import java.util.HashMap;
import java.util.Map;
public class HashMapUtil {
/**
* The largest power of two that can be represented as an {@code int}.
*
* @since 10.0
* 00000000 00000000 00000000 00000001 = 1
* 01000000 00000000 00000000 00000000 = 1073741824(10亿多)
* Integer.MAX_VALUE = 2147483647(21亿多)
*/
private static final int MAX_POWER_OF_TWO = 1 << (Integer.SIZE - 2);//(10亿多)
/**
* 来源google-guava
* @author:heshengjin qq:2356899074
@date 2019年10月31日 上午10:21:37
*/
public static int capacity(int expectedSize) {
if (expectedSize < 3) {
return expectedSize + 1;
}
if (expectedSize < MAX_POWER_OF_TWO) {
// This is the calculation used in JDK8 to resize when a putAll
// happens; it seems to be the most conservative calculation we
// can make. 0.75 is the default load factor.
return (int) ((float) expectedSize / 0.75F + 1.0F);
}
return Integer.MAX_VALUE; // any large value
}
@SuppressWarnings({ "serial", "unused" })
public static void main(String[] args) {
//创建的时候初始化值
Map<String, Integer> map = new HashMap<String, Integer>(capacity(100)){{
for(int i = 0,ilen = 100;i <= ilen; i++){
put("HashMapTest_"+i, i);
}
}};
}
}
