为什么最开始的时候要get(0),而后来就get（2）和（3）了
Classes[] classes={new Classes("3","泛函分析" ),new Classes("4", "大学物理")};
classesToSelect.addAll(Arrays.asList(classes));
Classes temp3=(Classes) classesToSelect.get(2);
Classes temp4=(Classes) classesToSelect.get(3);
System.out.println("添加了两门课程："+temp3.id+temp3.name+temp4.id+temp4.name);
Classes[] classes2={new Classes("5","泛函分析加强" ),new Classes("6", "大学物理2")};
classesToSelect.addAll(2, Arrays.asList(classes2));
Classes temp5=(Classes) classesToSelect.get(2);
Classes temp6=(Classes) classesToSelect.get(3);
System.out.println("添加了两门课程啊："+temp5.id+temp5.name+temp6.id+temp6.name);
