如何根据对象的属性对对象进行排序？ [英] How To Sort Object based on Object's attribute?

问题描述

除了排序方法外，我还有其他所有工作。我需要根据学生的第一个属性对HashMap中的学生进行排序。我需要在添加HashMap中的所有学生之后发生排序方法，而不是在添加时添加。

  package HashMap; 
 
 import java.io.InputStream; 
 import java.util.HashMap; 
 import java.util.HashSet; 
 import java.util.Scanner; 
 
 public class clubMapping {
 HashMap< String，HashSet< Student>> map = new HashMap< String，HashSet< Student>>（）; 
 $ b public clubMapping（String clubName）{
 InputStream is = getClass（）。getClassLoader（）。getResourceAsStream（
student.txt）; 
扫描仪扫描=新扫描仪（是）; （）扫描.next（），扫描。 ）; 
 scan.close（）; 
 System.out.println（map）; 
 System.out.println（）; 
 System.out.println（map.get（clubName））; 
} 
 
 public void add（String last，String first，Integer id，String club）{
 HashSet< Student> set = new HashSet< Student>（）; 
 if（！map.containsKey（club））{
 set.add（new Student（last，first，id））; 
 map.put（club，set）; 
} else {
 set = map.get（club）; 
 set.add（new Student（last，first，id））; 
 
 
 
 public static void main（String [] args）{
 new clubMapping（Math）; 
} 
} 
 
软件包HashMap; 
 
公共班级学生{
字符串最后，第一个; 
整数ID; 
 
 public Student（String l，String f，Integer i）{
 last = l; 
 first = f; 
 id = i; 
} 
 
 public String toString（）{
 return last ++ first ++ id; 
 
 
 
 $ div $解析方案

你应该为学生组使用 TreeSet 而不是 HashSet 。

  public void add（String last，String first，Integer id， String club）{
 TreeSet< Student> set = new TreeSet< Student>（studentComp）; 
 if（！map.containsKey（club））{
 set.add（new Student（last，first，id））; 
 map.put（club，set）; 
} else {
 set = map.get（club）; 
 set.add（new Student（last，first，id））; 
 
 
 
 
 
 
 $ b 
但是你必须定义一个用于排序的比较器，如下所示。
  private static Comparator< Student> studentComp = new Comparator< Student>（）{
 @Override 
 public int compare（Student s1，Student s2）{
 return（s1.last.compareTo（s2.last））; 
} 
}; 
  
另外，您应该将您的学生属性封装在getter / setter方法中，而不是直接以我已经完成了上面的内容（正如你的班级目前允许的那样）。
 
I got everything else to work except the sort method. I need to sort the Students in the HashMap based on the Student's first attribute. I need the sort method to happen after I added all of the students in the HashMap, not while it's being added.     
package HashMap;

        import java.io.InputStream;
        import java.util.HashMap;
        import java.util.HashSet;
        import java.util.Scanner;

        public class clubMapping {
            HashMap<String, HashSet<Student>> map = new HashMap<String, HashSet<Student>>();

            public clubMapping(String clubName) {
                InputStream is = getClass().getClassLoader().getResourceAsStream(
                        "student.txt");
                Scanner scan = new Scanner(is);
                while (scan.hasNext())
                    add(scan.next(), scan.next(), Integer.parseInt(scan.next()),
                            scan.next());
                scan.close();
                System.out.println(map);
                System.out.println();
                System.out.println(map.get(clubName));
            }

            public void add(String last, String first, Integer id, String club) {
                HashSet<Student> set = new HashSet<Student>();
                if (!map.containsKey(club)) {
                    set.add(new Student(last, first, id));
                    map.put(club, set);
                } else {
                    set = map.get(club);
                    set.add(new Student(last, first, id));
                }
            }

            public static void main(String[] args) {
                new clubMapping("Math");
            }
        }

    package HashMap;

    public class Student {
        String last, first;
        Integer id;

        public Student(String l, String f, Integer i) {
            last = l;
            first = f;
            id = i;
        }

        public String toString() {
            return last + " " + first + " " + id;
        }
    }

解决方案
You should use a TreeSet instead of a HashSet for the student set.  A tree set is implicitly sorted, a hash set is not.
public void add(String last, String first, Integer id, String club) {
    TreeSet<Student> set = new TreeSet<Student>(studentComp);
    if (!map.containsKey(club)) {
        set.add(new Student(last, first, id));
        map.put(club, set);
    } else {
        set = map.get(club);
        set.add(new Student(last, first, id));
    }
}
But you must define a comparator to use for the sorting, as shown below.
private static Comparator<Student> studentComp = new Comparator<Student>() {
    @Override
    public int compare(Student s1, Student s2) {
        return (s1.last.compareTo(s2.last));
    }
};
Also, you should probably encapsulate your student attributes in getter/setter methods, rather than accessing them directly as I've done above (as your class currently allows).

                        
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