Encountering null pointer array when looping though object. How do I catch it?

Question

I am encountering a null pointer when I loop through my array. Is there a test I can run to find out what might be causing this error... and maybe why there is no value when there should? The system stops at mapTiles[0][33]. Here's the code:

import java.lang.*;
import java.util.*;
import java.io.*;

public class Map {

MapTile[][] mapTiles;
String imageMap;
String rawMap;

// constructor 
public Map() {
    imageMap = "Map_DragonShrine.jpg";
    rawMap = "Dragon_Shrine.map";
    mapTiles = new MapTile[22][34];
}


// methods
public void loadMapFile() {

    rawMap = file2String(rawMap);

    // array used to hold columns in a row after spliting by space
    String[] mapCols = null;
    // split map using 'bitmap' as delimiter
    String[] mapLines = rawMap.split("bitmap");  
    // assign whatever is after 'bitmap'
    rawMap = mapLines[1];
    // split string to remove comment on the bottom of the file
    mapLines = rawMap.split("#");
    // assign final map
    rawMap = mapLines[0].trim();
    mapLines = rawMap.split("\\n+");

    for(int x = 0; x < mapLines.length; x++) {
        rawMap = mapLines[x] ;
        mapCols = rawMap.split("\\s+");            
        for(int y = 0; y < mapCols.length; y++) {
            mapTiles[x][y] = new MapTile(mapCols[y]);   
        }            
    }   
}       

public void getTileFeatures() {
}

public void checkFeature() {
    for(int x = 0; x < mapTiles.length; x++) {
        for(int y = 0; y < mapTiles[x].length; y++) {
            makeTile(x, y, mapTiles[x][y].getFeature());
            //  java.lang.NullPointerException at mapTiles[0][33]
        }
    }
}

public String file2String(String filename) 
{          
    try{ 
        FileReader fileReader = new FileReader(filename); 
        String fileContents = ""; 
        int i ; 

        while((i =  fileReader.read())!=-1){ 
            char ch = (char)i; 

            fileContents = fileContents + ch;  
        }//end while 

        // System.out.println(fileContents); 
        return fileContents; 
    }
    catch(Exception e){
        System.out.println("File failed to load:" + e); 
        return "Error " + e; 
    } 

}//end file2String

private void makeTile(int x, int y, String features)
{
    System.out.println("line1 " + y + " test");
    //         mapTiles[x][y] = new MapTile();//Assumes your Map class has an array of MapTile objects called mapTiles
    if(features.contains("w")){   mapTiles[x][y].SOLID = true; }
    System.out.println("line2 " + y + " test");
    if(features.contains("s")){   mapTiles[x][y].STATUE = true; }
    System.out.println("line3 " + y + " test");
    if(features.contains("A")){   mapTiles[x][y].VICTORY_A = true; }
    System.out.println("line4 " + y + " test");
    if(features.contains("B")){   mapTiles[x][y].VICTORY_B = true; }
    System.out.println("line5 " + y + " test");
    if(features.contains("a")){   mapTiles[x][y].START_A = true; }
    System.out.println("line6 " + y + " test");
    if(features.contains("b")){   mapTiles[x][y].START_B = true; }
    System.out.println("line7 " + y + " test");
    if(features.contains("h")){   mapTiles[x][y].HAUNTED = true; }
    System.out.println("line8 " + y + " test");
    if(features.contains("d")){   mapTiles[x][y].DIFFICULT = true; }
    System.out.println("line9 " + y + " test");
    if(features.contains("c")){   mapTiles[x][y].SACRED_CIRCLE = true; }
    System.out.println("line10 " + y + " test");
    if(features.contains("u")){   mapTiles[x][y].SUMMONING_CIRCLE = true; }
    System.out.println("line11 " + y + " test");
    if(features.contains("x")){   mapTiles[x][y].EXIT_A = true; }
    System.out.println("line12 " + y + " test");
    if(features.contains("z")){   mapTiles[x][y].EXIT_B = true; }
    System.out.println("line13 " + y + " test");
}//End makeTile

}

Answer 1

You should initialize the MapTile 2D array using your mapLines object. Right now you're assuming it will always be 22x34.

This could easily cause a cause a null pointer where some map tiles aren't initialized. It could also cause an IndexOutOfBounds.

Your error is going to come down to that. Somewhere you are not initializing that index in the mapTile array.

Answer 2

Use Unit Testing tools like JUnit,Mockito to test your class. This way you can set up MapTile[][] with different values and run test against it. Information on Mockito can be found here