java.lang.ClassCastException: java.util.HashMap cannot be cast to 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