What is a comparator? What interface must it implement?

The Comparator interface in Java plays a significant role when it comes to sorting collections of custom objects. It resides within the java.util package and serves as a functional interface defining a single method, compare(T o1, T o2). Unlike Comparable, which requires modifying the class of the objects to be compared, Comparator allows for creating external, separate logic for sorting, which can be highly beneficial for predefined classes or when multiple sorting orders are required.

Implementing the Comparator interface is straightforward. Declare a class that implements Comparator and provide a concrete implementation of the compare method, which determines the order of the objects. This flexibility allows for sorting objects based on various attributes, not just their natural order. For instance, if you have a class Person with attributes like name and age, you can create multiple Comparator implementations to sort by either attribute, without changing the Person class itself.

When dealing with objects in Java, there might be instances where the default sorting order does not meet the specific requirements of an application. Custom sorting becomes necessary when objects need to be sorted based on non-default or multiple criteria. Unlike primitives, objects can contain numerous fields, and a custom sorting order can be chosen to sort these objects based on any combination of these fields.

To achieve custom sorting, Java developers utilize the Comparator interface. By creating a class that implements this interface, developers can define their own logic within the compare method. Consequently, collections of these objects, like ArrayList or TreeSet, can be sorted by invoking Collections.sort() or TreeSet's constructor respectively, passing the custom Comparator as an argument. For example, you could sort a list of books by their title or even their publication year by defining an appropriate Comparator for each attribute.

The crux of using a Comparator lies within the implementation of the compare method. This method is where the logic for the ordering of the objects is defined. It accepts two objects of the same type and returns an integer value that determines their order. A return value of less than zero indicates the first object should go before the second, greater than zero means it should go after, and zero signifies that they are equal in terms of order.

The compare method provides the flexibility to sort objects in any desired order – ascending, descending, or even based on multiple criteria. Consider a collection of employee objects, where each employee has a department and a salary. By implementing different Comparators, you could sort the collection by salary, department, or even a combination where department is the primary sort key and salary is the secondary.

Best Practices

Write clear and concise compare methods, and remember that consistency with equals is important for certain collections. Also, handling of nulls should be considered to avoid NullPointerException. While using Lambda expressions can make the Comparator even more succinct, remember the primary goal is clarity and proper function.