How to convert a Collection to List?

问题:

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 Collection of the values using:

Collection coll = themap.values();

Which naturally works fine.

Main Question: I now want to know how I can convert/cast (not sure which is correct) coll into a List so it can be sorted?

I then intend to iterate over the sorted List object, which should be in order and get the appropriate keys from the TreeBidiMap (themap) using themap.getKey(iterator.next()) where the iterator will be over the list of doubles.

回答1:

List list = new ArrayList(coll);
Collections.sort(list);

As Erel Segal Halevi says below, if coll is already a list, you can skip step one. But that would depend on the internals of TreeBidiMap.

List list;
if (coll instanceof List)
  list = (List)coll;
else
  list = new ArrayList(coll);

回答2:

Something like this should work, calling the ArrayList constructor that takes a Collection:

List theList = new ArrayList(coll);

回答3:

I think Paul Tomblin's answer may be wasteful in case coll is already a list, because it will create a new list and copy all elements. If coll contains many elemeents, this may take a long time.

My suggestion is:

List list;
if (coll instanceof List)
  list = (List)coll;
else
  list = new ArrayList(coll);
Collections.sort(list);

回答4:

I believe you can write it as such:

coll.stream().collect(Collectors.toList())

回答5:

Collections.sort( new ArrayList( coll ) );

回答6:

@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());
}

回答7:

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

Otherwise, you can create a custom collection. 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 keySet() {
        return map.keySet();
    }
    public Multiset values() {
        return multiset;
    }
    // many more methods to be implemented, e.g. count, isEmpty etc.
}

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 have to invent something more complex.

回答8:

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

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