HashMap 和 LinkedHashMap 类中 keySet().iterator().next() 的时间复杂度？答案

【问题标题】：Time complexity of keySet().iterator().next() in classes HashMap and LinkedHashMap?HashMap 和 LinkedHashMap 类中 keySet().iterator().next() 的时间复杂度？
【发布时间】：2014-12-13 15:09:41
【问题描述】：

我试图弄清楚使用方法keySet().iterator().next() 获取 HashMap 和 LinkedHashMap java 集合类中的第一个键的时间复杂度是多少。 HashMap 中的顺序并不重要。我只想选择第一个可用的密钥。

我一直在挖掘这两个类的源代码，看起来像这样：

在 HashMap 中，它遍历所有条目，直到找到非 null 一。因此，最坏的情况是 O(N)。
在 LinkedHashMap 中看起来是 O(1)。

有人可以确认或更正这些陈述吗？

我知道 Apache Commons Collections 中的 LinkedMap 类有方法 firstKey() 和 lasKey()。这些方法的时间复杂度是多少？

更新：

根据我所做的测试，HashMap 和 LinkedHashMap 似乎都在 O(1) 中执行，因为当元素数量以 2 的幂增加时，执行时间不会增加：

import org.junit.Test;

import java.util.*;

import static org.junit.Assert.*;

public class LinkedHashMapTest {

    private static final Random rnd = new Random();

    @Test
    public void testHashMapFirstKeyTime() throws Exception {
        System.out.println("**testHashMapFirstKeyTime:**");
        firstKeyTime(new HashMap<Key, Integer>(), 22);
    }

    @Test
    public void testLinkedHashMapFirstKeyTime() throws Exception {
        System.out.println("**testLinkedHashMapFirstKeyTime:**");
        firstKeyTime(new LinkedHashMap<Key, Integer>(), 22);
    }

    private void firstKeyTime(Map<Key, Integer> map, int bits) {
        int m = 0;
        map.clear();
        for (int i = 1; i <= bits; i++) {
            int n = 1 << i;
            for (int j = m; j < n; j++) {
                Key key = new Key();
                key.s = String.valueOf(rnd.nextInt());
                key.d = new Date(System.currentTimeMillis());
                key.i = rnd.nextInt();
                map.put(key, j);
            }
            m = n;

            long startTime = System.currentTimeMillis();
            Key firstKey = map.keySet().iterator().next();
            long duration = System.currentTimeMillis() - startTime;
            System.out.printf("Retrieving first key %s in a map of size %10d took %d ms%n", firstKey, map.size(), duration);
            assertTrue(duration < 5); // less than 5ms
        }
    }

    private static class Key {
        String s;
        Date d;
        int i;

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;

            Key key = (Key) o;

            if (i != key.i) return false;
            if (!d.equals(key.d)) return false;
            if (!s.equals(key.s)) return false;

            return true;
        }

        @Override
        public int hashCode() {
            int result = s.hashCode();
            result = 31 * result + d.hashCode();
            result = 31 * result + i;
            return result;
        }

        @Override
        public String toString() {
            return String.format("Key{s='%1$11s', d=%2$tDT%2$tT.%2$tL%2$tz, i=%3$11d}", s, d, i);
        }
    }
}

testHashMapFirstKeyTime：

Retrieving first key Key{s=' 1985523727', d=12/14/14T02:00:24.335+0100, i=  643333406} in a map of size          2 took 0 ms
Retrieving first key Key{s=' 1985523727', d=12/14/14T02:00:24.335+0100, i=  643333406} in a map of size          4 took 0 ms
Retrieving first key Key{s='  767922762', d=12/14/14T02:00:24.346+0100, i=  431427041} in a map of size          8 took 0 ms
Retrieving first key Key{s=' -256241316', d=12/14/14T02:00:24.347+0100, i=-1263480851} in a map of size         16 took 0 ms
Retrieving first key Key{s=' -256241316', d=12/14/14T02:00:24.347+0100, i=-1263480851} in a map of size         32 took 0 ms
Retrieving first key Key{s=' -935843053', d=12/14/14T02:00:24.349+0100, i=  438592480} in a map of size         64 took 0 ms
Retrieving first key Key{s='-1067014413', d=12/14/14T02:00:24.352+0100, i= -621892808} in a map of size        128 took 0 ms
Retrieving first key Key{s='-1067014413', d=12/14/14T02:00:24.352+0100, i= -621892808} in a map of size        256 took 0 ms
Retrieving first key Key{s='-1988805714', d=12/14/14T02:00:24.353+0100, i=  -61029749} in a map of size        512 took 0 ms
Retrieving first key Key{s='-1926538837', d=12/14/14T02:00:24.353+0100, i=-1072972113} in a map of size       1024 took 0 ms
Retrieving first key Key{s=' -406648261', d=12/14/14T02:00:24.362+0100, i=  201747454} in a map of size       2048 took 0 ms
Retrieving first key Key{s='  913423510', d=12/14/14T02:00:24.373+0100, i=  532927328} in a map of size       4096 took 0 ms
Retrieving first key Key{s='-1514850500', d=12/14/14T02:00:24.390+0100, i=-1849450899} in a map of size       8192 took 0 ms
Retrieving first key Key{s='  913423510', d=12/14/14T02:00:24.373+0100, i=  532927328} in a map of size      16384 took 0 ms
Retrieving first key Key{s='  418571021', d=12/14/14T02:00:24.419+0100, i=  913773732} in a map of size      32768 took 0 ms
Retrieving first key Key{s=' -807723833', d=12/14/14T02:00:24.450+0100, i= 1204270612} in a map of size      65536 took 0 ms
Retrieving first key Key{s='  239598471', d=12/14/14T02:00:24.483+0100, i=  271236296} in a map of size     131072 took 0 ms
Retrieving first key Key{s=' 1332893667', d=12/14/14T02:00:24.607+0100, i= -494154632} in a map of size     262144 took 0 ms
Retrieving first key Key{s='  857896380', d=12/14/14T02:00:25.779+0100, i=  771055858} in a map of size     524288 took 0 ms
Retrieving first key Key{s='  422311855', d=12/14/14T02:00:25.828+0100, i= 1193799319} in a map of size    1048576 took 0 ms
Retrieving first key Key{s='  422311855', d=12/14/14T02:00:25.828+0100, i= 1193799319} in a map of size    2097152 took 1 ms
Retrieving first key Key{s='  222881497', d=12/14/14T02:00:34.934+0100, i= 1772777984} in a map of size    4194304 took 0 ms

testLinkedHashMapFirstKeyTime：

Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size          2 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size          4 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size          8 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size         16 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size         32 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size         64 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size        128 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size        256 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size        512 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size       1024 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size       2048 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size       4096 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size       8192 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size      16384 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size      32768 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size      65536 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size     131072 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size     262144 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size     524288 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size    1048576 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size    2097152 took 0 ms
Retrieving first key Key{s='    7710621', d=12/14/14T02:00:35.474+0100, i= -239716357} in a map of size    4194304 took 0 ms

【问题讨论】：

标签： java collections time-complexity

【解决方案1】：

查看代码，找到 HashMap（在HashIterator 类中）：

     final Entry<K,V>  [More ...] nextEntry() {
         if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
         Entry<K,V> e = next;
         if (e == null)
             throw new NoSuchElementException();
         if ((next = e.next) == null) {
             Entry[] t = table;
             while (index < t.length && (next = t[index++]) == null)
                 ;
         }
         current = e;
     }

对于 LinkedHashMap，您可以找到（在 LinkedHashIterator 类中）：

    Entry<K,V>  [More ...] nextEntry() {
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
        if (nextEntry == header)
            throw new NoSuchElementException();
        Entry<K,V> e = lastReturned = nextEntry;
        nextEntry = e.after;
        return e;
    }

这似乎证实了你的说法。

关于阿帕奇的LinkedMap：

public Object  [More ...] firstKey() {
    if (size == 0) {
        throw new NoSuchElementException("Map is empty");
    }
    return header.after.getKey();
}

public Object  [More ...] lastKey() {
    if (size == 0) {
        throw new NoSuchElementException("Map is empty");
    }
    return header.before.getKey();
}

似乎是 O(1)。

【讨论】：

要清楚一点：HashIterator -> O(N) LinkedHashIterator -> O(1) 只是因为来自 OP 的“更新”消息而感到困惑，说 HashIterator 是 O(1) 。谢谢！