Question

我发现了（iterator？）ConcurrentLinkedHashMap的奇怪行为。如果key很长，则通过迭代entrySet / keySet获得的元素是奇怪/意外的顺序。如果钥匙短，一切都好。

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());
    }
}

提供了这样的输出：

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

这很奇怪。如果我让钥匙更长 - 结果是不同的。

这是一个错误吗？我怎样才能得到正确的结果？（第二个＆＃39;打印＆＃39;结果应与第一个相同）

Answer 1

与java.util.LinkedHashMap不同，此类不提供可预测的迭代顺序。（Javadoc）

文档相当清楚 - 所以没有错误

关于条目在迭代时的顺序，ConcurrentLinkedHashMap提供关键保留的ascending和descending顺序。这可能并不总是你的目标，但至少对于给定的测试场景，给出下面的代码返回一个有序的输出：

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

ConcurrenLinkedtHashmap迭代器提供的元素顺序取决于密钥大小？

1 个答案: