问题

我的需求是查询出来一组数据后，按照其中的某个属性进行groupBy分组，分组后要保证顺序不变。
但是实际用groupBy进行分组后，返回的数据是杂乱无章的，没有按照原来list 的顺序返回。

排查

首先去api中查找问题原因，查看Java的java.util.stream 包 Collectors 类 groupingBy 方法实现，结果如下：

	//一个参数
	public static <T, K> Collector<T, ?, Map<K, List<T>>>
    groupingBy(Function<? super T, ? extends K> classifier) {
        return groupingBy(classifier, toList());
    }
	
	//实际调用方法
    public static <T, K, A, D>
    Collector<T, ?, Map<K, D>> groupingBy(Function<? super T, ? extends K> classifier,
                                          Collector<? super T, A, D> downstream) {
        return groupingBy(classifier, HashMap::new, downstream);
    }
	
	//两个参数
    public static <T, K, A, D>
    Collector<T, ?, Map<K, D>> groupingBy(Function<? super T, ? extends K> classifier,
                                          Collector<? super T, A, D> downstream) {
        return groupingBy(classifier, HashMap::new, downstream);
    }
    
	//三个参数
	public static <T, K, D, A, M extends Map<K, D>>
    Collector<T, ?, M> groupingBy(Function<? super T, ? extends K> classifier,
                                  Supplier<M> mapFactory,
                                  Collector<? super T, A, D> downstream) {
        Supplier<A> downstreamSupplier = downstream.supplier();
        BiConsumer<A, ? super T> downstreamAccumulator = downstream.accumulator();
        BiConsumer<Map<K, A>, T> accumulator = (m, t) -> {
            K key = Objects.requireNonNull(classifier.apply(t), "element cannot be mapped to a null key");
            A container = m.computeIfAbsent(key, k -> downstreamSupplier.get());
            downstreamAccumulator.accept(container, t);
        };
        BinaryOperator<Map<K, A>> merger = Collectors.<K, A, Map<K, A>>mapMerger(downstream.combiner());
        @SuppressWarnings("unchecked")
        Supplier<Map<K, A>> mangledFactory = (Supplier<Map<K, A>>) mapFactory;

        if (downstream.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)) {
            return new CollectorImpl<>(mangledFactory, accumulator, merger, CH_ID);
        }
        else {
            @SuppressWarnings("unchecked")
            Function<A, A> downstreamFinisher = (Function<A, A>) downstream.finisher();
            Function<Map<K, A>, M> finisher = intermediate -> {
                intermediate.replaceAll((k, v) -> downstreamFinisher.apply(v));
                @SuppressWarnings("unchecked")
                M castResult = (M) intermediate;
                return castResult;
            };
            return new CollectorImpl<>(mangledFactory, accumulator, merger, finisher, CH_NOID);
        }
    }

可以发现 groupingBy调用是内部自己创建了一 HashMap（HashMap::new）。因为 HashMap,是无无序的，是根据key的hashcode进行hash，然后放入指定位置，所以按照一定顺序put进HashMap时与遍历出HashMap的顺序不同。

因此我们直接调用三个参数的groupingBy方法mapFactory，传入有顺序的Map（LinkedHashMap） 就可以了。

示例

import java.util.ArrayList;
import java.util.Comparator;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Test {

	public static void main(String[] args) {
		List<Map<String, Object>> list = new ArrayList<Map<String, Object>>();

		for (int i = 0; i < 3; i++) {
			Map<String, Object> map = new HashMap<String, Object>();
			map.put("index", 3);
			map.put("category", "类别3");
			map.put("other", i);
			list.add(map);
		}

		for (int i = 0; i < 3; i++) {
			Map<String, Object> map = new HashMap<String, Object>();
			map.put("index", 1);
			map.put("category", "类别1");
			map.put("other", i);
			list.add(map);
		}

		for (int i = 0; i < 3; i++) {
			Map<String, Object> map = new HashMap<String, Object>();
			map.put("index", 2);
			map.put("category", "类别2");
			map.put("other", i);
			list.add(map);
		}

		for (int i = 3; i < 6; i++) {
			Map<String, Object> map = new HashMap<String, Object>();
			map.put("index", 2);
			map.put("category", "类别1");
			map.put("other", i);
			list.add(map);
		}

		System.out.println(list);

		Map<String, List<Map<String, Object>>> step1 = list.stream().collect(Collectors.groupingBy(o -> {
			return o.get("category").toString();
		}));

		System.out.println(step1);

		// 手动控制排序方式
		Map<String, List<Map<String, Object>>> step2 = list.stream().sorted(Comparator.comparingInt(o -> {
			return -(int) o.get("index");
		})).collect(Collectors.groupingBy(o -> {
			return o.get("category").toString();
		}, LinkedHashMap::new, Collectors.toList()));

		System.out.println(step2);
	}

}

运行结果

[{other=0, index=3, category=类别3}, {other=1, index=3, category=类别3}, {other=2, index=3, category=类别3}, {other=0, index=1, category=类别1}, {other=1, index=1, category=类别1}, {other=2, index=1, category=类别1}, {other=0, index=2, category=类别2}, {other=1, index=2, category=类别2}, {other=2, index=2, category=类别2}, {other=3, index=2, category=类别1}, {other=4, index=2, category=类别1}, {other=5, index=2, category=类别1}]

{类别1=[{other=0, index=1, category=类别1}, {other=1, index=1, category=类别1}, {other=2, index=1, category=类别1}, {other=3, index=2, category=类别1}, {other=4, index=2, category=类别1}, {other=5, index=2, category=类别1}], 类别3=[{other=0, index=3, category=类别3}, {other=1, index=3, category=类别3}, {other=2, index=3, category=类别3}], 类别2=[{other=0, index=2, category=类别2}, {other=1, index=2, category=类别2}, {other=2, index=2, category=类别2}]}

{类别3=[{other=0, index=3, category=类别3}, {other=1, index=3, category=类别3}, {other=2, index=3, category=类别3}], 类别2=[{other=0, index=2, category=类别2}, {other=1, index=2, category=类别2}, {other=2, index=2, category=类别2}], 类别1=[{other=3, index=2, category=类别1}, {other=4, index=2, category=类别1}, {other=5, index=2, category=类别1}, {other=0, index=1, category=类别1}, {other=1, index=1, category=类别1}, {other=2, index=1, category=类别1}]}