Java8 Stream使用flatMap合并List_卓立的博客-CSDN博客_stream.flatmap
之前也写过很多篇关于Java8使用的文章了，但是回顾一下，好像还没介绍过Java8 Stream的flatMap操作，昨天刚好在工作中遇到一个场景，发现flatMap简直太方便了，这里总结一下flatMap的常规使用。附带讲一下，使用Java8实现集合的并、交、差操作，其实之前也讲过一种使用Guava的实现方式，具体请参考Guava集合工具
flatMap
首先看一下一种场景，存在一个Map<Integer， ListContainer>，ListContainer中存在一个List<AClass>成员变量。有这样一个需求，讲Map中values中所有的List<AClass>组合成一个List<AClass>。我们也许会这样操作：
List<AClass> resultAClassList = Lists.newArrayList();for (ListContainer tmp : map.values()){ resultAClassList.addAll(tmp.getLst());}这还是只存在一层List的情况，如果存在多层，for还需要嵌套，使用起来很不方便。后来查到，Java8 Stream的flatMap操作可以很好地适用这种场景，首先看一下flatMap方法定义：
<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);参数是一个Function函数式接口，提供T到Stram的转换。其实参考方法实现，flatMap就是将Function转化后的Stram合并成一个Stream。下面看一下使用示例，完成上述相同的功能：
@Testpublic void mergeMapValuesTest(){ Map<Integer, ListContainer> map = Maps.newHashMap(); List<AClass> aClassList1 = Lists.newArrayList(); AClass aClass = new AClass(1, "zhuoli1", "haha1"); aClassList1.add(aClass); aClassList1.add(new AClass(2, "zhuoli2", "haha2")); aClassList1.add(new AClass(3, "zhuoli3", "haha3")); List<AClass> aClassList2 = Lists.newArrayList(); aClassList2.add(aClass); aClassList2.add(new AClass(5, "zhuoli5", "haha5")); aClassList2.add(new AClass(6, "zhuoli6", "haha6")); /*交集*/ /*[AClass(id=1, name=zhuoli1, description=haha1)]*/ List<AClass> intersectResult = aClassList1.stream().filter(aClassList2::contains).collect(Collectors.toList()); System.out.println(intersectResult); /*并集*/ List<AClass> unionResult = Stream.of(aClassList1, aClassList2).flatMap(Collection::stream).distinct().collect(Collectors.toList()); assertEquals(unionResult.size(), 5); System.out.println(unionResult); /*差集*/ /*[AClass(id=2, name=zhuoli2, description=haha2), AClass(id=3, name=zhuoli3, description=haha3)]*/ List<AClass> differenceResult = aClassList1.stream().filter(x -> !aClassList2.contains(x)).collect(Collectors.toList()); System.out.println(differenceResult); map.put(1, new ListContainer(aClassList1)); map.put(2, new ListContainer(aClassList2)); /*合并多个list*/ List<AClass> aClassListResult = map.values().stream().flatMap(listContainer -> listContainer.getLst().stream()).collect(Collectors.toList()); /*注意跟并集的区别*/ assertEquals(aClassListResult.size(), 6); System.out.println(aClassListResult);}分享一个flatMap的复杂操作，实现List<Data1>和List<Data2>根据Id进行连接，将连接结果输出为一个List<OutputData>：
@Data@AllArgsConstructorpublic class Data1 { private int id; private String name; private int amount;} @Data@AllArgsConstructorpublic class Data2 { private int id; private String name; private String type;} @Data@AllArgsConstructorpublic class OutputData { private int id; private String name; private String type; private int amount;} @Testpublic void intersectByKeyTest(){ List<Data2> listOfData2 = new ArrayList<Data2>(); listOfData2.add(new Data2(10501, "JOE" , "Type1")); listOfData2.add(new Data2(10603, "SAL" , "Type5")); listOfData2.add(new Data2(40514, "PETER", "Type4")); listOfData2.add(new Data2(59562, "JIM" , "Type2")); listOfData2.add(new Data2(29415, "BOB" , "Type1")); listOfData2.add(new Data2(61812, "JOE" , "Type9")); listOfData2.add(new Data2(98432, "JOE" , "Type7")); listOfData2.add(new Data2(62556, "JEFF" , "Type1")); listOfData2.add(new Data2(10599, "TOM" , "Type4")); List<Data1> listOfData1 = new ArrayList<Data1>(); listOfData1.add(new Data1(10501, "JOE" ,3000000)); listOfData1.add(new Data1(10603, "SAL" ,6225000)); listOfData1.add(new Data1(40514, "PETER" ,2005000)); listOfData1.add(new Data1(59562, "JIM" ,3000000)); listOfData1.add(new Data1(29415, "BOB" ,3000000)); List<OutputData> result = listOfData1.stream() .flatMap(x -> listOfData2.stream() .filter(y -> x.getId() == y.getId()) .map(y -> new OutputData(y.getId(), x.getName(), y.getType(), x.getAmount()))) .collect(Collectors.toList()); System.out.println(result); /*difference by key*/ List<Data1> data1IntersectResult = listOfData1.stream().filter(data1 -> listOfData2.stream().map(Data2::getId).collect(Collectors.toList()).contains(data1.getId())).collect(Collectors.toList()); System.out.println(data1IntersectResult);}faltMapToInt
首先看一下flatMapToInt方法定义：
IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper);跟flatMap不同的是，参数Function函数式接口提供由T到IntStream的转化，方法返回值是IntStream。
@Testpublic void flatMapToIntTest() { List<List<String>> listOfLists = Arrays.asList( Arrays.asList("1", "2"), Arrays.asList("5", "6"), Arrays.asList("3", "4") ); IntStream intStream = listOfLists.stream() .flatMapToInt(childList -> childList.stream() .mapToInt(Integer::new)); int sum = intStream.peek(System.out::println).sum(); System.out.println("sum: " + sum);}Stream接口中还存在类似的方法flatMapToDouble、flatMapToLong，使用方法跟flatMapToInt是一样的，这里就不多介绍了，仅罗列一下方法定义：
DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper);LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper);示例代码：码云 – 卓立 – Java8 flatMap示例
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创建于: 2022-02-14 15:24:58
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