Checking if a collection is empty in Java which is the best method

Question

I have two ways of checking if a List is empty or not  if  CollectionUtils isNotEmpty listName      and   if  listName    null  amp  amp  listName size      0    My arch tells me that the former is better than latter  But I think the latter is better   Can anyone please clarify it

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Apache Commons  CollectionUtils isNotEmpty Collection  is a NULL-SAFE check  Returns TRUE is the Collection List is not-empty and not-null Returns FALSE if the Collection is Null  Example   List lt String gt  properties   new ArrayList        if  CollectionUtils isNotEmpty properties          process the list   else       list is null or empty     Refer  https   commons apache org proper commons-collections apidocs org apache commons collections4 CollectionUtils html isNotEmpty java util Collection

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A good example of where this matters in practice is the ConcurrentSkipListSet implementation in the JDK  which states      Beware that  unlike in most collections  the size method is not a constant-time operation    This is a clear case where isEmpty   is much more efficient than checking whether size    0   You can see why  intuitively  this might be the case in some collections  If it s the sort of structure where you have to traverse the whole thing to count the elements  then if all you want to know is whether it s empty  you can stop as soon as you ve found the first one

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CollectionUtils isNotEmpty checks if your collection is not null and not empty  This is better comparing to double check but only if you have this Apache library in your project  If you don t then use   if list    null  amp  amp   list isEmpty

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I would use the first one  It is clear to see right away what it does  I dont think the null check is necessary here

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if  CollectionUtils isNotEmpty listName    Is the same as   if listName    null  amp  amp   listName isEmpty     In first approach listName can be null and null pointer exception will not be thrown  In second approach you have to check for null manually  First approach is better because it requires less work from you  Using  size      0 is something unnecessary at all  also i learned that it is slower than using  isEmpty

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If you have the Apache common utilities in your project rather use the first one  Because its shorter and does exactly the same as the latter one  There won t be any difference between both methods but how it looks inside the source code   Also a empty check using   listName size      0   Is discouraged because all collection implementations have the  listName isEmpty     function that does exactly the same   So all in all  if you have the Apache common utils in your classpath anyway  use  if  CollectionUtils isNotEmpty listName      in any other case use  if listName    null  amp  amp  listName isEmpty      You will not notice any performance difference  Both lines do exactly the same

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To Check collection is empty  you can use method   count    Example   DBCollection collection   mMongoOperation getCollection  sequence        if collection count      0            SequenceId sequenceId   new SequenceId  id   0           mMongoOperation save sequenceId

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Unless you are already using CollectionUtils I would go for List isEmpty    less dependencies   Performance wise CollectionUtils will be a tad slower  Because it basically follows the same logic but has additional overhead   So it would be readability vs  performance vs  dependencies  Not much of a big difference though

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You should absolutely use isEmpty    Computing the size   of an arbitrary list could be expensive  Even validating whether it has any elements can be expensive  of course  but there s no optimization for size   which can t also make isEmpty   faster  whereas the reverse is not the case   For example  suppose you had a linked list structure which didn t cache the size  whereas LinkedList lt E gt  does   Then size   would become an O N  operation  whereas isEmpty   would still be O 1    Additionally of course  using isEmpty   states what you re actually interested in more clearly

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isEmpty          Returns true if this list contains no elements    http   docs oracle com javase 1 4 2 docs api java util List html

User · Answer

Use CollectionUtils isEmpty Collection coll   Null-safe check if the specified collection is empty  Null returns true   Parameters  coll - the collection to check  may be null  Returns  true if empty or null

[java] Checking if a collection is empty in Java: which is the best method?

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