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comparing null with null

Posted on 2011-03-13
Medium Priority
Last Modified: 2012-05-11
I have the following scenario
1) i read values off a db table (recordset) to a arraylist
2) Same query is run on a different version of DB onto a another arraylist
3) the 2 array lists are comparaed value by value

Now the db table contains null values which get stored in the arraylist. When the values are compared (arraylist1.get(n).equals(arraylist2.get(n))) i get nullpointexception.

I found a workaround online

 public static boolean equals(Object a, Object b) {  
             return a==null ? b==null : a.equals(b);  

and am currently using equals(arraylist1, arraylist2) for comparision. Is this a foolproof workaround? Please suggest.
Question by:PearlJamFanatic
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LVL 40

Accepted Solution

Gurvinder Pal Singh earned 400 total points
ID: 35125688
this should work as long as Object class has overridden hashCode and equals method
LVL 92

Assisted Solution

objects earned 400 total points
ID: 35125692
why don't you just use

boolean same = list1.equals(list2);
LVL 47

Assisted Solution

for_yan earned 400 total points
ID: 35125694
Seems like a good method, but I'm a little bit comfused are you using it to caompare
ArrayList's object by object or you are using it to compare objects within the list?

I was also thinking that perhaps method of class Object

public boolean equals(Object obj)  

was implemented fully in ArrayList so that

if you just say,
maybe it will perform comparison one by one even checking for the nulls for you and return the required
booolean value staisfying your neds - have you tried it?  Never tried it myelf  - just a thought.

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Author Comment

ID: 35125704
i think i made a mistake above. i am using
equals(arraylist1.get(n), arraylist2.get(n))
where arraylist2.get(n) is a string. I was just wondering if this will work in the scenario  arraylist1.get(n) is null and arraylist2.get(n) is a string and vice versa.

LVL 47

Expert Comment

ID: 35125736
In general it seems that your method should work because if the
first object is null, then you just check if the second object is null, however
if the fiorst object is not null, then your method a.equals(b) should not throw exception
and shoul yield false even if b is null - so I think method is fine.

On the other hand if your ArrayList's contain only objects of the types defined
by java libraries, like String, etc. or even if your ArrayLists contain custom objects
for which you overrode correctly the equals method, then this much more simple suggestion
arraylist1.equals(arraylist2) should most probably work even better.

Do your arraylists contain custom objects of the types which you defined yourself?

LVL 20

Assisted Solution

by:Sathish David Kumar N
Sathish David  Kumar N earned 400 total points
ID: 35125921

I have a question why you need null values ? any reason behind this ?

You can avoid null values in query itself

you can add where condition is not null

example : that is role coum means select * from user  where role is  not null

Author Comment

ID: 35126581
Arraylist does not contain customm objects. It contains int, string and date. The reason I went for value by value comparision is that I need to know which item (row, column) didn't match.
LVL 86

Assisted Solution

CEHJ earned 400 total points
ID: 35126613
The best thing is probably to initialize the object anyway. Assuming all three fields are involved in equals (they probably aren't) then if the query returns null, the below should allow the object to behave properly:
X x = new X(Integer.MIN_VALUE, "null", new Date(Long.MAX_VALUE));

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LVL 92

Expert Comment

ID: 35126629
> . I was just wondering if this will work in the scenario  arraylist1.get(n) is null and arraylist2.get(n) is a string and vice versa.

yes, passing null to equals() will work fine
you should actually write unit tests to check it :)
LVL 20

Expert Comment

by:Sathish David Kumar N
ID: 35126639
why u comapre with null ? you change that as ""(speace ) and comapre


while ur puting into array list jsut check each value like

arraylist.add(a!=null?a:"") ; then you can easy to comapre !!
LVL 92

Expert Comment

ID: 35126640
its the same method used by ArrayList.equals()

  480       public boolean equals(Object object) {
  481           if (this == object) {
  482               return true;
  483           }
  484           if (object instanceof List) {
  485               List<?> list = (List<?>) object;
  486               if (list.size() != size()) {
  487                   return false;
  488               }
  490               Iterator<?> it1 = iterator(), it2 = list.iterator();
  491               while (it1.hasNext()) {
  492                   Object e1 = it1.next(), e2 = it2.next();
  493                   if (!(e1 == null ? e2 == null : e1.equals(e2))) {
  494                       return false;
  495                   }
  496               }
  497               return true;
  498           }
  499           return false;
  500       }
LVL 86

Expert Comment

ID: 35126687
>>why u comapre with null ? you change that as ""(speace ) and comapre

Yes, that's the  same principle as my suggestion, but it might be necessary to treat other fields too
LVL 20

Expert Comment

by:Sathish David Kumar N
ID: 35126743
>>>>>>>e1 == null ? e2 == null : e1.equals(e2)))

as per code if e1 null means it automatically take e2 is null is this correct ??
LVL 92

Expert Comment

ID: 35126759
> as per code if e1 null means it automatically take e2 is null is this correct ??

no if e1 is null, it will return whether e2 is null
ie. if e1 and e2 are null then return true
if only e1 is null then return false
otherwise call equals

works fine, no need to insert dummy values in the array. They only result in conversion and are error prone.

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