[PATCH] 8247402: Rewrite the implementation requirements for Map::compute()

[Martin Buchholz]

import java.util.HashMap;
import java.util.Map;
import java.util.function.BiFunction;

@SuppressWarnings("serial")
public class MapsComputeNull {

    static class HashMap1<K,V> extends HashMap<K,V> {
        @Override public V compute(
                K key,
                BiFunction<? super K, ? super V, ? extends V>
remappingFunction) {
            final Map<K,V> map = this;
            V oldValue = map.get(key);
            V newValue = remappingFunction.apply(key, oldValue);
            if (newValue != null) {
                map.put(key, newValue);
            } else {
                map.remove(key);
            }
            return newValue;
        }
    }

    static class HashMap2<K,V> extends HashMap<K,V> {
        @Override public V compute(
                K key,
                BiFunction<? super K, ? super V, ? extends V>
remappingFunction) {
            final Map<K,V> map = this;
            V oldValue = map.get(key);
            V newValue = remappingFunction.apply(key, oldValue);
            if (newValue != null) {
                map.put(key, newValue);
            } else if (oldValue != null) {
                map.remove(key);
            }
            return newValue;
        }
    }

    public static void main(String[] args) throws Throwable {
        test(new HashMap<Object, Object>());
        test(new HashMap1<Object, Object>());
        test(new HashMap2<Object, Object>());
    }

    static void test(Map<Object, Object> map) {
        Object key = 42;
        map.put(key, null);
        map.compute(key, (k, v) -> null);
        System.out.printf("%s %s%n",
                          map.getClass().getSimpleName(),
                          map.containsKey(key));
    }
}

On Thu, Jun 11, 2020 at 10:06 PM 林自均 <johnlinp at gmail.com> wrote:
>
> Hi Martin,
>
> Thanks for the review! However, the corner case you mentioned seems to
> be related to the design choice of Map::compute(), and it's out of the
> scope of this change. What do you think?
>
> Best,
> John Lin
>
> Martin Buchholz <martinrb at google.com> 於 2020年6月12日 週五 上午11:42寫道：
> >
> > 林自均, pleased to meet you!
> >
> > This rewrite seems clear and clean.
> >
> > Would it be even clearer if we did
> >
> > if (newValue != null) {
> >   map.put(key, newValue);
> > } else {
> >   map.remove(key);
> > }
> >
> > Hmmm, what about the corner case where the map permits null values and
> > the old value is null?
> >
> > On Thu, Jun 11, 2020 at 7:45 PM 林自均 <johnlinp at gmail.com> wrote:
> > >
> > > Hi all,
> > >
> > > This is my first time contribution. Please let me know if I did
> > > something wrong. Thanks. Regards
> > >
> > > I'm working on this bug: https://bugs.openjdk.java.net/browse/JDK-8247402
> > >
> > > The original problem is that the implementation requirements for
> > > Map::compute() lacks of return statements for some cases. However, I
> > > found that there are some other problems in it:
> > >
> > > 1. The indents are 3 spaces, while most of the indents are 4 spaces.
> > > 2. The if-else is overly complicated and can be simplified.
> > >
> > > My proposed patch that generated by `hg export -g` is:
> > >
> > > # HG changeset patch
> > > # User John Lin <johnlinp at gmail.com>
> > > # Date 1591923561 -28800
> > > #      Fri Jun 12 08:59:21 2020 +0800
> > > # Node ID 03c9b5c9e632a0d6e33a1f13c98bb3b31b1bf659
> > > # Parent  49a68abdb0ba68351db0f140ddac793b1c391bd5
> > > 8247402: Rewrite the implementation requirements for Map::compute()
> > >
> > > diff --git a/src/java.base/share/classes/java/util/Map.java
> > > b/src/java.base/share/classes/java/util/Map.java
> > > --- a/src/java.base/share/classes/java/util/Map.java
> > > +++ b/src/java.base/share/classes/java/util/Map.java
> > > @@ -1113,17 +1113,12 @@ public interface Map<K, V> {
> > >       * <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.remove(key);
> > >       * }
> > > +     * return newValue;
> > >       * }</pre>
> > >       *
> > >       * <p>The default implementation makes no guarantees about detecting if the
> > >
> > > Best,
> > > John Lin