Shekhar Ray

Comparable interface is used when sorting order can be added inside the same class and the objects can be sorted according to natural order of fields.

Comparable interface is used when sorting information isn’t available inside the object itself or a custom sorting logic needs to be implemented.

Although you don't need to, but a class can implement both Comparable and Comparator interface. In such scenario, the Comparator takes precedence over Comparable.

Reference:
TopJavaTutorial