Question

I have a class that has an ArrayList attribute, and another class that has a 2D Array of that class:

public class someThingsList {

    private List<someThings> lst;

    public someThingsList () {
        this.lst = new ArrayList<someThings>();
    }
    public addThing(someThings) {
        someThings s = someThings;
        this.lst.add(s);
    }
}

and

public class x implements y {

    private static someThingsList[][] field;

    public x(int h, int w) {
    x.field = new someThingsList[h][w];
    for (someThingsList[] lst1 : x.field)
        for (someThingsList lst2 : lst1)
            lst2 = new someThingsList();
}

and when I have for example a new x object with (2,2) as h and w, and try to do

x.fields[0][0].addThing(thing);

I get a NullPointerException error, any idea why?

Answer 1

Unfortunately you can’t use enhanced for-each loop with arrays when you want to mutate them. lst2 in your example is just another reference (to null), modifying it won’t modify the original array. Try this:

for(int i = 0; i < field.length; ++i)
    for(int j = 0; j < field[i].length; ++j)
        field[i][j] = new someThingsList();

or alternatively:

for(int i = 0; i < h; ++i)
    for(int j = 0; j < w; ++j)
        x.field[i][j] = new someThingsList();

As a matter of fact, this problem is not specific to arrays. Check the following code:

List<String> strings = //...

for(String s: strings) {
    s += "Bar";
}

Even though it looks as if you were modifying the original strings list, you are barely modifing temporary s variable that is recreated and ignore after each loop.