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代码例子

//测试数据类
@Data
@Accessors(chain = true)
static class Test {
    Integer id;
    List<String> list;
}

List<Test> testList = new ArrayList<>();
testList.add(new Test().setId(1).setList(Stream.of("1a", "1b", "1c").collect(Collectors.toList())));
testList.add(new Test().setId(1).setList(Stream.of("1d", "1e").collect(Collectors.toList())));
testList.add(new Test().setId(2).setList(Stream.of("2a", "2b", "2f").collect(Collectors.toList())));
testList.add(new Test().setId(2).setList(Stream.of("2c", "2d", "2e").collect(Collectors.toList())));
testList.add(new Test().setId(3).setList(Stream.of("3x", "3y", "3z").collect(Collectors.toList())));

Map<Integer, List<String>> collect =
        testList.stream()
                .collect(Collectors.groupingBy(
                        Test::getId,
                        Collectors.collectingAndThen(
                                Collectors.mapping(Test::getList, Collectors.toList()),
                                i -> i.stream().flatMap(List::stream).collect(Collectors.toList())
                        )
                ));
System.out.println("collect = " + collect);
//运行结果 collect = {1=[1a, 1b, 1c, 1d, 1e], 2=[2a, 2b, 2f, 2c, 2d, 2e], 3=[3x, 3y, 3z]}