public?void?testAdd()
	{//創(chuàng)建一個課程對象，并通過調(diào)用add方法，添加到備選課程List中
		Course?cr1=new?Course("1","數(shù)據(jù)結(jié)構(gòu)");
	????courseToSelect.add(cr1);
	????System.out.println("添加到備選課程");
	????Course?temp=(Course)courseToSelect.get(0);
	????System.out.println("添加了課程:"+temp.id+temp.name);
	????
	????Course?cr2=new?Course("2","C語言");
	????courseToSelect.add(0,?cr2);
	????Course?temp2=(Course)courseToSelect.get(0);
	????System.out.print("添加了課程："+temp2.id+temp2.name);
	????
	????courseToSelect.add(cr1);
	????System.out.println("添加到備選課程");
	????Course?temp0=(Course)courseToSelect.get(2);
	????System.out.println("添加了課程:"+temp0.name);
	????
	????//以下方法會拋出數(shù)組下標(biāo)越界異常
//	????Course?cr3=new?Course("3","test");
//	????courseToSelect.add(4,cr3);
	Course[]?course={new?Course("3","離散數(shù)學(xué)"),new?Course("4","匯編語言")};
	courseToSelect.addAll(Arrays.asList(course));
	Course?temp3=(Course)courseToSelect.get(2);
	Course?temp4=(Course)courseToSelect.get(3);

	System.out.println("添加了兩門課程:"+temp3.id+":"+
	temp3.name+":"+temp4.id+":"+temp4.name);
	Course[]?course2={new?Course("5","高等數(shù)學(xué)"),new?Course("6","大學(xué)英語")};
	courseToSelect.addAll(2,?Arrays.asList(course2));
	Course?temp5=(Course)courseToSelect.get(2);
	Course?temp6=(Course)courseToSelect.get(3);
	System.out.println("添加了兩門課程"+temp5.id+":"+
	temp5.name+":"+temp6.id+":"+temp6.name);}

為什么我的temp.id輸出值是null呢？（testAdd方法已貼）