Null Pointer Excpetion when trying to test if array is empty java

Question

I was tyring to test if my array is empty, but I always get a null pointer exception for some reason. I don't understand since my test actually test if the value is null. Here is the code where it gets stucks. I get the null pointer on the While loop:

private int findIndex(String key) {
 int index = calculateHashCode(key);
 while (table[index] != null && !table[index].getKey().equals(key)) {
 index = (index + 1) % TABLE_SIZE;
 }
 return index;
 }

EDIT

table is declared like this:

static HashEntry[] table;

• MahlerFive: Where is table declared? It's likely that table is null.

Answer 1

If table is null. The act of performing table[index] dereferences the array, which causes a NullPointerException. A null array is not a empty array. A null array roughly corresponds to that fact that there is no array at all, and the table variable points to nothing.

Please make the distinction between a null array, an empty array and a null element in a non-empty array.

Array is null:

table == null

Array is empty:

table.length == 0

Element 0 in array is null:

table[0] == null

Similarly, you cannot call methods from a null object. If table[index] returns a null element, calling getKey() upon it could result in NullPointerException. Same goes if getKey() is null and then trying to call equals() on that.

Please note an uninitialized array is implicitly null. Make sure you actually initialize the array.

Answer 2

The NullPointer Exception could be because either:

1. table is null, or
2. table[index].getKey() returns null

In case 1, the evaluation of the expression table[index] != null is a read of an item from the table array. This will throw a NullPointerException if table is null.

In case 2, if table[index].getKey() returns null, then the expression table[index].getKey().equals(key) is calling a method (equals) on a null object reference which results in the exception.

Answer 3

Your table isn't initialised, declare as

static HashEntry[] table = new HashEntry[TABLE_SIZE];

so that it isn't null.

Also, if HashEntry.getKey() can return null, make sure you check for this (or just flip the equals, i.e. !key.equals(table[index].getKey())