RFR: 8247402: Documentation for Map::compute contains confusing implementation requirements

[Pavel Rappo]

On Mon, 30 Nov 2020 13:52:17 GMT, Pavel Rappo <prappo at openjdk.org> wrote:

>> @pavelrappo Please see my updated CSR below. Thanks.
>> 
>> # Map::compute should have the implementation requirement match its default implementation
>> 
>> ## Summary
>> 
>> The implementation requirement of Map::compute does not match its default implementation. Besides, it has some other minor issues. We should fix it.
>> 
>> ## Problem
>> 
>> The documentation of the implementation requirements for Map::compute has the following problems:
>> 1. It doesn't match its default implementation.
>> 1. It lacks of the return statements for most of the if-else cases.
>> 1. The indents are 3 spaces, while the convention is 4 spaces.
>> 1. The if-else is overly complicated and can be simplified.
>> 1. The surrounding prose contains incorrect statements.
>> 
>> ## Solution
>> 
>> Rewrite the documentation of Map::compute to match its default implementation and solve the above mentioned problems.
>> 
>> ## Specification
>> 
>> diff --git a/src/java.base/share/classes/java/util/Map.java b/src/java.base/share/classes/java/util/Map.java
>> index b1de34b42a5..b30e3979259 100644
>> --- a/src/java.base/share/classes/java/util/Map.java
>> +++ b/src/java.base/share/classes/java/util/Map.java
>> @@ -1107,23 +1107,17 @@ public interface Map<K, V> {
>>       *
>>       * @implSpec
>>       * The default implementation is equivalent to performing the following
>> -     * steps for this {@code map}, then returning the current value or
>> -     * {@code null} if absent:
>> +     * steps for this {@code map}:
>>       *
>>       * <pre> {@code
>>       * V oldValue = map.get(key);
>>       * V newValue = remappingFunction.apply(key, oldValue);
>> -     * if (oldValue != null) {
>> -     *    if (newValue != null)
>> -     *       map.put(key, newValue);
>> -     *    else
>> -     *       map.remove(key);
>> -     * } else {
>> -     *    if (newValue != null)
>> -     *       map.put(key, newValue);
>> -     *    else
>> -     *       return null;
>> +     * if (newValue != null) {
>> +     *     map.put(key, newValue);
>> +     * } else if (oldValue != null || map.containsKey(key)) {
>> +     *     map.remove(key);
>>       * }
>> +     * return newValue;
>>       * }</pre>
>>       *
>>       * <p>The default implementation makes no guarantees about detecting if the
>
> @johnlinp, thanks for updating the CSR draft; it is much better now.
> 
> @stuart-marks, I think we could further improve this snippet. This `if` statement seems to use an optimization:
> 
> if (oldValue != null || map.containsKey(key))
> 
> I don't think we should include an optimization into the specification unless that optimization also improves readability. Is this the case here? Could this be better?
> 
> if (map.containsKey(key))

I would even go as far as to rewrite that snippet like this:

if (newValue == null) {
    remove(key);
} else {
    put(key, newValue);
}
return newValue;

This rewrite is possible thanks to the following properties of `Map.remove(Object key)`:

1. A call with an unmapped `key` has no effect.
2. A call with a mapped `key` has the same semantics regardless of the value that this key is mapped to.

In particular, (2) covers `null` values.

To me, this rewrite reads better; however, I understand that readability is subjective and that snippets used in `@implSpec` might be subject to additional requirements.

-------------

PR: https://git.openjdk.java.net/jdk/pull/714