java - 基于对象的不同可空字段的比较器

Question

我有一个 Employee 对象，它包含两个字段 name 和 jobTitle。对于员工对象排序，首先应优先考虑 jobTitle，如果 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;
}

我使用链式比较器与 JobTitlecomparator 和 NameComparator 来实现这一点:

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

但我没有得到预期的结果。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

谁能帮我实现这个目标？

Answer 1

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。比较职位和名字可以很容易地完成

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

如何处理 null 值也内置于 nullsLast 中和 nullsFirst方法。这些方法将现有比较器包装到 null 安全比较器中，将 null 值放在末尾或开头。

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);

比较器由 comparing 创建带有 null 安全比较器的职位头衔将 null 值放在最后 ( see also )。对于相同的标题，它是 thenComparing员工姓名。