On Wed, 23 Mar 2022 18:41:59 GMT, XenoAmess <d...@openjdk.java.net> wrote:
> 8186958: Need method to create pre-sized HashMap
I do have a local test to make sure the 3 functions I provided can produce
equal capacity HashMap, but I think it does not need to be added into jdk.
import java.lang.reflect.Array;
import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.Map;
public class CalculateHashMapCapacityFunctionTest {
/**
* Calculate initial capacity for HashMap based classes, from expected size.
*
* @param expectedSize expected size
* @return initial capacity for HashMap based classes.
* @since 19
*/
private static int calculateHashMapCapacity1(int expectedSize) {
return (int) Math.ceil(expectedSize / 0.75);
}
/**
* Calculate initial capacity for HashMap based classes, from expected size.
*
* @param expectedSize expected size
* @return initial capacity for HashMap based classes.
* @since 19
*/
private static int calculateHashMapCapacity2(int expectedSize) {
if (expectedSize >= 1610612736) {
return Integer.MAX_VALUE;
}
return (expectedSize + (expectedSize + 2) / 3);
}
/**
* Calculate initial capacity for HashMap based classes, from expected size.
*
* @param expectedSize expected size
* @return initial capacity for HashMap based classes.
* @since 19
*/
private static int calculateHashMapCapacity3(int expectedSize) {
if (expectedSize >= 805306368) {
return (1 << 30);
}
return (expectedSize + (expectedSize + 2) / 3);
}
public static void main(String[] args) throws Exception {
for (int i = Integer.MAX_VALUE; i >= 0; --i) {
Map<Object, Object> map1 = new
HashMap<>(calculateHashMapCapacity1(i));
map1.put("1", "1");
int length1 = getArrayLength(map1);
Map<Object, Object> map2 = new
HashMap<>(calculateHashMapCapacity2(i));
map2.put("1", "1");
int length2 = getArrayLength(map2);
Map<Object, Object> map3 = new
HashMap<>(calculateHashMapCapacity3(i));
map3.put("1", "1");
if (i % 1000000 == 0) {
System.out.println("running : " + i);
}
int length3 = getArrayLength(map3);
if (length1 != length2 || length1 != length3) {
throw new RuntimeException("wrong: " + i);
}
}
}
static Field FIELD;
static {
try {
FIELD = HashMap.class.getDeclaredField("table");
FIELD.setAccessible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
public static int getArrayLength(Map<Object, Object> map) throws Exception {
Object table = FIELD.get(map);
return Array.getLength(table);
}
}
-------------
PR: https://git.openjdk.java.net/jdk/pull/7928
