Suggestion for a more sensible implementation of EMPTY_MAP

[Roger Riggs]

Hi Stuart,

Not withstanding all the details. It would be useful (and possibly 
expected) that an EMPTY_MAP
could be substituted for a Map with no entries.  As it stands, the 
caller needs know whether any optional
possibly modifying operations will be used and decide whether to create 
an empty map or an EMPTY_MAP.
That makes using an EMPTY_MAP a risk and less useful and a cautious 
programmer will avoid it.

$.02, Roger


On 6/13/19 8:42 PM, Stuart Marks wrote:
>
>
> On 6/10/19 7:34 AM, Abraham Marín Pérez wrote:
>> I agree that it makes sense for EMPTY_MAP to have the same logic as 
>> Map.of() or unmodifiable(new HashMap()), which means that my 
>> suggestion cannot be applied to just EMPTY_MAP... but maybe that’s 
>> ok: maybe we can change all of them? If we keep the default 
>> implementation for Map.computeIfPresent in EMPTY_MAP, Map.of(), 
>> unmodifiableMap, etc., instead of overriding it to throw an 
>> exception, then:
>>
>> - For cases where the key isn’t present (regardless of empty or 
>> non-empt map), the call will be a safe no-op.
>> - For cases where the key is present, the call will still throw UOE 
>> via the implementation of put()
>>
>> I believe this would still respect the spec for Map.computeIfPresent 
>> while providing a more forgiving implementation (in the sense that it 
>> will avoid throwing an exception when possible).
>
> Hi Abraham,
>
> The specification does not mandate one behavior or another. Either 
> behavior is permitted, and in fact both behaviors are present in the JDK.
>
> The second and more significant point is raised by your last statement 
> suggesting a "more forgiving implementation." The question is, do we 
> actually want a more forgiving implementation?
>
> The collections specs define certain methods as "optional", but it's 
> not clear whether this means that calling such a method should always 
> throw an exception ("strict" behavior) or whether the method should 
> throw an exception only when it attempts to do something that's 
> disallowed ("lenient" behavior).
>
> Take for example the putAll() method. What happens if we do this?
>
>     Map<String, String> map1 = Collections.emptyMap();
>     map1.putAll(otherMap);
>
> The answer is that it depends on the state of otherMap. If otherMap is 
> empty, then the operation succeeds (and does nothing). However, if 
> otherMap is non-empty, then the operation will throw 
> UnsupportedOperationException. That's an example of lenient behavior. 
> What's notable about this is that you can't predict the outcome unless 
> you know the state of otherMap.
>
> Now, how about this example?
>
>     Map<String, String> map2 = Map.of();
>     map2.putAll(otherMap);
>
> This always throws UnsupportedOperationException, regardless of the 
> state of otherMap. I'd call this strict behavior.
>
> More recent implementations, including the Java 8 default methods, and 
> the Java 9 unmodifiable collections (Map.of et al), have all preferred 
> strict behavior. This is because less information about the state of 
> the system is necessary in order to reason about the code, which leads 
> to fewer bugs, or at least earlier detection of bugs. It's unfortunate 
> that emptyMap().putAll() has this lenient behavior, but we're stuck 
> with it for compatibility reasons.
>
> Now there is a slight wrinkle with computeIfPresent(). If we have
>
>     Map<String, String> map = Collections.emptyMap();
>     map.computeIfPresent(key, remappingFunction);
>
> then as you point out, this can never modify the map, since the key is 
> never present. Thus, there isn't any behavior that's dependent upon 
> additional state.
>
> But, as Michael observed, there is now an inconsistency with Map.of() 
> and Collections.unmodifiableMap(). So, continuing with your 
> suggestion, we might change those implementations to allow 
> computeIfPresent() as well. Thus,
>
>     Map<String, String> map1 = Collections.emptyMap();
>     Map<String, String> map2 = Map.of();
>     Map<String, String> map3 = Collections.unmodifiableMap(new 
> HashMap<>());
>
> and then
>
>     {map1,map2,map3}.computeIfPresent(key, remappingFunction);
>
> all are no-ops. Well, maybe. What if we had retained a reference to 
> the HashMap wrapped by the unmodifiableMap, and then we added an 
> element? Now it contains a mapping; what should happen then? Should 
> computeIfPresent() throw an exception because it *might* attempt to 
> modify the map? Or should it throw an exception *only* if it attempts 
> to modify the map? State-dependent behavior has slipped back in.
>
> We also need to consider the impact on the other compute* methods. 
> Consider:
>
>     Map<String, String> map = Collections.emptyMap();
>     map.compute(key, (k, v) -> null);
>
> This can never change the map, but it currently throws UOE. Should it 
> be changed to a no-op as well?
>
> **
>
> What we have now is strict rule: calling mutator methods on 
> unmodifiable collections throws an exception. It's simple to describe 
> and reason about, and it's (mostly) consistent across various 
> collections. However, it prohibits some operations that sometimes 
> people want to do.
>
> If we were to make the system more lenient, or more forgiving, then 
> weird inconsistencies and conditional behaviors pop up in other 
> places. This makes the system more complicated and harder to reason 
> about, and that leads to more bugs.
>
> s'marks
>