How To Sort Object based on Object's attribute?

问题

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;
        }
    }

回答1:

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).

来源：https://stackoverflow.com/questions/13773662/how-to-sort-object-based-on-objects-attribute

标签

java

hashmap