java.lang.ClassCastException：java.util.HashMap无法强制转换为java.lang.Comparable

Question

I was trying to sort my ArrayList.

java.util.ArrayList arList= new java.util.ArrayList();  
   arList=getList();    
   java.util.Collections.sort(arList);

where my getList() function here

public ArrayList getList() throws Exception
{
ArrayList listItems = new ArrayList();
//Query executing here..............!
            while (rs.next()) 
                {
                HashMap hashList = new HashMap();
                hashList.put("name",rs.getString(1));
                hashList.put("id",rs.getBigDecimal(2));
                listItems.add(hashList);
                }
          return listItems;
}

But I am facing error : java.lang.ClassCastException: java.util.HashMap cannot be cast to java.lang.Comparable

Answer 1

First Thing -

java.util.ArrayList arList= new java.util.ArrayList();  
arList=getList();    

You create the first instance unnecessary.

Second Thing -

ArrayList listItems = new ArrayList();

This list contains list HashMap And HashMap does not implements Comparable.

java.util.Collections.sort(arList);

public static <T extends Comparable<? super T>> void sort(List<T> list)

List contain( T ) must extends Comparable.

that is why it populate java.lang.ClassCastException: java.util.HashMap cannot be cast to java.lang.Comparable

Answer 2

In order to use Collections.sort() , Lists (and arrays) of objects that implement this interface Comparable . http://docs.oracle.com/javase/6/docs/api/java/lang/Comparable.html

Answer 3

First of all, your code seems weird, you are adding a newly created HashMap object into your list in each iteration of the loop. Anyway, I'll not discuss it. Maybe you should specify your ArrayList like this: ArrayList<HashMap> But that is not enough. If you have a list of a specific type and wanna sort them, then your type (in this case HashMap) should implement Comparable interface. HashMap doesn't implement Comparable interface. Read the API: http://docs.oracle.com/javase/6/docs/api/java/util/HashMap.html

You should implement your own Comparator as a solution, I guess.

Answer 4

Check this link

All elements in the list must implement the Comparable interface. Furthermore, all elements in the list must be mutually comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the list).

This should say it all.

Answer 5

Of course the compiler error is trying to convey you in the best way it can that it cannot compare HashMaps , that you have inside your list..

First you should declare your list as generic list like this: -

List<HashMap<String, String>> hashList = new ArrayList<>();

If you want to sort List<HashMap> , you need to have a Comparator to sort.. You cannot sort it using natural ordering.. Because your HashMap does not implement Comparable interface..

You can only compare two instances if they are comparable..

*EDIT: - Well, I think you will need two Comparators here.. One for your List and one for your HashMap inside the List .. But I'm not pretty sure about it whether it would work..

Answer 6

Collections.sort(List<T>) requires an object T of type Comparable . That means any object that implements Comparable interface will work. Refer [this]( http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#sort%28java.util.List%29 ):

 public static <T extends Comparable<? super T>> void sort(List<T> list)

In your case you are trying to sort an ArrayList that contains a HashMap , which does not implements Comparable interface. So. the code fails.

Answer 7

HashMap does not implement the Comparable interface (http://docs.oracle.com/javase/6/docs/api/). As far as I know the Collections.sort()-Method can only sort natural values.But you can write your own comparator and sort your list as you wish. The Collections.sort()-Method is overloaded therefore

Answer 8

For something to be sorted, it must implement the Comparable interface, which is something which the HashMap does not.

As a result, when you try to sort your list of HashMaps your code fails.

If you really need to be able to compare HashMaps, you could create your own HashMap, something like so:

public class MyHashMap<K,V> extends HashMap<K, V> implements Comparable<HashMap<K,V>>
{
    ...
    public int compareTo(HashMap<K,V> comparer)
    {
        ...
    }
}

Answer 9

Yes, Because java.util.Collections.sort(arList); arList contains the HashMap hashList = new HashMap(); unfortunately hashMap did not implement the Comparable or Comparator interface. and Collections.sort() method will sort on the implementation of

 Comparable or Comparator interface.

Answer 10

Guess the error happened on line Collections.sort().

If you want to use sort, the objects in the list must be Comparable. Otherwise how java is gonna sort it?

HashMap is not Comparable. You can extend HashMap and implement Comparable interface.

Answer 11

This way you can achieve a Sorted Map

public class MapUsingSort {
public static void main(String[] args) {

Map<Integer, String> abc = new HashMap<>();
abc.put(3, "a");
abc.put(6, "b");
abc.put(1, "c");
abc.put(4, "h");
abc.put(10, "k");
abc.put(9, "x");

 // Map is stored in ArrayList
List<Entry<Integer,String>> sortedEntries = new ArrayList<Entry<Integer,String>>(abc.entrySet());


Collections.sort(sortedEntries, new Comparator<Entry<Integer,String>>() {
    @Override
    public int compare(Entry<Integer, String> a, Entry<Integer, String> b) 
    {
        //Sorting is done here make changes as per your need 
        // swap a and b for descending order 

        return a.getKey().compareTo(b.getKey());   
    }
   });

for (Object object : sortedEntries) {

    //print your data in your own way
    System.out.println((Map.Entry)object);
   }
  }
}

For further in detail visit HashMap

*

This is my first answer on stackOverflow hope it helps!