Returning null instead of an actual array, collection or map forces callers of the method to explicitly test for nullity, making them
more complex and less readable. Moreover, in many cases, null is used as a synonym for empty.Noncompliant Code Example
public static List getAllResults[] {
return null; // Noncompliant
}
public static Result[] getResults[] {
return null; // Noncompliant
}
public static Map getValues[] {
return null; // Noncompliant
}
public static void main[String[] args] {
Result[] results = getResults[];
if [results != null] { // Nullity test required to prevent NPE
for [Result result: results] {
/* ... */
}
}
List allResults = getAllResults[];
if [allResults != null] { // Nullity test required to prevent NPE
for [Result result: allResults] {
/* ... */
}
}
Map values = getValues[];
if [values != null] { // Nullity test required to prevent NPE
values.forEach[[k, v] -> doSomething[k, v]];
}
}
Compliant Solution
public static List getAllResults[] {
return Collections.emptyList[]; // Compliant
}
public static Result[] getResults[] {
return new Result[0]; // Compliant
}
public static Map getValues[] {
return Collections.emptyMap[]; // Compliant
}
public static void main[String[] args] {
for [Result result: getAllResults[]] {
/* ... */
}
for [Result result: getResults[]] {
/* ... */
}
getValues[].forEach[[k, v] -> doSomething[k, v]];
}
See
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A List instance can contain items that represent null .
Can a list be null C#?
In C# programs, a List reference can be null. This is not the same as it being empty and having zero elements. We must often test for null lists.
How do you declare a list as null in Java?
Example 1
- import java.util.*;
- public class CollectionsEmptyListExample1 {
- public static void main[String[] args] {
- //Create an empty List.
- List EmptyList = Collections.emptyList[];
- System.out.println["Empty list: "+EmptyList];
- }
- }
How do you check if a list is null?
You should do if[test!= null] instead [Checking for null first]. The method isEmpty[] returns true, if an ArrayList object contains no elements; false otherwise [for that the List must first be instantiated that is in your case is null ].
Is an empty list null?
An empty collection isn't the same as null . An empty collection is actually a collection, but there aren't any elements in it yet. null means no collection exists at all.
Does isEmpty check for null?
isEmpty[] Returns true if the string is null or empty.
Is Empty list C#?
Check if a list is empty in C#
- Using Enumerable. Any[] method [ System. Linq ] ...
- Using Enumerable.FirstOrDefault[] method [ System.Linq ] The Enumerable.FirstOrDefault[] method returns the first element of a sequence. ...
- Using Enumerable. Count[] method [ System.
Is an empty list null Java?
An empty collection isn't the same as null . An empty collection is actually a collection, but there aren't any elements in it yet. null means no collection exists at all.
Does collection isEmpty check for NULL?
9 Answers. If you use the Apache Commons Collections library in your project, you may use the CollectionUtils. isEmpty and MapUtils. isEmpty[] methods which respectively check if a collection or a map is empty or null [i.e. they are "null-safe"].
Can a null list be an empty list?
Similarly, a list that contains null items is a list, and is not an empty list. Because it has items in it; it doesn't matter that those items are themselves null. As an example, a list with three null values in it, and nothing else: what is its length? Its length is 3. The empty list's length is zero. And, of course, null doesn't have a length.
How to check if a list is null in C #?
List in c# has a Count property. It can be used like so: if [myList == null] // Checks if list is null // Wasn't initialized else if [myList.Count == 0] // Checks if the list is empty myList.Add ["new item"]; else // List is valid and has something in it // You could access the element in the list if you wanted
Can an ArrayList be null with only nulls?
No. An ArrayList can be empty [or with nulls as items] an not be null. It would be considered empty. You can check for am empty ArrayList with: ArrayList arrList = new ArrayList[]; if[arrList.isEmpty[]] { // Do something with the empty list here. } Or if you want to create a method that checks for an ArrayList with only nulls:
How can I verify if a list is null in Java?
How can I verify if a List is null in Java ? A List instance can't be null, an instance is always something. A List type variable can be null and to test this, use the expression A List instance can by empty, meaning the list doesn't contain any values.
Related Posts:
CollectionUtils class of Apache Commons Collections library provides various utility methods for common operations covering wide range of use cases. It helps avoid writing boilerplate code. This library is very useful prior to jdk 8 as similar functionalities are now provided in Java 8's Stream API.
Checking non-empty list
isNotEmpty[] method of CollectionUtils can be used to check if a list is not empty without worrying about null list. So null check is not required to be placed everywhere before checking the size of the list.
Declaration
Following is the declaration for
org.apache.commons.collections4.CollectionUtils.isNotEmpty[] method −
public static boolean isNotEmpty[Collection coll]Parameters
coll − The collection to check, may be null.
Return Value
True if non-null and non-empty.
Example
The following example shows the usage of org.apache.commons.collections4.CollectionUtils.isNotEmpty[] method. We'll check a list is empty or not.
import java.util.List; import org.apache.commons.collections4.CollectionUtils; public class CollectionUtilsTester { public static void main[String[] args] { List list = getList[]; System.out.println["Non-Empty List Check: " + checkNotEmpty1[list]]; System.out.println["Non-Empty List Check: " + checkNotEmpty1[list]]; } static List getList[] { return null; } static boolean checkNotEmpty1[List list] { return ![list == null || list.isEmpty[]]; } static boolean checkNotEmpty2[List list] { return CollectionUtils.isNotEmpty[list]; } }Output
The output is given below −
Non-Empty List Check: false Non-Empty List Check: falseChecking empty list
isEmpty[] method of CollectionUtils can be used to check if a list is empty without worrying about null list. So null check is not required to be placed everywhere before checking the size of the list.
Declaration
Following is the declaration for
org.apache.commons.collections4.CollectionUtils.isEmpty[] method −
public static boolean isEmpty[Collection coll]Parameters
coll − The collection to check, may be null.
Return Value
True if empty or null.
Example
The following example shows the usage of org.apache.commons.collections4.CollectionUtils.isEmpty[] method. We'll check a list is empty or not.
import java.util.List; import org.apache.commons.collections4.CollectionUtils; public class CollectionUtilsTester { public static void main[String[] args] { List list = getList[]; System.out.println["Empty List Check: " + checkEmpty1[list]]; System.out.println["Empty List Check: " + checkEmpty1[list]]; } static List getList[] { return null; } static boolean checkEmpty1[List list] { return [list == null || list.isEmpty[]]; } static boolean checkEmpty2[List list] { return CollectionUtils.isEmpty[list]; } }Output
Given below is the output of the code −
Empty List Check: true Empty List Check: true