Class that describes a immutable, ordered set

Question

I need some class/interface name that describes an immutable, ordered set (in input order, like LinkedHashSet). I can of course just use the class like this:

class Foo {
    public final Set<Long> frozenOrderedSet;

    public Foo(List<Long> input) {
        frozenOrderedSet = Collections.unmodifiableSet(new LinkedHashSet(input));
    }
}

But this would not clarify my approach. I would like to make it clear to everyone reading the source that the Set is unmodifiable and unique while maintaining it's order with for(Long l : set){}.

Answer 1

Guava's ImmutableSet provides a high-performance, immutable Set with reliable, user-specified iteration order. There are also variations like ImmutableSortedSet.

Answer 2

The simplest way would be extend Set to create a custom immutable Set.

public CustomImmutableSet(){ return Collections.unmodifiableSet(new LinkedHashSet(input)); }

This way it will make it clear to everyone reading the source that the Set is unmodifiable and unique

Answer 3

Bozho in question Java Immutable Collections:

Unmodifiable collections are usually read-only views (wrappers) of other collections. You can't add, remove or clear them, but the underlying collection can change.

Immutable collections can't be changed at all - they don't wrap another collection - they have their own elements.

Here's a quote from guava's ImmutableList

Unlike Collections.unmodifiableList(java.util.List), which is a view of a separate collection that can still change, an instance of ImmutableList contains its own private data and will never change.

So, basically, in order to get an immutable collection out of a mutable one, you have to copy its elements to the new collection, and disallow all operations.

So Basically you might create subclass of class TreeSet and override all :add, remove etc. methods to just throw an Exception saying that this is immutable collection. Also you would have to create copying constructor in this class with one parameter TreeSet setToCopyFrom.