Sort HashMap basis on Object's member variable from value

Question

Have one class

class Employee {
    int id;
    String name;
}

and one map which contains this object in value

Map<Integer, Employee> map = new HashMap<Integer, Employee>();

Now I want to sort above map one the basis of Employee's name. Means when I iterate this map using Map.Entry, Employee objects must retrive alphabetically.

Thanks in advance

Solution 1

Michaël suggested one link which is Sort a Map<Key, Value> by values (Java) . I did some change in it. and it works for me

class ValueComparator implements Comparator<Integer> {

    Map<Integer, Employee> base;
    public ValueComparator(Map<Integer, Employee> base) {
        this.base = base;
    }

    // Note: this comparator imposes orderings that are inconsistent with equals.    
    public int compare(Integer a, Integer b) {
        return ((Employee)base.get(a)).compareTo(base.get(b));
    }

}

class Employee implements Comparable {
    public String name;
    public int id;

    Employee(int id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public int compareTo(Object obj) {
        return this.name.compareTo(((Employee)obj).name);
    }

    public String toString() {
        return name;
    }
}

For solution please refere above mension link too.

Thanks for all who have reply.

Solution 2

You can't sort a HashMap, but you can sort its entries obtained with entrySet().

public class MapSort {
    private static class Employee {
        public String name;

        public Employee(String name) {
            this.name = name;
        }

        @Override
        public String toString() {
           return name;
        }
    }

    public static void main(String[] args) {
        Map<Integer, Employee> map = new HashMap<Integer, Employee>();

        map.put(1, new MapSort.Employee("x"));
        map.put(2, new MapSort.Employee("a"));
        map.put(3, new MapSort.Employee("f"));

        List<Map.Entry<Integer, Employee>> entryList = new ArrayList<Map.Entry<Integer, Employee>>(map.entrySet());

            Collections.sort(
                    entryList, new Comparator<Map.Entry<Integer, Employee>>() {
                @Override
                public int compare(Map.Entry<Integer, Employee> integerEmployeeEntry,
                                   Map.Entry<Integer, Employee> integerEmployeeEntry2) {
                    return integerEmployeeEntry.getValue().name
                            .compareTo(integerEmployeeEntry2.getValue().name);
                }
            }
        );

        System.out.println(entryList);
    }
}

After sorting you can put back your entries in a map that supports ordering, for example LinkedHashMap.

It depends on your use case: if you need to keep the map always sorted it's simpler to employ a TreeMap that comes with an additional overhead. If you need just an one time sorting, you can use a HashMap with the above code.

Solution 3

Use a TreeMap with a custom Comparator using this constructor:

http://docs.oracle.com/javase/6/docs/api/java/util/TreeMap.html#TreeMap(java.util.Comparator)