java List多字段排序方法

java中对List接口的实例按实体的字段排序。如下例：

package se;import java.util.ArrayList;import java.util.Collections;import java.util.Comparator;import java.util.List;class Employee {    private int level;    private int salary;    private int years;    public int getLevel() {        return level;    }    public void setLevel(int level) {        this.level = level;    }    public int getSalary() {        return salary;    }    public void setSalary(int salary) {        this.salary = salary;    }    public int getYears() {        return years;    }    public void setYears(int years) {        this.years = years;    }    @Override    public String toString() {        return "Employee [level=" + level + ", salary=" + salary + ", years="                + years + "]";    }}public class CollectionsListSortTest {    // 按级别工资入职年数递减排序    public static void main(String[] args) {        List<Employee> list = new ArrayList<Employee>();        Employee e = new Employee();        e.setLevel(1);        e.setSalary(300);        e.setYears(4);        list.add(e);        e = new Employee();        e.setLevel(1);        e.setSalary(500);        e.setYears(4);        list.add(e);        e = new Employee();        e.setLevel(2);        e.setSalary(500);        e.setYears(4);        list.add(e);        e = new Employee();        e.setLevel(1);        e.setSalary(300);        e.setYears(5);        list.add(e);        Collections.sort(list, new Comparator<Employee>() {            public int compare(Employee e1, Employee e2) {                int a = e1.getLevel() - e2.getLevel();                if (a != 0) {                    return a > 0 ? -1 : 1;                }                a = e1.getSalary() - e2.getSalary();                if (a != 0) {                    return a > 0 ? -1 : 1;                }                a = e1.getYears() - e2.getYears();                return a > 0 ? -1 : 1;            }        });        for (Employee em : list) {            System.out.println(em);        }    }}