What does self comparable interface do in Collections Class?

Question

While practicing Reflection I came to know about SelfComparable Interface in Collections class

interface java.util.Collections$SelfComparable

What does this interface use for ?

Answer 1

It doesn't do anything. It is private so you can't import it.

It is really a comment that the type is "SelfComparable" and is not actually used.

Nothing implement this interface. The code which uses it relies on the fact it will be discarded at runtime.

public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) {
    if (comp==null)
        return (T)max((Collection<SelfComparable>) (Collection) coll);

could have been

public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) {
    if (comp==null)
        return (T)max(/*SelfComparable*/ (Collection) coll);

as it will be ignored at runtime.

Answer 2

From source:

private interface SelfComparable extends Comparable<SelfComparable> {}

This is nothing more than a marker over Comparable<SelfComparable>, which basically means that it is a marker for comparables that compare to self. Its use is somewhat superfluous.

It's used as:

return (T)min((Collection<SelfComparable>) (Collection) coll);

on line 662 where it basically casts a collection to Collection, and then performs a cast for the generic parameter to be a SelfComparable which just extends Comparable.