Java has two ways to define an order. Comparable<T> gives a class its natural ordering from inside the class, through one method, int compareTo(T other). Comparator<T> defines an ordering from outside the class, through int compare(T a, T b), so you can have many orders for one type. Both return a negative number if the first argument comes first, zero if they are equal in order, and a positive number if the first comes after.
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