Java Comparable interface is used to order the objects of user-defined class.This interface is found in java.lang package and contains only one method named compareTo(Object). It provide single sorting sequence only i.e. you can sort the elements on based on single data member only. For example it may be rollno, name, age or anything else.
public int compareTo(Object obj): is used to compare the current object with the specified object.
Collections class provides static methods for sorting the elements of collections. If collection elements are of Set or Map, we can use TreeSet or TreeMap. But We cannot sort the elements of List. Collections class provides methods for sorting the elements of List type elements.