Java8 : About order of Map elements returned by Collectors. 分组依据

Question

想就遇到的例子修改的下面一段代码求教

我想知道当输入列表元素序列发生变化时，生成的 Map 中的输出序列是否会发生变化。但是，似乎所有情况下的输出都是相同的(我在 IDE 上反复运行该程序)。我可以就生成的 Map 元素的顺序是否可以预期寻求建议？使用枚举或字符串时，序列(按字母顺序/非字母顺序？)是否会有任何差异，或者元素序列只是随机的？

public class TestCountry {

    public enum Continent {ASIA, EUROPE}

    String name;
    Continent region;

    public TestCountry (String na, Continent reg) {
        name = na;
        region = reg;
    }

    public String getName () {
        return name;
    }

    public Continent getRegion () {
        return region;
    }

    public static void main(String[] args) {
        List<TestCountry> couList1 = Arrays.asList (
                new TestCountry ("Japan", TestCountry.Continent.ASIA),
                new TestCountry ("Italy", TestCountry.Continent.EUROPE),
                new TestCountry ("Germany", TestCountry.Continent.EUROPE));

        List<TestCountry> couList2 = Arrays.asList (
                new TestCountry ("Italy", TestCountry.Continent.EUROPE),
                new TestCountry ("Japan", TestCountry.Continent.ASIA),
                new TestCountry ("Germany", TestCountry.Continent.EUROPE));

        Map<TestCountry.Continent, List<String>> mapWithRegionAsKey1 = couList1.stream ()
                    .collect(Collectors.groupingBy (TestCountry ::getRegion,
                        Collectors.mapping(TestCountry::getName, Collectors.toList())));

        Map<String, List<Continent>> mapWithNameAsKey1 = couList1.stream ()
                    .collect(Collectors.groupingBy (TestCountry::getName,
                        Collectors.mapping(TestCountry::getRegion, Collectors.toList())));

        Map<TestCountry.Continent, List<String>> mapWithRegionAsKey2 = couList2.stream ()
                    .collect(Collectors.groupingBy (TestCountry ::getRegion,
                        Collectors.mapping(TestCountry::getName, Collectors.toList())));

        Map<String, List<Continent>> mapWithNameAsKey2 = couList2.stream ()
                    .collect(Collectors.groupingBy (TestCountry::getName,
                        Collectors.mapping(TestCountry::getRegion, Collectors.toList())));

        System.out.println("Value of mapWithRegionAsKey1 in couList1: " + mapWithRegionAsKey1);
        System.out.println("Value of mapWithNameAsKey1 in couList1: " + mapWithNameAsKey1);

        System.out.println("Value of mapWithRegionAsKey2 in couList2: " + mapWithRegionAsKey2);
        System.out.println("Value of mapWithNameAsKey2 in couList2: " + mapWithNameAsKey2);
    }
}

/*
 * Output:
Value of mapWithRegionAsKey1 in couList1: {ASIA=[Japan], EUROPE=[Italy, 
Germany]}
Value of mapWithNameAsKey1 in couList1: {Japan=[ASIA], Italy=[EUROPE], 
Germany=[EUROPE]}
Value of mapWithRegionAsKey2 in couList2: {ASIA=[Japan], EUROPE=[Italy, 
Germany]}
Value of mapWithNameAsKey2 in couList2: {Japan=[ASIA], Italy=[EUROPE], 
Germany=[EUROPE]}
 */

Answer 1

Collectors.groupingBy() 当前返回一个 HashMap。这是一个实现细节，可能会在未来的版本中发生变化。

HashMap 的条目没有排序，但它们迭代的顺序(当打印 HashMap 时)是确定的，并且不依赖于插入订单。

因此，无论输入 List 中元素的顺序如何，您都会看到相同的输出。不过，您不应该依赖该顺序，因为它是一个实现细节。

如果您关心输出 Map 中条目的顺序，您可以修改代码以生成 LinkedHashMap(默认顺序为插入顺序)或一个 TreeMap(对键进行排序的地方)。

顺便说一句，如果您将输入更改为

    List<TestCountry> couList1 = Arrays.asList (
            new TestCountry ("Japan", TestCountry.Continent.ASIA),
            new TestCountry ("Italy", TestCountry.Continent.EUROPE),
            new TestCountry ("Germany", TestCountry.Continent.EUROPE));

    List<TestCountry> couList2 = Arrays.asList (
            new TestCountry ("Germany", TestCountry.Continent.EUROPE),
            new TestCountry ("Italy", TestCountry.Continent.EUROPE),
            new TestCountry ("Japan", TestCountry.Continent.ASIA)
            );

您将在输出中得到不同的顺序:

Value of mapWithRegionAsKey1 in couList1: {ASIA=[Japan], EUROPE=[Italy, Germany]}
Value of mapWithNameAsKey1 in couList1: {Japan=[ASIA], Italy=[EUROPE], Germany=[EUROPE]}
Value of mapWithRegionAsKey2 in couList2: {ASIA=[Japan], EUROPE=[Germany, Italy]}
Value of mapWithNameAsKey2 in couList2: {Japan=[ASIA], Italy=[EUROPE], Germany=[EUROPE]}

这是因为各个输出 List 的元素(在当前实现中是 ArrayList)是根据插入顺序排序的，这取决于关于您输入的 List 的顺序。