如何检查数组是否为空?

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

我有一个没有元素的int数组我正在检查它是否为空。

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

例如，为什么下面代码中的if语句从不为真?

int[] k = new int[3];if(k==null){    System.out.println(k.length);}

11 个解决方案

#1

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

空数组和空数组之间有一个关键的区别。这是对null的测试。

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:

“空”在这里没有官方意义。我选择将empty定义为0个元素:

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:

“空”的另一定义是如果所有元素都为空:

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

or

或

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

#2

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

isnotempty (testArrayName)来自包org.apache.common .lang3确保数组不是空的

#3

Look at its length:

看看它的长度:

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

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

虽然在同一时间检查空值更安全:

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

#4

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

我来自。net背景。但是，java/c#是差不多的。

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.

如果实例化一个非基元类型(在您的例子中是数组)，它就不是null。例如int[] numbers = new int[3];在这种情况下，分配空间&每个元素的默认值为0。

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

它将是空的，当你不更新它。如。

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

#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.

int数组初始化为零，因此它实际上不会包含null。只有对象的数组最初将包含null。

#6

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.

这里的重点是变量k不是空的因为它指向数组。数组本身是否为空并不重要。只有当变量k没有指向任何东西时，post中的零检验才会为真。

#7

I tested as below. Hope it helps.

我测试如下。希望它可以帮助。

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");        }   

#8

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.

没有元素的int数组不一定是空的。它只有在尚未分配时才为空。有关Java数组的更多信息，请参见本教程。

You can test the array's length:

您可以测试数组的长度:

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

#9

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.

您还可以通过查找数组的长度来检查数组中是否有任何元素，然后将其放入if-else语句中，以检查它是否为null。

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

#10

    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

这就像我检查一个int数组是否为空一样。虽然当数组中的ints实际上为零时，这将不起作用。它对{1,2,3}有效，如果{2,0}它仍然返回false，但是{0}将返回true

#11

I believe that what you want is

我相信你想要的是

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.

你把它更新了，所以它永远不会是零。