返回SortedSet [英] Returning a SortedSet

问题描述

当我运行以下代码时：

 学生student1 = new student（Billy，13） 
学生student2 =新学生（Bob，12）; 
学生student3 =新学生（Belle，11）; 
学生student4 =新学生（Barry，10）; 
学生student5 =新学生（Brian，10）; 
学生student6 =新学生（Bane，13）; 
 Collection< Student> students = new HashSet< student>（）; 
 students.add（student1）; 
 students.add（student2）; 
 students.add（student3）; 
 students.add（student4）; 
 students.add（student5）; 
 students.add（student6）; 
（学生学生：学生）
 {
 String name = student.getName（）; 
 System.out.println（name）; 
} 
  

它将打印出我的学生对象的名称列表。现在我想按字母顺序排列。我认为它会像使用TreeSet或SortedSet一样简单。

像这样：

 学生student1 = ，13）; 
学生student2 =新学生（Bob，12）; 
学生student3 =新学生（Belle，11）; 
学生student4 =新学生（Barry，10）; 
学生student5 =新学生（Brian，10）; 
学生student6 =新学生（Bane，13）; 
 Collection< Student> students = ** new TreeSet< Student>（）; ** 
 students.add（student1）; 
 students.add（student2）; 
 students.add（student3）; 
 students.add（student4）; 
 students.add（student5）; 
 students.add（student6）; 
（学生学生：学生）
 {
 String name = student.getName（）; 
 System.out.println（name）; 
} 
  

但这只是抛出异常：

 线程main中的异常java.lang.ClassCastException：helloworld.Student不能转换为java.lang.Comparable 
 at java.util.TreeMap。 put（TreeMap.java:542）
 at java.util.TreeSet.add（TreeSet.java:238）
 at helloworld.Main.main（Main.java:60）
  
 
 
 
 Java结果：1 
 
 
 
学生类：
  public int compareTo（Student other）
 {
 return this.getName .compareTo（other.getName（））; 
} 
  
 
 
解决方案
订购？如果你的意思是按他们添加的顺序，那么只需使用 LinkedHashSet 。如果你想要某种排序，那么你必须描述 Student  s应该如何通过 Student 实现可比较的< Student> 或提供比较器< Student> 。
 
 
 
如果你是按字母排序，那么你应该修改 Student 类，例如：
  class Student implements Comparable< Student> {
 ... 
 public int compareTo（Student other）{
 return getName（）。compareTo（other.getName（））; 
} 
} 
  
 
When I run the following code:
  Student student1 = new Student("Billy", 13);
  Student student2 = new Student("Bob", 12);
  Student student3 = new Student("Belle", 11);
  Student student4 = new Student("Barry", 10);
  Student student5 = new Student("Brian", 10);
  Student student6 = new Student("Bane", 13);
  Collection<Student> students = new HashSet<Student>();
  students.add(student1);
  students.add(student2);
  students.add(student3);
  students.add(student4);
  students.add(student5);
  students.add(student6);
  for(Student student : students)
  {
    String name = student.getName();
    System.out.println(name);
  }
It will print out a list of names for my student objects. Now I'd like to do them in alphabetical order. I thought it would be as simple as using a TreeSet or SortedSet. 

Like this: 
 Student student1 = new Student("Billy", 13);
 Student student2 = new Student("Bob", 12);
 Student student3 = new Student("Belle", 11);
 Student student4 = new Student("Barry", 10);
 Student student5 = new Student("Brian", 10);
 Student student6 = new Student("Bane", 13);
 Collection<Student> students = **new TreeSet<Student>();**
 students.add(student1);
 students.add(student2);
 students.add(student3);
 students.add(student4);
 students.add(student5);
 students.add(student6);
 for(Student student : students)
 {
    String name = student.getName();
    System.out.println(name);
 }
But this just throws the exception:
  Exception in thread "main" java.lang.ClassCastException: helloworld.Student cannot be cast to java.lang.Comparable
    at java.util.TreeMap.put(TreeMap.java:542)
    at java.util.TreeSet.add(TreeSet.java:238)
    at helloworld.Main.main(Main.java:60)
Java Result: 1

I have added a compareTo method in the student class too: 
 public int compareTo(Student other)
 {
   return this.getName().compareTo(other.getName());
 }

解决方案
What do you mean by "order"?  If you mean in the order they were added, then just use LinkedHashSet.  If you want some kind of sorting, then you have to describe how Students should be sorted by having Student implement Comparable<Student> or by providing a Comparator<Student>.

If you meant alphabetical order, then you should modify the Student class like so:
class Student implements Comparable<Student> {
   ...
   public int compareTo(Student other) {
     return getName().compareTo(other.getName());
   }
}


                        
这篇关于返回SortedSet的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！