Java 8流，用于基于特定字段对对象列表进行排序 [英] java 8 stream for sorting a list of objects based on particular field

问题描述

我有一个根据时间戳对对象进行排序的方案.这些类如下:

I have a scenario to sort the objects based on timestamp. The classes are as follows:

class Employee 
{
    private String name;
    private List<Address> addresses;

    //...
    //Getters and setters for all fields
}

public class Address
{
    private String city;
    private Timestamp startDate;
    private Timestamp endDate;
    private String typeOfResidence;

    //...
    //Getters and setters for all the fields
}

对于一名员工来说，有两种可能性 1.它将有一个地址列表. 2.地址列表可以为空

For an employee, there are 2 possibilities 1. it will have a list of address. 2. the list of address can be null

地址类具有字段typeOfResidence，该字段可以具有诸如Residence，Office，ForeignHome之类的值.

The address class has a field typeOfResidence which can have values such as Residence, Office, ForeignHome.

每个Employee可以具有地址列表，一个地址将是住所"，另一个地址是办公室"，依此类推.可以有多个住宅地址，但只有一个办公室地址.

Each Employee can have list of address, one address will be Residential, other Office and so on. There can be multiple Residential addresses but only one Office address.

我想根据typeDyResidence = Office的地址的startDate对员工列表进行排序.

I want to sort the list of employees based on startDate of Address whose typeOfResidence=Office.

我写了以下代码:

private void sortEmployeeListBasedOnStartDate(List<Employee> employeeList)
{
    Comparator<Address> byTimeStamp = Comparator.comparing(
        Address::getStarteDate,
        (ts1, ts2) -> ts1.toGregorianCalendar().compareTo(ts2.toGregorianCalendar())
    );

    employeeList.stream()
        .map(x -> getLatestAddressByType(x, "Office"))
        .filter(y -> y!=null)
        .sorted(byTimeStamp.reversed())
        .collect(Collectors.toList());
}

public Address getLatestAddressByType(Employee employee, String type)
{
    if(role != null && brokerageManagedProposalType != null)
    {
         return getUserAddress(employee).stream()
            .filter(address-> address.getTypeOfResidence().equals(type))
            .findFirst()
            .orElse(null);
    }
    return null;
}

public List<Address> getUserAddress(Employee employee)
{
    if (!NullChecker.isNull(employee) && !NullChecker.isNull(employee.getAddress()))
    {
        return employee.getAddress();
    }
    return Collections.emptyList();
}

这似乎不起作用.员工未排序.请帮助我使其正常工作.

Somehow this does not seem to be working. The employees are not sorted. Please help me to make it working.

推荐答案

如果要就地排序，可以使用Collections.sort代替流.

If you want in-place sorting, you can use Collections.sort instead of streams.

Collections.sort(employeeList, 
    (e1,e2) -> byTimeStamp.compare(
        getLatestAddressByType(e2, "Office"),
        getLatestAddressByType(e1, "Office") );

类似地，可以按如下方式就地解决null过滤问题:

Similarly, the null-filtering issue can be solved in-place as follows:

employeeList.removeIf(e -> getLatestAddressByType(e, "Office") == null);

(btw，getLatestAddressByType应该是Employee的实例方法)

(btw, getLatestAddressByType should probably be an instance method of Employee)

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