我正在写一个字典,在Map<String, Index>
中大量使用字符串作为键。我关心的是HashMap
和TreeMap
中的哪一个在地图中搜索关键字时会有更好(更快)的性能?
发布于 2011-08-14 22:43:52
假设没有太多的排序,hashmap将提供o(1)性能(对于大量的排序,这可能会降级到O(n),其中N是任何单个存储桶中的条目(排序)的数量)。另一方面,如果您想要有某种平衡的树结构,产生O(logN)检索,则使用TreeMaps。因此,这真的取决于您的特定用例。但是,如果您只想访问元素,而不管它们的顺序,请使用HashMap
发布于 2014-05-12 19:49:00
public class MapsInvestigation {
public static HashMap<String, String> hashMap = new HashMap<String, String>();
public static TreeMap<String, String> treeMap = new TreeMap<String, String>();
public static ArrayList<String> list = new ArrayList<String>();
static {
for (int i = 0; i < 10000; i++) {
list.add(Integer.toString(i, 16));
}
}
public static void main(String[] args) {
System.out.println("Warmup populate");
for (int i = 0; i < 1000; i++) {
populateSet(hashMap);
populateSet(treeMap);
}
measureTimeToPopulate(hashMap, "HashMap", 1000);
measureTimeToPopulate(treeMap, "TreeMap", 1000);
System.out.println("Warmup get");
for (int i = 0; i < 1000; i++) {
get(hashMap);
get(treeMap);
}
measureTimeToContains(hashMap, "HashMap", 1000);
measureTimeToContains(treeMap, "TreeMap", 1000);
}
private static void get(Map<String, String> map) {
for (String s : list) {
map.get(s);
}
}
private static void populateSet(Map<String, String> map) {
map.clear();
for (String s : list) {
map.put(s, s);
}
}
private static void measureTimeToPopulate(Map<String, String> map, String setName, int reps) {
long start = System.currentTimeMillis();
for (int i = 0; i < reps; i++) {
populateSet(map);
}
long finish = System.currentTimeMillis();
System.out.println("Time to populate " + (reps * map.size()) + " entries in a " + setName + ": " + (finish - start));
}
private static void measureTimeToContains(Map<String, String> map, String setName, int reps) {
long start = System.currentTimeMillis();
for (int i = 0; i < reps; i++) {
get(map);
}
long finish = System.currentTimeMillis();
System.out.println("Time to get() " + (reps * map.size()) + " entries in a " + setName + ": " + (finish - start));
}
}
给出了以下结果:
Warmup populate
Time to populate 10000000 entries in a HashMap: 230
Time to populate 10000000 entries in a TreeMap: 1995
Warmup get
Time to get() 10000000 entries in a HashMap: 140
Time to get() 10000000 entries in a TreeMap: 1164
发布于 2011-08-14 22:43:12
对于访问,HashMap为O(1) (通常);TreeMap为O(log )(保证)。
这假设您的键对象是不可变的,并且具有正确编写的equals和hashCode方法。有关如何正确覆盖equals和hashCode的信息,请参阅Joshua Bloch的"Effective“chapter 3。
https://stackoverflow.com/questions/7057430
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