arrays - Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1 -

i'm getting error:

exception in thread "main" java.lang.arrayindexoutofboundsexception: -1  @ snakebox.neighbor(snakebox.java:150)  @ snakebox.findsnake(snakebox.java:86)  @ pg6a.main(pg6a.java:28) 

the class being used is:

/** class can used manipulate 2d boxes of "snakes" pg6.  */    import java.util.*;    import java.io.*;    public class snakebox {  // instance variables   private char[][] box;   private int rows, columns;   private int snakecount;   private int startrow, startcol;   private int endrow, endcol;     private boolean finish;     private int x, y;     private string todisplay;  /** create , initialize snakebox reading file.     @param filename external name of plain text file */   public snakebox(string filename) throws ioexception{      scanner filescan = new scanner(new filereader(filename));      rows = filescan.nextint();         columns = filescan.nextint();         snakecount = filescan.nextint();         filescan.nextline();         box = new char[rows][columns];      (int i=0; < rows; i++) {         string line = filescan.nextline();         (int j=0; j < columns; j++) {            char character = line.charat(j);            box[i][j] = character;         }      }     }     /** create new snakebox of given size , snake count @param rows height of box @param cols number of columns @param snakes how many snakes in box     */   public snakebox(int rows, int cols, int snakes) {      this.rows = rows;      this.columns = cols;      this.snakecount = snakes;    }  /** display box on screen.  */   public void display() {      string todisplay = "";       (int row = 0; row < rows; row++) {         (int column = 0; column < columns; column++) {            todisplay += box[row][column];         }         todisplay += '\n';      }           system.out.print(todisplay);    }  /** find next snake, skinning change s .  */   public void findsnake() {     // check make sure there still snakes skin       if (finish = true) {                   int row, col, nb;     // find s search       outerloop:       (int k=0; k < rows; k++) {           (int l=0; l < columns; l++) {           if (box[k][l] == 's') {           startrow = k;           startcol = l;           endrow = k;           endrow = l;           break outerloop;           }        }       }     // set initial s position both start , end      // search end, updating go      {         nb = neighbor(endrow, endcol);         switch (nb) {            case 1: endrow--; endcol--;                break;            case 2: endrow--;                break;            case 3: endrow--; endcol++;                break;            case 4: endcol--;                break;            case 5: endcol++;                break;            case 6: endcol--; endrow++;                break;            case 7: endrow++;                break;            case 8: endrow++; endcol++;                break;         }      } while (nb != 0);      // search start, updating go     {         nb = neighbor(startrow, startcol);         switch (nb) {            case 1: startrow--; startcol--;                break;            case 2: startrow--;                break;            case 3: startrow--; startcol++;                break;            case 4: startcol--;                break;            case 5: startcol++;                break;            case 6: startcol--; startrow++;                break;            case 7: startrow++;                break;            case 8: startrow++; startcol++;                break;         }      } while (nb != 0);      // update snake count       snakecount = snakecount - 1;               //display start/end points of snake, display   }    /** change position s . , find neighboring s if 1 exists.     @param x row number of position change     @param y column number of position change     @return 0 if no s neighbor found, or number grid      indicating position of found neighbor            1 2 3            4 s 5            6 7 8 */   }     private int neighbor(int x, int y) {        box[x][y] = '.';          if (box[x--][y--] == 's')          return 1;          if (box[x--][y] == 's')          return 2;          if (box[x--][y++] == 's')         return 3;          if (box[x][y--] == 's')          return 4;          if (box[x][y++] == 's')          return 5;          if (box[x++][y--] == 's')          return 6;          if (box[x++][y] == 's')          return 7;          if (box[x++][y++] == 's')          return 8;          else              return 0;           }  /** display endpoints of snake skinned.  */   public void printends() {   system.out.print("(" + x + "," + y + ")");   }  /** find out how many full snakes in box.     @return snake count */   public int getcount() {     return snakecount;         }  /** check whether snakes have been skinned, based on snake count.     @return true if done, false otherwise */   public boolean finished() {     if (snakecount == 0) {     finish = true;     }     else {     finish = false;     }           return finish;   }     } 

also, driver program (pg6) is:

 import java.util.*;  import java.io.*;   public class pg6a {    public static void main(string[] args) throws ioexception {      scanner keyboard = new scanner(system.in);       system.out.println("please enter name of file snake box resides, , try our best eliminate snakes in snake box!");      string filename;      filename = keyboard.nextline();      snakebox snakes = new snakebox(filename);      snakes.display();       while (snakes.finished() == false) {         snakes.findsnake();         snakes.printends();         snakes.display();         system.out.print(snakes.getcount());      }                         } } 

its because of if statements in neighbor().

if (box[x--][y--] == 's') - whether evaluates true or false, decrement x & y.

similarly, if statements, keep decrementing or incrementing x & y. that's why @ point, value of either x or y or both go below 0, giving index of -1 in array. , hence arrayindexoutofboundsexception.

you can fix using x-1 & y+1 in if statements, instead of increment/decrement operators.

this line snakebox.neighbor(snakebox.java:150) says error exists in neighbor method , looking @ if statements in neighbor() method, tells have possibly gone wrong.