What does “Mutually Comparable” mean?

Question

I read that :

whenever a collection need to be sorted, the elements must be mutually comparable.

I wrote the below code and it

Answer 1

Mutually comparable means that instances of both classes must be comparable to the other.

In case of type A and type B:

• A must implement Comparable where T extends B (or is B)
• B must implement Comparable where U extends A (or is A)
• The comparison must be consistent. Consistent means that if a>b then it must be so b. And if a then it must be so b>a. And if a==b then it must be so b==a.

So that the following code can be written:

A a = ...;
B b = ...;

int rab = a.compareTo(b);  // A is comparable to B
int rba = b.compareTo(a);  // B is comparable to A

// And also consistent:
if (   !(rab<0 && rba>0)
    || !(rab>0 && rba<0)
    || !(rab==0 && rba!=0))
    System.out.println("Not consistent!");

If type A and type B are the same, then a and b are obviously mutually comparable.

Sorting algorithms usually require the elements to be mutually comparable so they can compare any 2 of the sortable elements to each other.

Back to your question

Your case is special. Your 2 objects obj1 and obj2 are mutually comparable according to the definition. Your obj1 and obj2 instances are mutually comparable, but your b and c classes are not.

However, if you would pass another instance of either type a or type b, all elements would be not. Because if we would select 2 instances from the collection of the same type, they would not implement Comparable to the same type.

obj1.compareTo(obj2); // This is valid, b implements Comparable
obj2.compareTo(obj1); // This is also valid, c implements Comparable

obj1.compareTo(obj1); // This is NOT valid! b does not implement Comparable