我需要在Java中实现数据透视表,并且知道如何使用Java 8 Streams功能。 Web上有很多很好的解决方案,但是我还需要更多,而且我不知道该怎么做:我需要创建一个更动态的表,理想情况下,您不知道必须汇总哪些列。
例如,如果我有以下列(“国家”,“公司”,“行业”,“雇员人数”),则必须输入:
度量的自定义聚合函数(例如总和)
聚合的可变顺序:例如,我要为Nation进行第一个聚合,然后将“ Nation”或“ Nation and Company”作为参数,并给“ Nation-> Company”作为参数。
换句话说,我不知道哪些字段用于聚合,基本上我需要一种方法来实现通用的GROUP BY SQL子句,如下所示:
// Given an the Arraylist ("Nation", "Company", "Industry","Number of employes") called data with some rows
Map<String, List<Object[]>> map = data.stream().collect(
Collectors.groupingBy(row -> row[0].toString() + "-" + row[1].toString()));
for (Map.Entry<String, List<Object[]>> entry : map.entrySet()) {
final double average = entry.getValue().stream()
.mapToInt(row -> (int) row[3]).average().getAsDouble();
这不是我所需要的,因为它太明确了。
我需要:
根据我从数据中提取的标头名称提供的值,将输入Arraylist拆分为子列表中的值(或更多,这取决于我必须分组的列数)
汇总每个子列表
合并子列表
有人可以帮助我吗?谢谢
最佳答案
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
class Input {
private String nation, company, industry;
private int employees;
public Input(String nation, String company, String industry, int employees) {
super();
this.nation = nation;
this.company = company;
this.industry = industry;
this.employees = employees;
}
public String getNation() {
return nation;
}
public void setNation(String nation) {
this.nation = nation;
}
public String getCompany() {
return company;
}
public void setCompany(String company) {
this.company = company;
}
public String getIndustry() {
return industry;
}
public void setIndustry(String industry) {
this.industry = industry;
}
public int getEmployees() {
return employees;
}
public void setEmployees(int employees) {
this.employees = employees;
}
@Override
public String toString() {
return String.format(
"Nation : %s, Company : %s, Industry : %s, Employees : %s",
nation, company, industry, employees);
}
}
public class CustomGroupBy {
// Generic GroupBy
static Map<String, List<Input>> groupBy(List<Input> input,
Function<Input, String> classifier) {
return input.stream().collect(Collectors.groupingBy(classifier));
}
public static void main(String[] args) {
List<Input> input = Arrays.asList(new Input("India", "A", "IT", 12),
new Input("USA", "B", "ELECTRICAL", 90), new Input("India",
"B", "MECHANICAL", 122), new Input("India", "B", "IT",
12), new Input("India", "C", "IT", 200));
// You need to pass this in parameter
Function<Input, String> groupByFun = i -> i.getNation() + "-"
+ i.getCompany();
// Example-1
Map<String, List<Input>> groupBy = groupBy(input, Input::getCompany);
// Example-2
Map<String, List<Input>> groupBy2 = groupBy(input, groupByFun);
System.out.println(groupBy2);
List<Double> averages = groupBy
.entrySet()
.stream()
.map(entry -> entry.getValue().stream()
.mapToInt(row -> row.getEmployees()).average()
.getAsDouble()).collect(Collectors.toList());
System.out.println(averages);
}
}
您可以通过传递功能接口来使其通用。仅供参考。