請看下面程序中MyDate newDay=new MyDate(this);this是什么作用?class MyDate{ private int day; private int month; private int year; //定義三個成員變量; public MyDate(int day,int month,int year){ this.day=day; this.month=month; this.year=year; } //構造方法; public MyDate(MyDate date){ this.day=date.day; this.month=date.month; this.year=date.year; //將參數Date類中的成員變量賦給MyDate類; } //構造方法; public int getDay(){ return day; }//方法; public void setDay(int day){ this.day=day;//參數Day賦給此類中的Ddy; } public MyDate addDays(int moreDay){ MyDate newDay=new MyDate(this); newDay.day=newDay.day+moreDay; return newDay; //返回整個類; } public void print(){ System.out.println("My Date: "+year+"-"+month+"-"+day); }}public class TestMyDate{ public static void main(String args[]){ MyDate myBirth=new MyDate(19,11,1987); //利用構造函數初始化; MyDate next=myBirth.addDays(7); //addDays()的返回值是類,將其返回值賦給變量next; next.print(); }}