基于对象的不同可空字段的比较器

我有一个Employee包含两个字段的对象name and jobTitle。为了对员工对象进行排序，第一优先级应该是jobTitle, if jobTitle为 null 则应根据名称排序。

下面是 Employee 对象

public class Employee {
    private String name;
    private String jobTitle;
}

我使用了链式比较器JobTitlecomparator and NameComparator为了达成这个：

public class EmployeeChainedComparator implements Comparator<Employee> {

    private List<Comparator<Employee>> listComparators;

    @SafeVarargs
    public EmployeeChainedComparator(Comparator<Employee>... comparators) {
        this.listComparators = Arrays.asList(comparators);
    }

    @Override
    public int compare(Employee emp1, Employee emp2) {
        for (Comparator<Employee> comparator : listComparators) {
            int result = comparator.compare(emp1, emp2);
            if (result != 0) {
                return result;
            }
        }
        return 0;
    }
}

public class EmployeeJobTitleComparator implements Comparator<Employee> {

    @Override
    public int compare(Employee emp1, Employee emp2) {
        if(emp1.getJobTitle() != null && emp2.getJobTitle() != null){
            return emp1.getJobTitle().compareTo(emp2.getJobTitle());
        } else {
            return 0;
        }
    }
}

public class EmployeeNameComparator implements Comparator<Employee> {

    @Override
    public int compare(Employee emp1, Employee emp2) {
        return emp1.getName().compareTo(emp2.getName());
    }
}

public class SortingMultipleAttributesExample {
    public static void main(String[] args) {
        List<Employee> listEmployees = new ArrayList<Employee>();
        listEmployees.add(new Employee("Tom", "Developer"));
        listEmployees.add(new Employee("Sam", null));
        listEmployees.add(new Employee("Tim", "Designer"));
        listEmployees.add(new Employee("Bob", null));
        listEmployees.add(new Employee("Peter", null));
        listEmployees.add(new Employee("Craig", "Programmer"));

        Collections.sort(listEmployees, new EmployeeChainedComparator(new EmployeeJobTitleComparator(), new EmployeeNameComparator()
                ));

        for(Employee emp : listEmployees){
            System.out.println("Employee Job: "+emp.getJobTitle()+" Employee Name: "+emp.getName());
        }
    }
}

现在我应该得到这样的输出

Employee Job: Designer Employee Name: Tim
Employee Job: Developer Employee Name: Tom
Employee Job: Programmer Employee Name: Craig
Employee Job: null Employee Name: Bob
Employee Job: null Employee Name: Peter
Employee Job null Employee Name: Sam

但我没有得到我预期的预期结果。我得到这样的输出

Employee Job Developer Employee Name Tom
Employee Job null Employee Name Sam
Employee Job Designer Employee Name Tim
Employee Job null Employee Name Bob
Employee Job null Employee Name Peter
Employee Job Programmer Employee Name Craig

谁能帮助我如何实现这一目标？

由于您使用的是 Java 8，因此您可以使用内置的比较器工具，而不是创建自己的比较器。比较职位名称和名称可以轻松完成

Comparator<Employee> comparator =
     Comparator.comparing(Employee::getJobTitle).thenComparing(Employee:getName);

如何处理null值也内置于nullsLast https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#nullsLast-java.util.Comparator- and nullsFirst https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#nullsFirst-java.util.Comparator-方法。这些方法将现有的比较器包装成null安全比较器，放置null值位于末尾或开头。

因此，您可以：

import static java.util.Comparator.comparing;
import static java.util.Comparator.naturalOrder;
import static java.util.Comparator.nullsLast;

// ...

Comparator<Employee> comparator = 
    comparing(Employee::getJobTitle, nullsLast(naturalOrder())).thenComparing(Employee::getName);

Collections.sort(listEmployees, comparator);

比较器是由创建的comparing https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#comparing-java.util.function.Function-java.util.Comparator-职位名称带有null安全比较器放置null最后的值（see also https://stackoverflow.com/questions/26350996/java-8-comparator-nullsfirst-naturalorder-confused）。对于平等的头衔来说，thenComparing https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#thenComparing-java.util.function.Function-员工的姓名。