Return empty arrays or collections, not nulls

Posted on 2011-04-26
Medium Priority
Last Modified: 2012-06-21
Return empty arrays or collections, not nulls

I was reading above staemtent. I have not understood the statement, concept behind clearly.
Please advise. Any ideas, sugestions, sample code, links, resources highly appreciated. thanks in advance
Question by:gudii9
LVL 23

Assisted Solution

cmalakar earned 200 total points
ID: 35468753
Obviously accessing members of null references will cause NullPointerException.
Hence to overcome from that, the statement suggests to return empty arrays or collections.
LVL 47

Assisted Solution

for_yan earned 400 total points
ID: 35468770

I'd think that this is because it is easier to use
empty collections for the calling program
If people foget to check for null -
null will trhrow exception, wheraes apmpty collection
will usually work in all codes - just traversing through nothing.
So if folks are expecting to get ArrayList from method - let them
have ArrayList wuith no elements rather
than returning null
LVL 28

Accepted Solution

dpearson earned 1400 total points
ID: 35470826
What this point says is it's better to do this:

public List<String> getNamesOfMyFriends() {
     if (haveNoFriends)
         return new ArrayList<String>() ;
         ... return the names of the friends

than to do this:

public List<String> getNamesOfMyFriends() {
     if (haveNoFriends)
         return null ;       <-- Notice the difference
         ... return the names of the friends

Both compile and work, so why is the first better?

It's because the caller can then write code like this:

List<String> names = getNamesOfMyFriends() ;
int numberOfFriends = names.size() ;

With the first version, this code always works.
With the second version, if the user has no friends, it will throw a null pointer exception here:
int numberOfFriends = names.size() ;    <-- names is null

So instead you have to write:
List<String> names = getNamesOfMyFriends() ;
int numberOfFriends = 0 ;
if (names != null) {
     numberOfFriends = names.size() ;

which is a lot more code, harder to read and more likely to be wrong.

Hope that explains it.


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