how we check for null array in java

Question

Hello,

I am new bie to java unable to check for null.

can you enlighten me on this.

I have int array which has no elements

I tried this code

int[] k = new int[3];

if(k==null)
{
    System.out.println(k.length);
}

but this condition always stay false and nerver prints "k.length"

Answer 1

There's a key difference between a null array and an empty array. This is a test for null.

int arr[] = null;
if (arr == null) {
  System.out.println("array is null");
}

"Empty" here has no official meaning. I'm choosing to define empty as having 0 elements:

arr = new int[0];
if (arr.length == 0) {
  System.out.println("array is empty");
}

An alternative definition of "empty" is if all the elements are null:

Object arr[] = new Object[10];
boolean empty = true;
for (int i=0; i<arr.length; i++) {
  if (arr[i] != null) {
    empty = false;
    break;
  }
}

or

Object arr[] = new Object[10];
boolean empty = true;
for (Object ob : arr) {
  if (ob != null) {
    empty = false;
    break;
  }
}

Answer 2

An int array without elements is not necessarily null. It will only be null if it hasn't been allocated yet. See this tutorial for more information about Java arrays.

You can test the array's length:

void foo(int[] data)
{
  if(data.length == 0)
    return;
}

Answer 3

I am from .net background. However, java/c# are more/less same.

If you instantiate a non-primitive type (array in your case), it won't be null.
e.g. int[] numbers = new int[3];
In this case, the space is allocated & each of the element has a default value of 0.

It will be null, when you don't new it up.
e.g.

int[] numbers = null; // changed as per @Joachim's suggestion.
if (numbers == null)
{
   System.out.println("yes, it is null. Please new it up");
}

Answer 4

Look at its length:

int[] i = ...;
if (i.length == 0) { } // no elements in the array

Though it's safer to check for null at the same time:

if (i == null || i.length == 0) { }

Answer 5

An int array is initialised with zero so it won't actually ever contain nulls. Only arrays of Object's will contain null initially.

Answer 6

I believe that what you want is

int[] k = new int[3];

if (k != null) {  // Note, != and not == as above
    System.out.println(k.length);
}

You newed it up so it was never going to be null.