如何按对象java中的属性值分组

Question

假设我有一个下面的对象，

class sample
{
private String type;
private List<String> a1;
private List<String> a2;
private List<String> a3;
}

以及上述类的值，

 Sample sample1 = new Sample("type1", Arrays.asList("first", "second"), Arrays.asList("first") , [] );
 Sample sample2 = new Sample("type2", Arrays.asList("first", "second"), Arrays.asList("third") , [] );

我期待下面的结果，

first - [ type1, type2 ], second -[ type1, type2 ], third - [ type2 ]

要求： "将 a1、a2、a3 值和地图类型的属性与其他属性分组"

请帮我解决这个问题

Answer 1

首先连接每个列表a1、a2和a3，将其映射到Map.entry，其中键是列表中的字符串，值是type（使用distinct，因为有时“first”同时出现在列表a1中和 a2)。然后用Map.Entry进行分组。

Sample sample1 = new Sample("type1", Arrays.asList("first", "second"), Arrays.asList("first"), Arrays.asList());
Sample sample2 = new Sample("type2", Arrays.asList("first", "second"), Arrays.asList("third"), Arrays.asList());

List<Sample> samples = Arrays.asList(sample1, sample2);

Map<String, List<String>> samplesMap = samples.stream()
        .flatMap(s -> Stream.of(s.getA1(), s.getA2(), s.getA3())
                .flatMap(l -> l.stream().map(a -> Map.entry(a, s.getType())))
                .distinct()
        )
        .collect(Collectors.groupingBy(Map.Entry::getKey,
                Collectors.mapping(Map.Entry::getValue, Collectors.toList())));