Order HashMap in alphabetical order by value

Question

I have HashMap<Object, Student>where Object is the student identifier, and Student is the object from Student.

How can I resort to HashMap by the name of students student->getName()?

Answer 1

HashMaps are internally disordered and cannot be sorted.

Instead, you can use SortedMap , such as TreeMap .
However, even a sorted card can only be sorted by its keys.

If you want to sort by values, you need to copy them to a sorted list.

Answer 2

, HashMap, , , -, . HashMap < String, Integer > Integer, , Javarevisited. HashMap < String, String > :

/*
 * Java method to sort Map in Java by value e.g. HashMap or Hashtable
 * throw NullPointerException if Map contains null values
 * It also sort values even if they are duplicates
 */
public static <K extends Comparable,V extends Comparable> Map<K,V> sortByValues(Map<K,V> map){
    List<Map.Entry<K,V>> entries = new LinkedList<Map.Entry<K,V>>(map.entrySet());

    Collections.sort(entries, new Comparator<Map.Entry<K,V>>() {

        @Override
        public int compare(Entry<K, V> o1, Entry<K, V> o2) {
            return o1.getValue().compareTo(o2.getValue());
            // to compare alphabetically case insensitive return this instead
            // o1.getValue().toString().compareToIgnoreCase(o2.getValue().toString()); 
        }
    });

    //LinkedHashMap will keep the keys in the order they are inserted
    //which is currently sorted on natural ordering
    Map<K,V> sortedMap = new LinkedHashMap<K,V>();

    for(Map.Entry<K,V> entry: entries){
        sortedMap.put(entry.getKey(), entry.getValue());
    }

    return sortedMap;
}

, :

Map<String, Integer> sorted = sortByValues(myOriginalHashMapObject);

: http://javarevisited.blogspot.com/2012/12/how-to-sort-hashmap-java-by-key-and-value.html#ixzz2akXStsGj

Answer 3

. , :

Collection<Student> students = map.values();

Collection.sort(new ArrayList<Student>(students)), new Comparator<Student>() {
    public int compare(Student s1, Student s2) {
        return s1.getName().compareTo(s2.getName());
    }
});

, , . ( , ?)

.

Answer 4

HashMaps . , . , SortedSet , .

class StudentComparator implements Comparator<Student> {
    int compare(Student s1, Student s2) {
       return s1.getName().compareTo(s2.getName());
    }
}

, , .

Answer 5

, .

ArrayList , , ArrayList, . :

Map<Object, Student> valueMap = new LinkedHashMap<String, String>();
List<Student> pairValueList = new ArrayList<PairValue>();

PairValue p;
for (Map.Entry<Object, Student> entry : map.entrySet()) {
  Object key = entry.getKey();
  Student value = entry.getValue();        
  p = new PairValue(key, value);
  pairValueList.add(p);
 }

Collections.sort(pairValueList, new Comparator<PairValue>() {
  @Override
  public int compare(PairValue c1, PairValue c2) {
    return c1.getLabel().compareTo(c2.getLabel());
  }
});

for (PairValue pv : pairValueList) {
  valueMap.put(pv.getValue(), pv.getStudent());
}

PairValue

    class PairValue {    

  private Object value;    
  private Student student;

  public PairValue(Object value, String student) {
    this.value = value;
    this.student= student;
  }

  public String getValue() {
    return value;
  }

  public String getStudent() {
    return student;
  }    
}

The way I solved some similar problem that I had in the past. Note that the returned map implementation must be LinkedHashMap.