How to convert HashMap<String, ArrayList<MyClass>> to ArrayList<MyClass> in java?

Question

I have googled but did not come accross converting HashMap<String, ArrayList<Class>> to ArrayList<Class>. Could someone help me please?

Basically, i want to change my method getStudentLst below from calling constructor and convert hash map to arrayList as below. I have tried many times but it keeps generating error.Where i am going wrong?

  ArrayList<Student> arrStudLst = new ArrayList<Student>(hmpStudent.keySet());
  Collectoins.sort(arrStudLst, new Comparator());
  return arrStudLst;

but it did not work and generate error "The constructor ArrayList(Set) is undefined

any help much appreciated! adan

/* works well but i want to avoid calling constructor */
public ArrayList<Student> getStudentLst(HashMap<String, ArrayList<Student>> hmpStudent)
{
     ArrayList<Student> arrStudLst = new ArrayList<Student>();   
     Set<String> keylst = hmpStudent.keySet();
     Student student;
     for(String keys: keylst)
     {         
        for(Student f: hmpStudent.get(keys))
        {       
         student = new Student(f.getName(),f.getGrade(), f.getTeacher());                arrStudLst.add(student);
        }
     }
     Collections.sort(arrStudLst,new StudentComparator());
     return arrStudLst;
}

What do you want to get to array list , keys or values ?

someone
– someone

2012-12-24 13:28:37 +00:00
Commented Dec 24, 2012 at 13:28 — someone
– someone, Commented Dec 24, 2012 at 13:28

JB Nizet · Accepted Answer · 2012-12-24 13:30:04Z

1

You tried to initialize the list with the keys of your map. But the keys are Strings, they're not Students.

You need to return a list containing every student in every value of the map. So the code would be:

Set<Student> allStudents = new HashSet<Student>(); // use a set to avoid duplicate students
for (List<Student> list : map.values()) {
    allStudents.addAll(list);
}
List<Student> allStudentsAsList = new ArrayList<Student>(allStudents);

answered Dec 24, 2012 at 13:30

JB Nizet

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1 Comment

user1926649 Over a year ago

I have tried all suggestions and all worked well. Thank you so much.

Evgeniy Dorofeev · Accepted Answer · 2012-12-24 13:30:15Z

0

I can try

ArrayList<MyClass> list = new ArrayList<>();
for (ArrayList<MyClass> l : map.values()) {
    list.addAll(l);
}

answered Dec 24, 2012 at 13:30

Evgeniy Dorofeev

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Comments

Rahul · Accepted Answer · 2012-12-24 13:43:11Z

0

The key is a String, So hmpStudent.keySet() will return a List<String> and not a List<Student>

 new ArrayList<Student>(hmpStudent.keySet());

Hence the above statement will fail. You can use the below mentioned method

public List<Student> getAllStudents(Map<String, ArrayList<Student> map){
   List<Student> allStudents = new ArrayList<Student>();
   for (List<Student> list : map.values()) {
       allStudents.addAll(list);
   }
   return allStudents;
}

answered Dec 24, 2012 at 13:43

Rahul

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Comments

Parvez · Accepted Answer · 2012-12-24 18:09:18Z

0

If your ArrayList in map is big but fewer elements in map then this is what you can try.

public ArrayList<Student> getStudentLst(HashMap<String, ArrayList<Student>> hmpStudent)
{
   List<Student> arrStudLst = new ArrayList<Student>();
   for(ArrayList<Student> studentLst : hmpStudent.values()) {
       arrStudLst.addAll(studentLst);
   }
     Collections.sort(arrStudLst,new StudentComparator());
     return arrStudLst;
}

edited Dec 24, 2012 at 18:09

answered Dec 24, 2012 at 17:32

Parvez

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2 Comments

JB Nizet Over a year ago

Why create this intermediate list of lists, instead of iterating directly through the collection of lists returned by map.values()?

Parvez Over a year ago

@JB Nizet: My mistake. I had something else in my mind but realized now that it is effectively doing the same thing that others have posted. I am not sure if deleting my answer is a right choice. Updated my answer now.

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