Java的List集合按照对象的多个属性排序

public 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 MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        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 e2, Employee e1) {
//                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;
//            }
//        });
        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;
            }
        });

//  Employee [level=2, salary=500, years=4]
//  Employee [level=1, salary=500, years=4]
//  Employee [level=1, salary=300, years=5]
//  Employee [level=1, salary=300, years=4]

// Employee [level=1, salary=300, years=4]
// Employee [level=1, salary=300, years=5]
// Employee [level=1, salary=500, years=4]
// Employee [level=2, salary=500, years=4]

//          [level=1, salary=300, years=4]
//          [level=1, salary=300, years=5]
//          [level=1, salary=500, years=4]
//          [level=2, salary=500, years=4]


        for (Employee em : list) {
            System.out.println(em);
        }

    }
}