How to Remove Duplicates from ArrayList in Java

ArrayList is the most popular implementation of List interface from Java's Collection framework, but it allow duplicates. Though there is another collection called Set which is primarily designed to store unique elements, there are situations when you receive a List e.g. ArrayList in your code and you need to ensure that it doesn't contain any duplicate before processing. Since with ArrayList you cannot guarantee uniqueness, there is no other choice but to remove repeated elements from ArrayList. There are multiple ways to do this, you can follow the approach we used for removing duplicates from array in Java, where we loop through array and inserting each element in a Set, which ensures that we discard duplicate because Set doesn't allow them to insert, or you can also use remove method of ArrayList to get rid of them, once you found that those are duplicates, but the simplest approach to remove repeated objects from ArrayList is to copy them to a Set e.g. HashSet and then copy it back to ArrayList. This will remove all duplicates without writing any more code. One thing to noted is that, if original order of elements in ArrayList is important for you, as List maintains insertion order, you should use LinkedHashSet because HashSet doesn't provide any ordering guarantee. BTW, if you are using deleting duplicates while iterating, make sure you use Iterator's remove() method and not the ArrayList one to avoid ConcurrentModificationException.  In this tutorial we will see this approach to remove duplicates.

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