java - ConcurrenLinkedtHashmap 迭代器提供的元素顺序取决于 key 大小？

Question

我发现(迭代器？)ConcurrentLinkedHashMap 的奇怪行为。如果键很长，通过迭代 entrySet/keySet 获得的元素会以奇怪/意外的顺序排列。如果 key 短，一切正常。

以下代码:

    public static void main(String[] args) {
    ConcurrentLinkedHashMap<String, String> map =
            new ConcurrentLinkedHashMap.Builder<String, String>().maximumWeightedCapacity(1000).build();
    for (int i = 0; i < 5; i++) {
        map.put(i + "", i + "");
    }
    print(map);
    map.clear(); 
    // NOW AGAIN THE SAME, BUT WITH LONG KEY
    for (int i = 0; i < 5; i++) {
        map.put(i + "aaaaaaaaaaaaaaaaaaaaaaaaaa" +
                "aaaaaaaaaaaaaaaaaaaaaaaaaa" +
                "aaaaaaaaaaaaaaaaaaaaaaaaaa", i + "");
    }
    print(map);
}

private static void print(ConcurrentLinkedHashMap<String, String> a) {
    Iterator iterator = a.entrySet().iterator();
    while (iterator.hasNext()) {
        System.out.println(" = " + iterator.next());
    }
}

提供这样的输出:

 = 0=0
 = 1=1
 = 2=2
 = 3=3
 = 4=4
 = 1aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa=1
 = 4aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa=4
 = 2aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa=2
 = 3aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa=3
 = 0aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa=0

这很奇怪。如果我使 key 更长-结果不同。

这是一个错误吗？我怎样才能得到正确的结果？(第二个“打印”结果应与第一个顺序相同)

Answer 1

Unlike java.util.LinkedHashMap, this class does not provide predictable iteration order. (Javadoc)

文档对此很清楚 - 所以没有错误

关于遍历条目时的顺序，ConcurrentLinkedHashMap 提供了 ascending和 descending保留 key 的顺序。这可能并不总是您的目标，但至少对于给定的测试场景，在给定以下代码的情况下返回有序输出:

private static void print(ConcurrentLinkedHashMap<String, String> a)
{
    Iterator iterator = a.entrySet().iterator();
    while (iterator.hasNext())
    {
        System.out.println(" = " + iterator.next());
    }
    Iterator<String> iter = a.ascendingKeySet().iterator();
    while (iter.hasNext()) 
    {
        String key = iter.next();
        System.out.println(key + " -> " + a.get(key));
    }
}

输出:

 = 0=0
 = 1=1
 = 2=2
 = 3=3
 = 4=4
0 -> 0
1 -> 1
2 -> 2
3 -> 3
4 -> 4
 = 1aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa=1
 = 4aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa=4
 = 2aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa=2
 = 3aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa=3
 = 0aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa=0
0aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa -> 0
1aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa -> 1
2aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa -> 2
3aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa -> 3
4aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa -> 4