How do I remove repeated elements from ArrayList?

Suppose we have a list of String like:

List<String> strList = new ArrayList<>(5);
// insert up to five items to list.        

Then we can remove duplicate elements in multiple ways.

Prior to Java 8

List<String> deDupStringList = new ArrayList<>(new HashSet<>(strList));

Note: If we want to maintain the insertion order then we need to use LinkedHashSet in place of HashSet

Using Guava

List<String> deDupStringList2 = Lists.newArrayList(Sets.newHashSet(strList));

Using Java 8

List<String> deDupStringList3 = strList.stream().distinct().collect(Collectors.toList());

Note: In case we want to collect the result in a specific list implementation e.g. LinkedList then we can modify the above example as:

List<String> deDupStringList3 = strList.stream().distinct()
                 .collect(Collectors.toCollection(LinkedList::new));

We can use parallelStream also in the above code but it may not give expected performace benefits. Check this question for more.

In Java 8:

List<String> deduped = list.stream().distinct().collect(Collectors.toList());

Please note that the hashCode-equals contract for list members should be respected for the filtering to work properly.

If you don't want duplicates in a Collection, you should consider why you're using a Collection that allows duplicates. The easiest way to remove repeated elements is to add the contents to a Set (which will not allow duplicates) and then add the Set back to the ArrayList:

Set<String> set = new HashSet<>(yourList);
yourList.clear();
yourList.addAll(set);

Of course, this destroys the ordering of the elements in the ArrayList.

Although converting the ArrayList to a HashSet effectively removes duplicates, if you need to preserve insertion order, I'd rather suggest you to use this variant

// list is some List of Strings
Set<String> s = new LinkedHashSet<>(list);

Then, if you need to get back a List reference, you can use again the conversion constructor.