我有地图下面的列表
List<Map<String, Object>> listBeforeGroup = new ArrayList<Map<String, Object>>();
Map<String, Object> m1 = new HashMap<String, Object>();
m1.put("company", "LG");
m1.put("billType", "A");
m1.put("billPeriod", "09-2018");
Map<String, Object> m2 = new HashMap<String, Object>();
m2.put("company", "LG");
m2.put("billType", "A");
m2.put("billPeriod", "09-2018");
Map<String, Object> m3 = new HashMap<String, Object>();
m3.put("company", "LG");
m3.put("billType", "A");
m3.put("billPeriod", "09-2018");
Map<String, Object> m4 = new HashMap<String, Object>();
m4.put("company", "LG");
m4.put("billType", "B");
m4.put("billPeriod", "01-2019");
Map<String, Object> m5 = new HashMap<String, Object>();
m5.put("company", "LG");
m5.put("billType", "B");
m5.put("billPeriod", "01-2019");
Map<String, Object> m6 = new HashMap<String, Object>();
m6.put("company", "Samsung");
m6.put("billType", "A");
m6.put("billPeriod", "10-2018");
Map<String, Object> m7 = new HashMap<String, Object>();
m7.put("company", "Samsung");
m7.put("billType", "A");
m7.put("billPeriod", "10-2018");
Map<String, Object> m8 = new HashMap<String, Object>();
m8.put("company", "Samsung");
m8.put("billType", "B");
m8.put("billPeriod", "11-2018");
listBeforeGroup.add(m1);listBeforeGroup.add(m2);
listBeforeGroup.add(m3);listBeforeGroup.add(m4);
listBeforeGroup.add(m5);listBeforeGroup.add(m6);
我如何获得这个输出?
//Desired Output - List<Map<String, Object>>
//{company=LG, billType=A, billPeriod=09-2018, count=3}
//{company=LG, billType=B, billPeriod=01-2019, count=2}
//{company=Samsung, billType=A, billPeriod=10-2018, count=2}
//{company=Samsung, billType=B, billPeriod=11-2018, count=1}
我想这一点,使用Java 8流,但我不能得到所需的输出
List<Map<String, Object>> listAfterGroup = listBeforeGroup.stream().map(m -> m.entrySet().stream().collect(Collectors.toMap(p -> p.getKey(), p - > p.getValue()))).collect(Collectors.toList());
并试图此,顺便说一句这个解决方案提供了一个地图,但我不希望这样
Map<Object, Long> listAfterGroup = listBeforeGroup.stream().flatMap(m -> m.entrySet().stream()).collect(Collectors.groupingBy(Map.Entry::getKey,Collectors.counting()));
我想GROUP BY键“billPeriod”例如地图,并通过数值计算的项目,然后生成地图的新列表。
你可以创建一个类Company
,然后后续的操作变得更加简单。
class Company {
String company;
String billType;
String billPeriod;
public Company(String company, String billType, String billPeriod) {
this.company = company;
this.billType = billType;
this.billPeriod = billPeriod;
}
// getters, setters, toString, etc
}
初始化列表:
List<Company> list = new ArrayList<>();
list.add(new Company("LG", "A", "09-2018"));
list.add(new Company("LG", "A", "09-2018"));
list.add(new Company("LG", "A", "09-2018"));
list.add(new Company("LG", "B", "01-2019"));
list.add(new Company("LG", "B", "01-2019"));
list.add(new Company("Samsung", "A", "10-2018"));
list.add(new Company("Samsung", "A", "10-2018"));
list.add(new Company("Samsung", "B", "11-2018"));
现在为例,可以通过公司名称组:
Map<String, Long> map = list.stream().collect(
Collectors.groupingBy(Company::getCompany,
Collectors.mapping((Company c) -> c, Collectors.counting())));
现在,它变得更加容易,你的愿望执行其他操作。另外,在这里,而不是创建8个地图你最终处理的只是1名单。
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