java – 如何有效地迭代’Map’中的每个条目?

如果我有一个实现MapJava接口的对象,并且我想遍历其中包含的每一对,那么通过该映射的最有效方法是什么?

元素的排序是否依赖于我为界面设计的特定映射实现?

答案



Map<String, String> map = ...
for (Map.Entry<String, String> entry : map.entrySet())
{
    System.out.println(entry.getKey() + "/" + entry.getValue());
}

总结其他答案并将其与我所知道的结合起来,我找到了10个主要方法来做到这一点(见下文)。另外,我写了一些性能测试(见下面的结果)。例如,如果我们想要查找地图的所有键和值的总和,我们可以这样写:

  1. 使用迭代器Map.Entry
    long i = 0;
    Iterator<Map.Entry<Integer, Integer>> it = map.entrySet().iterator();
    while (it.hasNext()) {
        Map.Entry<Integer, Integer> pair = it.next();
        i += pair.getKey() + pair.getValue();
    }
  2. 使用foreachMap.Entry
    long i = 0;
    for (Map.Entry<Integer, Integer> pair : map.entrySet()) {
        i += pair.getKey() + pair.getValue();
    }
  3. 从Java 8 使用forEach
    final long[] i = {0};
    map.forEach((k, v) -> i[0] += k + v);
  4. 使用keySetforeach
    long i = 0;
    for (Integer key : map.keySet()) {
        i += key + map.get(key);
    }
  5. 使用keySet迭代器
    long i = 0;
    Iterator<Integer> itr2 = map.keySet().iterator();
    while (itr2.hasNext()) {
        Integer key = itr2.next();
        i += key + map.get(key);
    }
  6. 使用forMap.Entry
    long i = 0;
    for (Iterator<Map.Entry<Integer, Integer>> entries = map.entrySet().iterator(); entries.hasNext(); ) {
        Map.Entry<Integer, Integer> entry = entries.next();
        i += entry.getKey() + entry.getValue();
    }
  7. 使用Java 8 Stream API
    final long[] i = {0};
    map.entrySet().stream().forEach(e -> i[0] += e.getKey() + e.getValue());
  8. 使用Java 8 Stream API并行
    final long[] i = {0};
    map.entrySet().stream().parallel().forEach(e -> i[0] += e.getKey() + e.getValue());
  9. 使用IterableMapApache Collections
    long i = 0;
    MapIterator<Integer, Integer> it = iterableMap.mapIterator();
    while (it.hasNext()) {
        i += it.next() + it.getValue();
    }
  10. 使用Eclipse(CS)集合的MutableMap
    final long[] i = {0};
    mutableMap.forEachKeyValue((key, value) -> {
        i[0] += key + value;
    });

性能测试(mode = AverageTime,系统= Windows 8.1 64位,Intel i7-4790 3.60 GHz,16 GB)

  1. 对于小地图(100个元素),得分0.308是最好的
    Benchmark                          Mode  Cnt  Score    Error  Units
    test3_UsingForEachAndJava8         avgt  10   0.308 ±  0.021  µs/op
    test10_UsingEclipseMap             avgt  10   0.309 ±  0.009  µs/op
    test1_UsingWhileAndMapEntry        avgt  10   0.380 ±  0.014  µs/op
    test6_UsingForAndIterator          avgt  10   0.387 ±  0.016  µs/op
    test2_UsingForEachAndMapEntry      avgt  10   0.391 ±  0.023  µs/op
    test7_UsingJava8StreamApi          avgt  10   0.510 ±  0.014  µs/op
    test9_UsingApacheIterableMap       avgt  10   0.524 ±  0.008  µs/op
    test4_UsingKeySetAndForEach        avgt  10   0.816 ±  0.026  µs/op
    test5_UsingKeySetAndIterator       avgt  10   0.863 ±  0.025  µs/op
    test8_UsingJava8StreamApiParallel  avgt  10   5.552 ±  0.185  µs/op
    1. 对于具有10000个元素的地图,得分37.606是最好的

      基准模式Cnt得分错误单位test10_UsingEclipseMap avgt 10 37.606±0.790μs/ op test3_UsingForEachAndJava8 avgt 10 50.368±0.887μs/ op test6_UsingForAndIterator avgt 10 50.332±0.507μs/ op test2_UsingForEachAndMapEntry avgt 10 51.406±1.032μs/ op test1_UsingWhileAndMapEntry avgt 10 52.538±2.431μs/ op test7_UsingJava8StreamApi avgt 10 54.464±0.712μs/ op test4_UsingKeySetAndForEach avgt 10 79.016±25.345μs/ op test5_UsingKeySetAndIterator avgt 10 91.105±10.220μs/ op test8_UsingJava8StreamApiParallel avgt 10 112.511±0.365μs/ op test9_UsingApacheIterableMap avgt 10 125.714±1.935μs/ op

  2. 对于有100000个元素的地图,得分11​​84.767是最好的
    Benchmark                          Mode   Cnt  Score        Error    Units
    test1_UsingWhileAndMapEntry        avgt   10   1184.767 ±   332.968  µs/op
    test10_UsingEclipseMap             avgt   10   1191.735 ±   304.273  µs/op
    test2_UsingForEachAndMapEntry      avgt   10   1205.815 ±   366.043  µs/op
    test6_UsingForAndIterator          avgt   10   1206.873 ±   367.272  µs/op
    test8_UsingJava8StreamApiParallel  avgt   10   1485.895 ±   233.143  µs/op
    test5_UsingKeySetAndIterator       avgt   10   1540.281 ±   357.497  µs/op
    test4_UsingKeySetAndForEach        avgt   10   1593.342 ±   294.417  µs/op
    test3_UsingForEachAndJava8         avgt   10   1666.296 ±   126.443  µs/op
    test7_UsingJava8StreamApi          avgt   10   1706.676 ±   436.867  µs/op
    test9_UsingApacheIterableMap       avgt   10   3289.866 ±  1445.564  µs/op

图(性能测试取决于地图大小)

表(性能测试取决于地图大小)

          100     600      1100     1600     2100
test10    0.333    1.631    2.752    5.937    8.024
test3     0.309    1.971    4.147    8.147   10.473
test6     0.372    2.190    4.470    8.322   10.531
test1     0.405    2.237    4.616    8.645   10.707
test2     0.376    2.267    4.809    8.403   10.910
test7     0.473    2.448    5.668    9.790   12.125
test9     0.565    2.830    5.952   13.220   16.965
test4     0.808    5.012    8.813   13.939   17.407
test5     0.810    5.104    8.533   14.064   17.422
test8     5.173   12.499   17.351   24.671   30.403

所有的测试都在GitHub上

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