How to remove duplicates from a list?

Question

I want to remove duplicates from a list but what I am doing is not working:

List<Customer> listCustomer = new ArrayList<Customer>();    
for (Customer customer: tmpListCustomer)
{
  if (!listCustomer.contains(customer)) 
  {
    listCustomer.add(customer);
  }
 }

Answer 1

java 8 update
you can use stream of array as below:

Arrays.stream(yourArray).distinct()
                    .collect(Collectors.toList());

Answer 2

I suspect you might not have Customer.equals() implemented properly (or at all).

List.contains() uses equals() to verify whether any of its elements is identical to the object passed as parameter. However, the default implementation of equals tests for physical identity, not value identity. So if you have not overwritten it in Customer, it will return false for two distinct Customer objects having identical state.

Here are the nitty-gritty details of how to implement equals (and hashCode, which is its pair - you must practically always implement both if you need to implement either of them). Since you haven't shown us the Customer class, it is difficult to give more concrete advice.

As others have noted, you are better off using a Set rather than doing the job by hand, but even for that, you still need to implement those methods.

Answer 3

The correct answer for Java is use a Set. If you already have a List<Customer> and want to de duplicate it

Set<Customer> s = new HashSet<Customer>(listCustomer);

Otherise just use a Set implemenation HashSet, TreeSet directly and skip the List construction phase.

You will need to override hashCode() and equals() on your domain classes that are put in the Set as well to make sure that the behavior you want actually what you get. equals() can be as simple as comparing unique ids of the objects to as complex as comparing every field. hashCode() can be as simple as returning the hashCode() of the unique id' String representation or the hashCode().