How to convert a Collection to List?

Question

I am using TreeBidiMap from the Apache Collections library. I want to sort this on the values which are doubles.

My method is to retrieve a

Answer 1

Here is a sub-optimal solution as a one-liner:

Collections.list(Collections.enumeration(coll));

Answer 2

Collections.sort( new ArrayList( coll ) );

Answer 3

What you request is quite a costy operation, make sure you don't need to do it often (e.g in a cycle).

If you need it to stay sorted and you update it frequently, you can create a custom collection. For example, I came up with one that has your TreeBidiMap and TreeMultiset under the hood. Implement only what you need and care about data integrity.

class MyCustomCollection implements Map<K, V> {
    TreeBidiMap<K, V> map;
    TreeMultiset<V> multiset;
    public V put(K key, V value) {
        removeValue(map.put(key, value));
        multiset.add(value);
    }
    public boolean remove(K key) {
        removeValue(map.remove(key));
    }
    /** removes value that was removed/replaced in map */
    private removeValue(V value) {
        if (value != null) {
            multiset.remove(value);
        }
    }
    public Set<K> keySet() {
        return Collections.unmodifiableSet(map.keySet());
    }
    public Collection<V> values() {
        return Collections.unmodifiableCollection(multiset);
    }
    // many more methods to be implemented, e.g. count, isEmpty etc.
    // but these are fairly simple
}

This way, you have a sorted Multiset returned from values(). However, if you need it to be a list (e.g. you need the array-like get(index) method), you'd need something more complex.

For brevity, I only return unmodifiable collections. What @Lino mentioned is correct, and modifying the keySet or values collection as it is would make it inconsistent. I don't know any consistent way to make the values mutable, but the keySet could support remove if it uses the remove method from the MyCustomCollection class above.

Answer 4

@Kunigami: I think you may be mistaken about Guava's newArrayList method. It does not check whether the Iterable is a List type and simply return the given List as-is. It always creates a new list:

@GwtCompatible(serializable = true)
public static <E> ArrayList<E> newArrayList(Iterable<? extends E> elements) {
  checkNotNull(elements); // for GWT
  // Let ArrayList's sizing logic work, if possible
  return (elements instanceof Collection)
      ? new ArrayList<E>(Collections2.cast(elements))
      : newArrayList(elements.iterator());
}