HashMap.putAll can cause redundant space waste

[Xeno Amess]

also find some other places have same problem.
If some of your already-in people aggree, I would create a pr, but
according to the rules seems I should wait for now.

Xeno Amess <xenoamess at gmail.com> 于2022年2月4日周五 18:42写道：

> import java.lang.reflect.Array;
> import java.lang.reflect.Field;
> import java.util.HashMap;
> import java.util.Map;
>
> public class TestMap {
>
>     public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
>         HashMap<Object, Object> a = new HashMap<>();
>         fill12(a);
>         HashMap<Object, Object> b = new HashMap<>(12);
>         fill12(b);
>         HashMap<Object, Object> c = new HashMap<>(a);
>         HashMap<Object, Object> d = new HashMap<>();
>         d.putAll(a);
>         System.out.println("a : " + getArrayLength(a));
>         System.out.println("b : " + getArrayLength(b));
>         System.out.println("c : " + getArrayLength(c));
>         System.out.println("d : " + getArrayLength(d));
>     }
>
>     public static void fill12(Map<Object, Object> map) {
>         for (int i = 0; i < 12; i++) {
>             map.put(i, i);
>         }
>     }
>
>     public static int getArrayLength(Map<Object, Object> map) throws NoSuchFieldException, IllegalAccessException {
>         Field field = HashMap.class.getDeclaredField("table");
>         field.setAccessible(true);
>         Object table = field.get(map);
>         return Array.getLength(table);
>     }
>
> }
>
> run this and we get the output:
>
> a : 16
> b : 16
> c : 32
> d : 32
>
> So I go see the codes.
>
> /**
>  * Implements Map.putAll and Map constructor.
>  *
>  * @param m the map
>  * @param evict false when initially constructing this map, else
>  * true (relayed to method afterNodeInsertion).
>  */
> final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
>     int s = m.size();
>     if (s > 0) {
>         if (table == null) { // pre-size
>             float ft = ((float)s / loadFactor) + 1.0F;
>             int t = ((ft < (float)MAXIMUM_CAPACITY) ?
>                      (int)ft : MAXIMUM_CAPACITY);
>             if (t > threshold)
>                 threshold = tableSizeFor(t);
>         } else {
>             // Because of linked-list bucket constraints, we cannot
>             // expand all at once, but can reduce total resize
>             // effort by repeated doubling now vs later
>             while (s > threshold && table.length < MAXIMUM_CAPACITY)
>                 resize();
>         }
>
>         for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
>             K key = e.getKey();
>             V value = e.getValue();
>             putVal(hash(key), key, value, false, evict);
>         }
>     }
> }
>
> yep I do think *((float)s / loadFactor) + 1.0F* here is wrong.
>
> It should be *Math.ceil((float)s / loadFactor)*
>
> So I wish to generate a pull request.
>
> Anyone interested?
>
>