Sorting a List of Maps dynamically in Ascending and Descending order

I am having Maps inside a List.

I need to sort the List based on the input dynamically by passing it as a parameter into the method sortData.

It is working as expected, but the problem is that am not able to sort the list in reverse order.

I’m getting an Error : The method get(String) is undefined for the type Object

Code

public class TestingClass {
    
    public static void main(String[] args) {
        
        List< LinkedHashMap<String,Object> > list = new ArrayList<>(); 
        
        /*Map 2*/
        LinkedHashMap<String,Object> lhm1 = new LinkedHashMap<>();
        lhm1.put("LONG_HEADER", 2l);
        lhm1.put("STRING_HEADER1", "C");
        lhm1.put("STRING_HEADER2", "D");

        /*Map 2*/

        LinkedHashMap<String,Object> lhm2 = new LinkedHashMap<>();
        lhm2.put("LONG_HEADER", 1l);
        lhm2.put("STRING_HEADER1", "E");
        lhm2.put("STRING_HEADER2", "F");
        
        /*Map 3*/       
        LinkedHashMap<String,Object> lhm3 = new LinkedHashMap<>();
        lhm3.put("LONG_HEADER", 3l);
        lhm3.put("STRING_HEADER1", "A");
        lhm3.put("STRING_HEADER2", "B");
        
        list.add(lhm1);
        list.add(lhm2);
        list.add(lhm3);
        
        List< LinkedHashMap<String,Object> > sortedList1 = sortData(list,"LONG_HEADER","ASC" );
        System.out.println("Output 1 After sorting" +sortedList1);
        List< LinkedHashMap<String,Object> > sortedList2 = sortData(list,"STRING_HEADER1","ASC" );
        System.out.println("Output 2 After sorting"+sortedList2);
        List< LinkedHashMap<String,Object> > sortedList3 = sortData(list,"STRING_HEADER2","ASC" );
        System.out.println("Output 3 After sorting"+sortedList3);
        
        /* It won't work */
        sortData(list,"LONG_HEADER","DESC" );
        sortData(list,"STRING_HEADER1","DESC" );
        sortData(list,"STRING_HEADER2","DESC" );
    }

    private static List<LinkedHashMap<String, Object>> sortData(List<LinkedHashMap<String, Object>> inputGridData,
            String sortBy, String sortOrder) {
        List<LinkedHashMap<String, Object>> lOutputGridDat = null;
        if (inputGridData.get(0).get(sortBy) instanceof Long) {
            if( "ASC".equals(sortOrder)) {
                lOutputGridDat = inputGridData.stream().sorted(Comparator.comparingLong(o -> ((Long) o.get(sortBy))))
                        .collect(Collectors.toList());
            }
            else {
                /*Here am getting error  if i use .reversed() method*/
                //Error : The method get(String) is undefined for the type Object
                lOutputGridDat = inputGridData.stream().sorted(Comparator.comparingLong(o -> ((Long) o.get(sortBy))).reversed())
                        .collect(Collectors.toList());
            }

        } else {
            if( "ASC".equals(sortOrder)) {
                lOutputGridDat = inputGridData.stream().sorted(Comparator.comparing(o -> ((String) o.get(sortBy))))
                        .collect(Collectors.toList());
            }
            else {
                /*Here am getting error  if i use .reversed() method*/
                // Error : The method get(String) is undefined for the type Object
                lOutputGridDat = inputGridData.stream().sorted(Comparator.comparing(o -> ((String) o.get(sortBy))).reversed())
                        .collect(Collectors.toList());
            }
        }
        return lOutputGridDat;
    }   
}

Map 1 After sorting

[{LONG_HEADER=1, STRING_HEADER1=E, STRING_HEADER2=F}, {LONG_HEADER=2, STRING_HEADER1=C, STRING_HEADER2=D}, {LONG_HEADER=3, STRING_HEADER1=A, STRING_HEADER2=B}]

Map 2 After sorting

[{LONG_HEADER=3, STRING_HEADER1=A, STRING_HEADER2=B}, {LONG_HEADER=2, STRING_HEADER1=C, STRING_HEADER2=D}, {LONG_HEADER=1, STRING_HEADER1=E, STRING_HEADER2=F}]

Map 3 After sorting

[{LONG_HEADER=3, STRING_HEADER1=A, STRING_HEADER2=B}, {LONG_HEADER=2, STRING_HEADER1=C, STRING_HEADER2=D}, {LONG_HEADER=1, STRING_HEADER1=E, STRING_HEADER2=F}]

Answer

Firstly, it’s worth to point out at some important issues with the code you’ve provided :

• As I’ve said, it’s not a good practice to use Object as generic type. Generics were introduced to enforce the type safety, using Object as generic type is as bad as don’t use generics at all.
• Don’t store the elements of different types together in a single collection. And avoid instanceof checks and type casting.
• Don’t write your code against concrete classes like LinkedHashMap – variable type has to be Map instead.

You can find more elaborate explanation on all account mentioned above on this site if you doubt if these suggestions have a value.

With regard to your question, comparators as defined in your’r code will not compile.

That’s you can fix it:

Comparator.<Map<String, Object>, String>comparing(
                o -> ((String) o.get(sortBy))).reversed()

When you’re chaining the methods, the compiler is unable to infer the type of the prameter o inside the comparing based on the type of the elements of the stream.

Parameter o is treated not as a Map but as Object therefore you can’t invoke get() on it.

Generic type information needs to be provided explicitly: <Map<String, Object>, String>. Where the first part – type of the argument passed into comparing (i.e. element of the stream), the second is a type of value that will be used for comparison (i.e. string).

for information on the syntax of generic methods, take a look at this tutorial