NPE when using get() from Arrays.asList - List

Question

I have tried to access an element of my Arrays.asList - List, but I get an NPE.

Main Class:

public class Game  {
    List<Cell> cells = null;


    public Game  () {
        cells = new ArrayList<>();
        cells.addAll(Arrays.asList(new Cell[80])); //Target: Add 80 additional list elements
    }

    public int getSelectedValue () {
        return cells.get(10).value;   //Here I get the NPE, but I need the 10th cell
    }

}

Subclass cell:

public class Cell {
    int row;
    int col;
    int value;
}

How can I instance a List with many objects (100 are required) and have access to those elements with the get() operator?

Answer 1

Asking a null for an object’s field throws exception

You established an array with 80 slots that are capable of pointing to Cell objects. But you never created any Cell objects. So all eighty slots point to nothing, they are null.

You then added that array of nulls to an empty ArrayList. Still no Cell objects in existence, so the ArrayList has no Cell objects, only nulls. You did create a List of 80 slots, but all slots are empty (null).

You then asked for the object being referenced from the 11th slot of the list (index of 10). There is an eleventh slot. But there is no object reference in the eleventh slot. So you retrieved a null from the list.

From that null you tried to access the value field defined in Cell. But you have no Cell object in hand. You have only a null in hand. A null means nothing. Trying to access a field (or call a method) on a null (on nothing) makes no sense. So Java throws a NullPointerException.

To verify if you have an object reference in hand rather than a null:

if( Objects.nonNull( eleventhCell ) ) {…}

Define a record using early-access Java 16, an abbreviated way to make your class.

record Cell ( int row, int col, int val ) {} ;

int initialCapacity = 80 ;
List< Cell > cells = new ArrayList<>( initialCapacity ) ;
for( int i = 1 ; i <= initialCapacity ; i ++ )
{
    cells.add(
        new Cell( 
            ThreadLocalRandom.current().nextInt( 1 , 100 ) ,
            ThreadLocalRandom.current().nextInt( 1 , 100 ) ,
            ThreadLocalRandom.current().nextInt( 1_000 , 10_000 ) 
        )
    );
}
Cell eleventhCell = cells.get( 10 ) ;
if( Objects.nonNull( eleventhCell ) )  // If in doubt, verify not null.
{ 
    int eleventhVal = eleventhCell.val ; 
}

---

By the way, value is a poor name for a field in a Java. We use that term as programmers all the time. Discussing “the value of value” will be quite confusing.

Answer 2

always add a null check on the object before calling a function on it in order to avoid NPE

for eg.

public int getSelectedValue () {
 if(cells !=null && cells.size() >0) 
   { 
    if(cells.get(10) !=null)
       return cells.get(10).value;   
   }
    else return null;
}