How to check if there are duplicate elements in an ArrayList in Java

Question

I have an ArrayList actors. When I print this actors variable, I get an output of like this :

Ram, cricket, Ram, chess

I want to take only only one "Ram" instead of duplication.

same with: http://stackoverflow.com/questions/15518107/how-to-check-if-there-are-duplicate-elements-in-an-arraylist-in-java — Foredoomed, Mar 20 '13 at 08:01
@Foredoomed I don't want to remove element. I just want to know if there is a duplicate entry in an ArrayList. — Prasad, Mar 20 '13 at 08:05
Sorry for the wrong url, it should be http://stackoverflow.com/questions/562894/java-detect-duplicates-in-arraylist — Foredoomed, Mar 20 '13 at 08:07
Prasad,Please see my answer to find if there is any duplicate — Abhijit Mazumder, Mar 20 '13 at 08:10

score 6 · Answer 1 · answered Mar 20 '13 at 07:57

You need to use a Set instead of a List.

From the docs:-

A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. As implied by its name, this interface models the mathematical set abstraction.

score 2 · Answer 2 · answered Mar 20 '13 at 07:58

2

You may like to use Set instead of ArrayList as it stores unique elements.

answered Mar 20 '13 at 07:58

Divya Motiwala

1,659
1
16
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score 1 · Answer 3 · edited Mar 09 '14 at 16:37

Why would you use arraylist?. Use Set.

Or if you want to know that if there are any duplicates use java.util.Collections.sort() to sort and then compare previous element to current element to find duplicates

or

public int getNumOfDuplicates(List<Integer> dupAraay)
{ 
  Set<Integer> yourSet = new HashSet();
  int dupFound = 0;

  for (Integer yourInt : dupAraay)
  {
   if (!set1.add(yourInt))
   {
    dupFound++;
   }
  }
  return dupFound;
}

score 1 · Answer 4 · answered Mar 20 '13 at 08:04

1

ArrayList<String> values = ... //Your values
HashSet<String> uniqueValues = new HashSet<>(values);
for (String value : uniqueValues) {
   ... //Do something
}

answered Mar 20 '13 at 08:04

MangeshBiradar

3,820
1
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score 1 · Answer 5 · answered Mar 20 '13 at 08:05

You should use a Set. Specifically, I suggest HashSet if order does not matter, LinkedHashSet if you want to preserve order of arrival.

If you really need an ArrayList for other reasons, you can remove duplicates by copying to a LinkedHashSet, and then back again. That is likely to be faster than sorting and testing.

score 1 · Answer 6 · answered Mar 20 '13 at 09:20

1

contains

this is method available in array list go for java docs.

answered Mar 20 '13 at 09:20

sagar

112
9

How to check if there are duplicate elements in an ArrayList in Java

6 Answers6