DisjSet Maze Builder 中的空指针异常，无法弄清楚原因

Question

我正在尝试使用 DisjSet 算法构建迷宫。我觉得我已经搞定了，但出于某种原因，我无法弄清楚为什么我在使用的 classes 之一中不断收到此空指针异常。任何帮助将不胜感激

这是迷宫class

package project3;

public class Maze2 {



private void nextRoom(int[] room, int wall, int n)
{

        int row = wall / (2 * n - 1);
        int column = wall % (2 * n - 1);


        if (column < n - 1)
        {

            room[0] = n * row + column;
            room[1] = n * row + column + 1;
        }
        else
        {

            column += 1 - n;

            room[0] = n * row + column;
            room[1] = n * (row + 1) + column;
        }
}

    private void mazeBuild(int m, int n)
    {


        DisjSets rooms = new DisjSets(m * n);

        int wallNumber = 2 * m * n - m - n;
        boolean[] is_wall = new boolean[wallNumber];
        Permutation wallUntested = new Permutation(wallNumber);

        for (int i = 0; i < wallNumber; ++i)
        {
            is_wall[i] = true;
        }




        while (rooms.s.length>1 )
            {
            int wall = wallUntested.next();
            int[] room = new int[2];
            nextRoom(room, wall, n);


            if (rooms.find(room[0]) != rooms.find(room[1]))
            {
                is_wall[wall] = false;
                rooms.union(room[0], room[1]);
            }
        }



        System.out.print("+  +");

        for (int i = 0; i < n - 1; ++i)
        {
            System.out.print("--+");
        }

        System.out.print("\n");

        int wall_counter = 0;

        for (int i = 0; i < m - 1; ++i)
        {


            System.out.print("|");

            for (int j = 0; j < n - 1; ++j)
            {
                if (is_wall[wall_counter])
                {
                    System.out.print("  |");
                }
                else
                {
                    System.out.print("   ");
                }

                ++wall_counter;
            }

            System.out.print("  |");
            System.out.print("\n");



            System.out.print("+");

            for (int j = 0; j < n; ++j)
            {
                if (is_wall[wall_counter])
                {
                    System.out.print("--+");
                }
                else
                {
                    System.out.print("  +");
                }

                ++wall_counter;
            }

            System.out.print("\n");
        }


        System.out.print("|");

        for (int j = 0; j < n - 1; ++j)
        {
            if (is_wall[wall_counter])
            {
                System.out.print("  |");
            }
            else
            {
                System.out.print("   ");
            }

            ++wall_counter;
        }

        System.out.print("  |");
        System.out.print("\n");



        for (int i = 0; i < n - 1; ++i)
        {
            System.out.print("+--");
        }

        System.out.print("+  +");
        System.out.print("\n");
    }


public static void main( String [] args){


Maze2 maze = new Maze2();


//input parameters here

maze.mazeBuild(3,3);



}
}

这是 DisjSet class

package project3;

public class DisjSets
{

// DisjSets class
//
// CONSTRUCTION: with int representing initial number of sets
//
// ******************PUBLIC OPERATIONS*********************
// void union( root1, root2 ) --> Merge two sets
// int find( x )              --> Return set containing x
// ******************ERRORS********************************
// No error checking is performed

/**
 * Disjoint set class, using union by rank and path compression.
 * Elements in the set are numbered starting at 0.
 * @author Mark Allen Weiss
 */

    /**
     * Construct the disjoint sets object.
     * @param numElements the initial number of disjoint sets.
     */
    public DisjSets( int numElements )
    {
        s = new int [ numElements ];
        for( int i = 0; i < s.length; i++ )
            s[ i ] = -1;
    }

    /**
     * Union two disjoint sets using the height heuristic.
     * For simplicity, we assume root1 and root2 are distinct
     * and represent set names.
     * @param root1 the root of set 1.
     * @param root2 the root of set 2.
     */
    public void union( int root1, int root2 )
    {
        if( s[ root2 ] < s[ root1 ] )  // root2 is deeper
            s[ root1 ] = root2;        // Make root2 new root
        else
        {
            if( s[ root1 ] == s[ root2 ] )
                s[ root1 ]--;          // Update height if same
            s[ root2 ] = root1;        // Make root1 new root
        }
    }

    /**
     * Perform a find with path compression.
     * Error checks omitted again for simplicity.
     * @param x the element being searched for.
     * @return the set containing x.
     */
    public int find( int x )
    {
        if( s[ x ] < 0 )
            return x;
        else
            return s[ x ] = find( s[ x ] );
    }

    public int [ ] s;



}

这是排列class

package project3;

import java.util.*;

public class Permutation{

 private static Random random;

int capacity;
int bound;
int[] array;

public Permutation(int n)
{
    this.capacity = n;
    this.bound =  (int) capacity;
    this.array = new int[capacity];
    for (int i = 0; i < capacity; ++i)
    {
        array[i] = i;
    }
}



public final int next()
{
    if (bound == 0)
    {
        return 0;
    }

    int index = random.nextInt() % bound;
    int result = array[index];

    --bound;
    array[index] = array[bound];

    return result;
}


public final void reset()
{
    for (int i = 0; i < capacity; ++i)
    {
        array[i] = i;
    }

    bound = capacity;
}

}

错误是

Exception in thread "main" java.lang.NullPointerException
at project3.Permutation.next(Permutation.java:33)
at project3.Maze2.mazeBuild(Maze2.java:50)
at project3.Maze2.main(Maze2.java:159)

并且都指向排列中的这一行代码 class

        int index = random.nextInt() % bound;

提前致谢

Answer 1

你需要这样的东西：

private static Random random = new Random();

或者您也可以将其设为最终版本：

private static final Random random = new Random();

Java 正在将随机初始化为 null，因此：

random.nextInt()

生成一个 NPE，因为您正试图在 null 上调用方法。

DisjSet Maze Builder 中的空指针异常，无法弄清楚原因

Null Pointer Excpetion In DisjSet Maze Builder and can't figure out why

java

maze

nullpointerexception

disjoint-sets