Array index out of bounds exception java while using loops

Question

Every few runs of my program it gives me the error

     at outlab6.Battleship.setShips(Battleship.java:75)
     at outlab6.Battleship.setBoard(Battleship.java:35)

But I thought that I was already setting the code up so that those shouldn't happen. Can someone tell me what I did wrong and why it's not ignoring the possibilities that make that possible?

package outlab6;

import java.util.Scanner;

public class Battleship {
  private int rows;
  private int cols;
  private Spot spot[][];
  Scanner input = new Scanner(System.in);

  public  Battleship(int rows, int cols){
    this.rows = rows;
    this.cols = cols;
  }
  public void setBoard(){
    spot = new Spot[rows][cols];
    for(int i = 0; i < rows; i++){
      for( int j = 0; j < cols; j++){
        spot[i][j] = new Spot();
      }
    }
    //setup board to be completely empty
    for(int i = 0; i < rows; i++){
      for(int j = 0; j < cols; j++){
        spot[i][j].setShip(0);
      }
    }
    //   //test code
    //   for(int i = 0; i < rows; i++){
    //     for(int j = 0; j < cols; j++){
    //       System.out.print(spot[i][j].getShip());
    //     }
    //     System.out.println();
    //   }
    setShips();
  }
  public void printBoard(boolean active){


  }
  public boolean over() {

    return false;
  }
  public void makeGuess() {
    input.nextInt();

  }
  public void printStatistics() {


  }
  public void setShips(){
    //this method creates and places the ships
    //start with carrier and move on down
    for(int i = 5; i > 1; i--){
      int col;
      int row;
      boolean valid = false;
      //set a direction
      int direction = (int)(Math.random()*2)+1;
      //System.out.println(direction);
      //get a valid spot
      while(!valid){
        //generate a location
        int chosenRow = (int)(Math.random()* rows)+1;
        int chosenCol = (int)(Math.random()* cols)+1;
        System.out.println("Row:" + chosenRow);
        System.out.println("Col:" + chosenCol);
        //check to see if spot is open

        //for horizontal ships
        if(chosenCol + i < cols){
          for(int j = 0; j < i; j++){
            if(spot[chosenRow][chosenCol + i].getShip() == 0){
              valid = true;
            }else{
              valid = false;
            }
          }
        }else{
          //go through again
        }

      }
    }
  }
}

I would have a look at these two line int chosenRow = (int)(Math.random()* rows)+1 and int chosenCol = (int)(Math.random()* cols)+1 I haven't run your code but that +1 might be screwing you up there.. — Michael Queue
– Michael Queue, Commented Nov 30, 2014 at 19:58
(int)(Math.random()* rows)+1 - imagine rows is 1. then every time you would get 1, arrays start at 0 of course so you would always get and index out of bounds. It should read (int)(Math.random()* rows). — ug_
– ug_, Commented Nov 30, 2014 at 19:58

Eran · Accepted Answer · 2014-11-30 19:58:03Z

1

Your problem is probably here :

    int chosenRow = (int)(Math.random()* rows)+1;
    int chosenCol = (int)(Math.random()* cols)+1;

This will give you a chosenRow between 1 and rows, and a chosenCol between 1 and cols. Having chosenRow == rows will bring you out of the array bounds when you try to access spot[chosenRow][chosenCol + i].

You should change it to :

    int chosenRow = (int)(Math.random()* rows);
    int chosenCol = (int)(Math.random()* cols);

answered Nov 30, 2014 at 19:58

Eran

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2 Comments

KevinRandall Over a year ago

That's it thank you. I completely forgot for some reason that arrays start at 0. Thank you very very much!

Michael Queue Over a year ago

@user3303616 remember that the 0th index is also a row and a column. You do not need to account for Math.random() coming out to 0.0

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