Java - Distinct List of Objects

Question

I have a list/collection of objects that may or may not have the same property values. What's the easiest way to get a distinct list of the objects with equal properties? Is one collection type best suited for this purpose? For example, in C# I could do something like the following with LINQ.

var recipients = (from recipient in recipientList
                 select recipient).Distinct();

My initial thought was to use lambdaj (link text), but it doesn't appear to support this.

Answer 1

return new ArrayList(new HashSet(recipients)); 

Answer 2

Use an implementation of the interface Set<T> (class T may need a custom .equals() method, and you may have to implement that .equals() yourself). Typically a HashSet does it out of the box : it uses Object.hashCode() and Object.equals() method to compare objects. That should be unique enough for simple objects. If not, you'll have to implement T.equals() and T.hashCode() accordingly.

See Gaurav Saini's comment below for libraries helping to implement equals and hashcode.

Answer 3

Place them in a TreeSet which holds a custom Comparator, which checks the properties you need:

SortedSet<MyObject> set = new TreeSet<MyObject>(new Comparator<MyObject>(){

    public int compare(MyObject o1, MyObject o2) {
         // return 0 if objects are equal in terms of your properties
    }
});

set.addAll(myList); // eliminate duplicates

Answer 4

Java 8:

recipients = recipients.stream()
    .distinct()
    .collect(Collectors.toList());

See java.util.stream.Stream#distinct.