how to remove unique elements from arrayList

Question

I have two lists, in one list values are 1,2,3

another list 2,3

I want to remove the values which are not matched in both lists.

2 and 3 are matched in both lists then 1 is not mached in both lists so I want to remove that value.

List original = [1,2,3];

List dummy = [2,3];

What if `dummy` have values that aren't in `original`? Do you want to get those too? — Mureinik, May 26 '18 at 09:53

score 3 · Answer 1 · answered May 26 '18 at 09:54

3

If it's possible to use sets instead, then you can just get the intersection between the sets (info):

Set<String> s1;
Set<String> s2;
s1.retainAll(s2); // s1 now contains only elements in both sets

Of course with sets you can't have duplicates and you'll lose ordering.

answered May 26 '18 at 09:54

Mark

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A `LinkedHashSet` even would maintain the insert order. And duplicates seem not to be conceived. So this is indeed a fine solution. – Joop Eggen May 26 '18 at 10:05
1

This answer changes the requirement (if the two lists contain `1`, it is not asked that we have only a single 1 as result) and it changes also the elements order. The question is "I want to remove the values which are not matched in both lists." – davidxxx May 26 '18 at 10:10
@davidxxx That was already clearly stated in the answer, but thanks for emphasizing – Mark May 26 '18 at 10:28

score 1 · Accepted Answer · answered May 26 '18 at 10:04

You don't need to use a Set to achieve your requirement.
Use retainAll() defined in Collection that any List implementation implements such as :

List<Integer> original = new ArrayList<>(Arrays.asList(1,2,3));
List<Integer> dummy = Arrays.asList(2,3);

original.retainAll(dummy);
System.out.println(original);

Output :

[2, 3]

score 0 · Answer 3 · answered May 26 '18 at 10:01

If you are using Java 8+ you can use :

original.removeIf(a -> !dummy.contains(a));

Here is an example with Java 10

var original = new ArrayList<>(List.of(1, 2, 3, 4));
var dummy = new ArrayList<>(List.of(2, 4, 3));

original.removeIf(a -> !dummy.contains(a));

System.out.println(original);
->[2, 3, 4]

how to remove unique elements from arrayList

3 Answers3