add(initialState);   // initial state into open list
    while (!openList.isEmpty()){
        Node n = openListP.remove(); 
        if (isGoal(n.state)){                      // check if goal 
            printPath(n);
            return;
        }
        else {
         expandH(n);     // expand to next frontier  
         closedList.add(n);
        }
    }
    if(openList.isEmpty())
        System.out.println(" unsolvable");

}

当我添加initialState时，我收到此错误

Exception in thread "main" java.lang.NullPointerException
at puzzle.Puzzle.Asearch(Puzzle.java:150)
at puzzle.Puzzle.solve(Puzzle.java:142)
at puzzle.Test.main(Test.java:11)

Answer 1

您必须将openList更改为openListP。

openList.add(initialState);   // initial state into open list

应该是

openListP.add(initialState);   // initial state into open list