How can I remove duplicates out of an ArrayList?

Question

for (int i = 0; i < arrayP.size(); i++) {

        if (arrayP.get(i) == What to put here?) {
              arrayP.remove(i)
        }
}

If I put i+1 it just compares side by side. How can I get it to run through each element?

Element 1 then run through the whole ArrayList. Then element 2 run through ArrayList.

@JarrodRoberson That question is simply about *detecting* whether duplicates are present, not about removing them. — arshajii, Sep 01 '13 at 01:02
[ArrayList#removeAll](http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#removeAll(java.util.Collection))? — MadProgrammer, Sep 01 '13 at 01:34

arshajii · Answer 1 · 2013-09-01T00:33:10.260

7

You can use a LinkedHashSet:

list = new ArrayList<>(new LinkedHashSet<>(list));

If the order of elements does not matter, you should use a normal HashSet instead. In fact, if it's logical to do so, you might consider using a Set instead of a List in the first place. Read about the differences between the two and pick the structure that's more appropriate.

edited Sep 01 '13 at 00:33

answered Sep 01 '13 at 00:27

arshajii

127,459
24
238
287

2

+1. Not only removed duplicates, but also **preserved** the initial order of elements in list. – informatik01 Sep 01 '13 at 00:30

Ahmed Adel Ismail · Answer 2 · 2013-09-01T00:35:47.117

0

try this

Set arrayPSet = new LinkedHashSet(arrayP);

now arrayPSet holds your array removing it's duplicate values, then you can use it, you can re-assign your list to this set once again, like

arrayP = new ArrayList(arrayPSet);

now duplicates are removes

just make sure to override the equals() and hashCode() methods if arrayP list holds objects created by you

ok, i updated it maybe this works, i didnt try the order issue, but there are TreeSet/Comparators/Comparables solutions , just try this modification first

edited Sep 01 '13 at 00:35

answered Sep 01 '13 at 00:29

Ahmed Adel Ismail

2,168
16
23

2

Fail. Changes the order of the elements of the ArrayList. – Stephen C Sep 01 '13 at 00:30
@StephenC: I don't see how that's a hard and fast requirement, per se... – Makoto Sep 01 '13 at 00:47

How can I remove duplicates out of an ArrayList?

2 Answers2