Set courseSet = new HashSet();
Set<String> courseSet1 = new HashSet<String>();
Set<String> h = new HashSet<String>(Arrays.asList("a", "b"));

List和Set集合是可以像String类型那样声明和初始化的，区别在于Set本身是一个接口，只能实例化它的实现类，比如上面贴的HashSet。像第三行那样声明并初始化使用也可以。

public class Student{
       public String id;
       public String name;
       public Set courses;

       public Student(String id, String name){
              this.id = id;
              this.name = name;
              this.courses = new HashSet();
       }
}