如何按对象的属性对List <object>进行排序？

Question

班级Task.java：

    import java.util.Date;

    public class Task {

        public Long taskId;
        public Date startDate;

        public Task() {

        }

        public Task(Long taskId, Date startDate) {  
            this.taskId = taskId;
            this.startDate = startDate;
        }

        public Long getTaskId() {
            return taskId;
        }

        public void setTaskId(Long taskId) {
            this.taskId = taskId;
        }

        public Date getStartDate() {
            return startDate;
        }

        public void setStartDate(Date startDate) {
            this.startDate = startDate;
        }   

    }



Class `Test.java`
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.List;

public class Test {

    public static void main(String[] args) throws ParseException {
        List<Task> subTaskList = new ArrayList<Task>();
        DateFormat dateFormatter = new SimpleDateFormat("yyyy-MM-dd");
        Task t1 = new Task(34L, dateFormatter.parse("2013-09-20"));
        Task t2 = new Task(31L, dateFormatter.parse("2015-09-10"));
        Task t3 = new Task(1L, dateFormatter.parse("2015-09-13"));
        Task t4 = new Task(34L, dateFormatter.parse("2014-02-23"));     
        subTaskList.add(t1);
        subTaskList.add(t2);
        subTaskList.add(t3);
        subTaskList.add(t4);
    }   

}

如何按subTaskList排序startDate？