使用Comparable接口的匿名内部类

Question

When I am trying to create anonymous inner using comparable interface i am getting compilation error.

//Code trying to create treeset using comparable
// compilation error
 TreeSet<String> treeSet5 = new TreeSet<String>(new Comparable<String>() {
        @Override
        public int compareTo(String o) {
            // TODO Auto-generated method stub
            return 0;
        }
    });
  // CE:The constructor TreeSet<String>(new Comparable<String>(){}) is undefined

I know for custom sorting we need to use comparator, but i am curious why we cannot create comparable anonymous class.

//Custom sorting: default sorting as String implements comparable
// below code is fine as its working as expected.
    TreeSet<String> treeSet2 = new TreeSet<String>(new Comparator<String>() {

        @Override
        public int compare(String o1, String o2) {
            return o2.compareTo(o1);

        }
    });

Answer 1

Comparable is used to create class that can be compared with himself.

Comparator is to be used with classes that do not implement Comparable (for whatever reason, coming from 3rd party for example), or you want to compare them differently from the original compareTo() method without inheriting them.

Hope that makes sense to you.

Here is come elaborate example: https://www.geeksforgeeks.org/comparable-vs-comparator-in-java/

Answer 2

1. Sorting can be done by comparing the two entity.
2. Sorting Util class like TreeSet requires Comparator since it has two parameters it can be compared with.
3. Comparable doesn't have enough information how to compare it other element.

---

Q : Now you will ask how it works when Sorting Item implements Comparable ?

A : In this case Sorting Item is the first Item to compare with and other element is passed to its method compareTo(String o)

Answer 3

Actually i realized below once i saw the TreeSet api.

the above error is because the TreeSet does not accept Comparable interface as constructor parameter, it accepts only accepts Comparator interface with implementation.

Treeset treeSet=new TreeSet();
//Creates empty tree set. All objects are inserted according to natural sorting order.
Treeset treeSet=new TreeSet(Comparator c);
//Creates empty treeSet object, objects are maintained according to defined Comparator.
TreeSet treeSet = new TreeSet(Collection c)