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 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

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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

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Java 10 introduced List copyOf which returns unmodifiable List while preserving the order  List lt Integer gt  list   List copyOf coll

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Here is a sub-optimal solution as a one-liner   Collections list Collections enumeration coll

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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

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Use streams  someCollection stream   collect Collectors toList

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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 lt K  V gt        TreeBidiMap lt K  V gt  map      TreeMultiset lt V gt  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 lt K gt  keySet             return Collections unmodifiableSet map keySet               public Collection lt V gt  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

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I believe you can write it as such   coll stream   collect Collectors toList

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Something like this should work  calling the ArrayList constructor that takes a Collection   List theList   new ArrayList coll

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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  lt E gt  ArrayList lt E gt  newArrayList Iterable lt   extends E gt  elements      checkNotNull elements      for GWT      Let ArrayList s sizing logic work  if possible   return  elements instanceof Collection          new ArrayList lt E gt  Collections2 cast elements           newArrayList elements iterator

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Collections sort  new ArrayList  coll

[java] How to convert a Collection to List?

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