How can I check whether an array is null / empty?

问题:

I have an int array which has no elements and I'm trying to check whether it's empty.

For example, why is the condition of the if-statement in the code below never true?

int[] k = new int[3];

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

回答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;
  }
}

回答2:

ArrayUtils.isNotEmpty(testArrayName) from the package org.apache.commons.lang3 ensures Array is not null or empty

回答3:

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) { }

回答4:

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");
}

回答5:

Method to check array for null or empty also is present on org.apache.commons.lang:

import org.apache.commons.lang.ArrayUtils;

ArrayUtils.isEmpty(array);

回答6:

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

回答7:

The point here very simply is that the variable k isn't null because it points to the array. It doesn't matter that the array itself is empty. The null test in your post would only evaluate to true if the variable k didn't point to anything.

回答8:

I tested as below. Hope it helps.

Integer[] integers1 = new Integer[10];
        System.out.println(integers1.length); //it has length 10 but it is empty. It is not null array
        for (Integer integer : integers1) {
            System.out.println(integer); //prints all 0s
        }

//But if I manually add 0 to any index, now even though array has all 0s elements
//still it is not empty
//        integers1[2] = 0;
        for (Integer integer : integers1) {
            System.out.println(integer); //Still it prints all 0s but it is not empty
            //but that manually added 0 is different
        }

//Even we manually add 0, still we need to treat it as null. This is semantic logic.

        Integer[] integers2 = new Integer[20];
        integers2 = null; //array is nullified
//        integers2[3] = null; //If I had int[] -- because it is priitive -- then I can't write this line. 
        if (integers2 == null) {
            System.out.println("null Array");
        }   

回答9:

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;
}

回答10:

You can also check whether there is any elements in the array by finding out its length, then put it into if-else statement to check whether it is null.

int[] k = new int[3];
if(k.length == 0)
{
//do something
}

回答11:

    public boolean empty() {
    boolean isEmpty = true;
    int i = 0;
    for (int j = 0; j < array.length; j++) {
        if (array[j] != 0) {
            i++;
        }
    }
    if (i != 0) {
        isEmpty = false;
    }
    return isEmpty;
}

This is as close as I got to checking if an int array is empty. Although this will not work when the ints in the array are actually zero. It'll work for {1,2,3}, and it'll still return false if {2,0} but {0} will return true

回答12:

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.