Sorting a map in java

Question

I have a tree map with a following key value pair combination

Map is declared as follows

Map<String, List<Bean>> outputMap = new TreeMap<String,List<Bean>>();

Corresponding code will be

for (String course_date : shiftSet) {
        Bean courseBean = null; 
        boolean  valueExists = false;

                for (Entry<String, List<Bean>> entry: courseMap.entrySet()){
                    valueExists = false;
                    String studentDetail = entry.getKey();
                    String [] studentSet =  StringUtils.getArray(studentDetail , ",");
                    String studentId = studentSet[0];
                    String courseId = studentSet[1];

                    for(Bean resultBean : entry.getValue()){

                        if(course_date.equalsIgnoreCase(resultBean.getCourseDate()){
                            valueExists = true;
                        }
                }
                        if(!valueExists ) {
                            courseBean = new Bean();
                            courseBean.setStudent(studentId);
                            courseBean.setCourse(courseId);
                            List<Bean> courseList = entry.getValue();
                            courseList.add(courseBean);
                            outputMap.put(studentId+courseId, courseList);
                         }
            }
        }

And finally the OutputMap needs to be inserted to courseMap

OutputMap result will be

{ADV001,STU02=   
  [Bean[course_id=ADV1, student_id=STU1_2,Day=Wed-Night,courseDate=12-Feb-2014],
   Bean[course_id=ADV1, student_id=STU1_2,Day=Tue-Day,courseDate=11-Feb-2014], 
   Bean[course_id=ADV1, student_id=STU1_2,Day=Tue-Night,courseDate=11-Feb-2014], 
   Bean[course_id=ADV1, student_id=STU1_2,Day=Wed-Day,courseDate=12-Feb-2014]] 

So Here i need to sort the outputMap based on courseDate?Kindly help on how the desired output can be achieved ?

Thanks in advance..

Answer 1

A TreeMap sorts the elements by their key, if courseDate is part of the values, you'd have to use a TreeSet, e.g.

import java.util.Set;
import java.util.TreeSet;

public class Example {

    public static void main(String[] args) {
        Set<Dummy> example = new TreeSet<>();

        example.add( new Dummy(7, "first dummy"));
        example.add( new Dummy(2, "second dummy"));
        example.add( new Dummy(3, "third dummy"));

        System.out.println(example);
    }

    static class Dummy implements Comparable<Dummy>{
        int courseDate;
        String name;

        Dummy(int courseDate, String name) {
            this.courseDate = courseDate;
            this.name = name;
        }

        @Override
        public String toString() {
            return String.format("[Dummy courseDate=%s, name=%s]", courseDate, name);
        }

        @Override
        public int compareTo(Dummy o) {
            // here you specify how the TreeSet will order your elements
            return Integer.compare(this.courseDate, o.courseDate);
        }
    }
}