java - 如何仅从所需的类(流)获取数据？

Question

Employee类:

public abstract class Employee extends Person {
    private final Manager manager;
    private final BigDecimal salary;
    
    protected Employee(String firstName, String surname, LocalDate birth_date, Manager _manager, BigDecimal _salary) {
        super(firstName, surname, birth_date);
        manager = _manager;
        salary = _salary;
        if (manager != null) {
            manager.getSubordinates().add(this);
        }
    }

    ...
}

Worker类:

public class Worker extends Employee {
    private final LocalDate employment_date;
    private BigDecimal bonus;
    
    public Worker(String firstName, String surname, LocalDate birth_date, Manager manager, BigDecimal salary,
                  LocalDate _employment_date, BigDecimal _bonus) {
        super(firstName, surname, birth_date, manager, salary);
        employment_date = _employment_date;
        bonus = _bonus;
    }

    ...
}

Manager类:

public final class Manager extends Worker {
    List<Employee> subordinates = new ArrayList<Employee>();
    
    public Manager(String firstName, String surname, LocalDate birth_date, Manager manager, BigDecimal salary,
                   LocalDate employment_date, BigDecimal bonus) {
        super(firstName, surname, birth_date, manager, salary, employment_date, bonus);
    }

    ...
}

Trainee类:

public class Trainee extends Employee {

    private final LocalDate start_date;
    private final short apprenticeship_length;
    
    public Trainee(String firstName, String surname, LocalDate birth_date, Manager manager, BigDecimal salary,
                   LocalDate _start_date, short _apprenticeship_length) {
        super(firstName, surname, birth_date, manager, salary);
        manager.getSubordinates().add(this);
        start_date = _start_date;
        apprenticeship_length = _apprenticeship_length;
    }
}

payrol类:

public final class PayrollEntry {

    private final Employee _employee;
    private final BigDecimal _salaryPlusBonus;
    
    public PayrollEntry(Employee employee, BigDecimal salary, BigDecimal bonus) {
        _employee = employee;
        _salaryPlusBonus = salary.add(bonus);
    }
}

我必须编写函数 List<PayrollEntry> payroll(List<Employee> employees) {} .正如你在上面看到的，只有 Worker和 Manager可以有奖金，Trainee另一方面不要，但它们都派生自类 Employee (顺便说一句，我无法更改类(class)层次结构中的任何内容，因为这是我的作业，而层次结构是由老师编写的)。我应该使用函数式编程技术来编写函数，这是我的尝试:

public static List<PayrollEntry> payroll(List<Employee> employees) {
    return employees
            .stream()
            .map(employee -> new PayrollEntry(employee, employee.getSalary(), ((Worker) employee).getBonus()))
            .collect(Collectors.toList());
}

我明白为什么它会给我一个 ClassCastException , 但我不知道还有其他方法可以使用 stream 来做到这一点.我想我可以用 for-each 来做每次循环检查是否为 Trainee或不，但我想知道是否有办法使用 stream .

Answer 1

对于 map() 的内容应该有什么限制？

否则你可以有类似的东西:

.map(employee -> {
    if (employee instanceof Worker) {
        return new PayrollEntry()....
    } else {
        return new PayrollEntry()....
    }
})