Empty ArrayList equals null

Question

Is an empty Arraylist  with nulls as its items  be considered as null  So  essentially would the below statement be true    if  arrayList    null     thanks

User · Answer

Just as zero is a number - just a number that represents none - an empty list is still a list, just a list with nothing in it. null is no list at all; it's therefore different from 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.

User · Answer

No  because it contains items there must be an instance of it  Its items being null is irrelevant  so the statment   arrayList     null     true

User · Answer

arrayList    null if there are no instance of the class ArrayList assigned to the variable arrayList  note the upercase for classes and the lowercase for variables    If  at anytime  you do arrayList   new ArrayList   then arrayList    null because is pointing to an instance of the class ArrayList  If you want to know if the list is empty  do   if arrayList    null  amp  amp   arrayList isEmpty         has items here  The fact that has items does not mean that the items are    null      You have to check the nullity for every item    else      either there is no instance of ArrayList in arrayList or the list is empty      If you don t want null items in your list  I d suggest you to extend the ArrayList class with your own  for example    public class NotNullArrayList extends ArrayList    Override public boolean add Object o        if o  null  throw new IllegalArgumentException  Cannot add null items to the list          else return super add o             Or maybe you can extend it to have a method inside your own class that re-defines the concept of  empty List    public class NullIsEmptyArrayList extends ArrayList    Override public boolean isEmpty       if super isEmpty    return true     else       Iterate through the items to see if all of them are null        You can use any of the algorithms in the other responses  Return true if all are null  false otherwise        You can short-circuit to return false when you find the first item not null  so it will improve performance          The last two approaches are more Object-Oriented  more elegant and reusable solutions   Updated with Jeff suggestion IAE instead of NPE

User · Answer

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   public static Boolean ContainsAllNulls ArrayList arrList        if arrList    null                for object a   arrList              if a    null  return false             return true

User · Answer

No  this will not work   The best you will be able to do is to iterate through all values and check them yourself   boolean empty   true  for  Object item   arrayList        if  item    null            empty   false          break

User · Answer

If you want to check whether the array contains items with null values  use this   private boolean isListOfNulls ArrayList lt String gt  stringList       for  String s  stringList          if  s    null  return false      return true      You could replace  lt String gt  with the corresponding type for your ArrayList

User · Answer

First of all  You can verify this on your own by writing a simple TestCase      empty Arraylist  with nulls as its items     Second  If ArrayList is EMPTY that means it is really empty  it cannot have NULL or NON-NULL things as element   Third     List list    new ArrayList        list add null       System out println list    null    would print false

[java] Empty ArrayList equals null

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