Does java.util.List.isEmpty() check if the list itself is null? [duplicate]
Does java.util.List.isEmpty()
check if the list itself is null
, or do I have to do this check myself?
For example:
List<String> test = null;
if (!test.isEmpty()) {
for (String o : test) {
// do stuff here
}
}
Will this throw a NullPointerException
because test is null
?
Solution 1:
You're trying to call the isEmpty()
method on a null
reference (as List test = null;
). This will surely throw a NullPointerException
. 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
).
Edit:
You may want to see this question.
Solution 2:
I would recommend using Apache Commons Collections
http://commons.apache.org/proper/commons-collections/javadocs/api-release/org/apache/commons/collections4/CollectionUtils.html#isEmpty(java.util.Collection)
which implements it quite ok and well documented:
/**
* Null-safe check if the specified collection is empty.
* <p>
* Null returns true.
*
* @param coll the collection to check, may be null
* @return true if empty or null
* @since Commons Collections 3.2
*/
public static boolean isEmpty(Collection coll) {
return (coll == null || coll.isEmpty());
}
Solution 3:
This will throw a NullPointerException
- as will any attempt to invoke an instance method on a null
reference - but in cases like this you should make an explicit check against null
:
if ((test != null) && !test.isEmpty())
This is much better, and clearer, than propagating an Exception
.
Solution 4:
No java.util.List.isEmpty()
doesn't check if a list is null
.
If you are using Spring framework you can use the CollectionUtils
class to check if a list is empty or not. It also takes care of the null
references. Following is the code snippet from Spring framework's CollectionUtils
class.
public static boolean isEmpty(Collection<?> collection) {
return (collection == null || collection.isEmpty());
}
Even if you are not using Spring, you can go on and tweak this code to add in your AppUtil
class.
Solution 5:
Invoking any method on any null reference will always result in an exception. Test if the object is null first:
List<Object> test = null;
if (test != null && !test.isEmpty()) {
// ...
}
Alternatively, write a method to encapsulate this logic:
public static <T> boolean IsNullOrEmpty(Collection<T> list) {
return list == null || list.isEmpty();
}
Then you can do:
List<Object> test = null;
if (!IsNullOrEmpty(test)) {
// ...
}