Re: Using

Rogan Dawes <>
Wed, 16 Aug 2006 15:42:39 +0200
Christopher Benson-Manica wrote:

I sincerely hope I'm missing something about how can be
used, but it seems that there is no way to update a single value
without traversing the map twice, at least not cleanly. As an
example, consider this code taken from the Sun autoboxing example:

public static void main(String[] args) {
  Map<String, Integer> m = new TreeMap<String, Integer>();
  for (String word : args) {
    Integer freq = m.get(word);
    m.put(word, (freq == null ? 1 : freq + 1));

Notice how both get() and put() are invoked. Clearly this means that
the map is traversed twice - once to find the value, and once to
figure out where to put a new value (which happens to be the same
place!). In C++ (where I come from), maps contain objects of type
pair<key, value>, and one can therefore retrieve the pair<> in
question and update the value with only one traversal of the map. Is
there any way to avoid two traversals in Java short of writing a kludgy
wrapper object such as

class WrapsAnInt {
  private int wrapped;

  public WrapsAnInt( int w ) { wrapped = w; }
  public void setWrapped( int w ) { wrapped = w; }
  public int getWrapped() { return wrapped; }

and put *those* in the map? If not, why didn't Java simply introduce
the concept of a pair<> like C++ did and fix the problem?

I notice that noone mentioned Map.Entry, which is one way of achieving
what Christopher is looking for.

However, the Map interface does not provide an optimized way of looking
up the entry in question, but rather expects the user to iterate over
all the entries until they find the one they are looking for.


Map.Entry<K,V> getEntry(Object key)

might be a useful method. Obviously I am not considering the additional
complexity that this adds to all implementations, etc.


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