ConcurrentHashMap/ConcurrentMap/Map.compute

[Arne Siegel]

On 14 Dec 2012 at 11:03, Doug Lea wrote:

> 
> I placed a full candidate version of Map.java at
> http://gee.cs.oswego.edu/dl/wwwtmp/apis/Map.java
> 

Hi Doug,

just digged into your proposed implementation, compiling it using the lambda-8-b67 compiler, 
and observed the following issues:

1) I needed to change 
import java.util.functions.*;
into
import java.util.function.*;

2) The 'default' keyword is missing in:
    V compute(K key,
              BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
...

3) Error in method signature:
    default V merge(K key, V value,
                    BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
must be changed into
    default V merge(K key, V value,
                    BiFunction<? super K, ? super V, ? extends V> remappingFunction) {

4) The javadoc of 'merge' contains an odd code example, as there's no put method with 3 
arguments:

 V oldValue = map.get(key);
 V newValue = !map.containsKey(key)? value :
              remappingFunction.apply(oldValue, value);
 if (newValue != null)
   map.put(key, oldValue, newValue);
 else
   map.remove(key, oldValue);

You could write map.replace instead, but that's not the whole story, because:

5) The implementation of merge goes into an infinite loop for keys not already existing in the 
map, due to the semantics of the 'replace' method. I tested it with this code:

        Map<Integer, Integer> m = ...;
        for (int n = 0; n <= 10; n++) {
            m.put(n, n);
        }
        for (int n = 0; n <= 15; n++) {
            m.merge(n, n * n, (k, v) -> k * v);
        }

To make it work, the merge method may be corrected the following way:

replace
                if (replace(key, oldValue, newValue))
                    return newValue;
by
                if (oldValue == null ? putIfAbsent(key, newValue) == null :
                    replace(key, oldValue, newValue))
                    return newValue;


Best regards
Arne Siegel