Does java util List isEmpty check if the list itself is null

Question

Does java util List isEmpty   check if the list itself is null  or do I have to do this check myself   For example   List lt String gt  test   null   if   test isEmpty          for  String o   test               do stuff here                       Will this throw a NullPointerException because test is null

User · Answer

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());
}

User · Answer

Yes  it will throw an Exception  maybe you are used to PHP code  where empty  element  does also check for isset  element   In Java this is not the case   You can memorize that easily because the method is directly called on the list  the method belongs to the list   So if there is no list  then there is no method  And Java will complain that there is no list to call this method on

User · Answer

Invoking any method on any null reference will always result in an exception   Test if the object is null first   List lt Object gt  test   null  if  test    null  amp  amp   test isEmpty                     Alternatively  write a method to encapsulate this logic   public static  lt T gt  boolean IsNullOrEmpty Collection lt T gt  list        return list    null    list isEmpty        Then you can do   List lt Object gt  test   null  if   IsNullOrEmpty test

User · Answer

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 lt   gt  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

User · Answer

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   amp  amp   test isEmpty      This is much better  and clearer  than propagating an Exception

User · Answer

You can use your own isEmpty  for multiple collection  method too  Add this your Util class   public static boolean isEmpty Collection    collections        for  Collection collection   collections            if  null    collection    collection isEmpty                return true            return false

User · Answer

In addition to Lion s answer i can say that you better use if CollectionUtils isNotEmpty test        This also checks for null  so no manual check is not needed

User · Answer

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

[java] Does java.util.List.isEmpty() check if the list itself is null?

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