Interfaces - Beginning Java 8 Fundamentals - page 690

Using a Comparator is easy. Create its object and pass it to the methods that take a collection of objects and
a comparator to compare them. For example, to sort an array of ComparablePerson objects, pass the array and a
FirstNameComparator to the static sort() method of the Arrays class.

ComparablePerson[] persons = create and populate the array...

// Sort the persons array based on first name
Comparator fnComparator = new FirstNameComparator();
Arrays.sort(persons, fnComparator);

You can use a similar logic to sort the array based on the last name.

// Sort the persons array based on last name
Comparator lnComparator = new LastNameComparator();
Arrays.sort(persons, lnComparator);

Prior to Java 8, if you wanted to the array based on the first name followed by the last name, you needed to create
another implementation of the Comparator interface. Thanks to Java 8's adding of default methods to interfaces,
you do not need to create a new implementation of the Comparator interface. The Comparator class contains a
thenComparing() default method declared as follows:

default Comparator<T> thenComparing(Comparator<? super T> other)

The thenComparing() method takes a Comparator as an argument and returns a new Comparator . The new
Comparator is used for ordering if the two objects being compared are equal using the original Comparator . The
following snippet of code combines the first name and last Comparators to create a new Comparator :

// Sort using first name, then last name
Comparator firstLastComparator = fnComparator.thenComparing(lnComparator);
Arrays.sort(persons, firstLastComparator);

■ You can chain the call to the thenComparing() method to create a Comparator that imposes the ordering on
several nested levels.

Tip

There is another useful addition to the Comparator interface in Java 8: a default method named reversed() .
The method returns a new Comparator that imposes the reverse ordering of the original Comparator . If you want to
sort the array of persons based on first name, then last name in descending order, you can do so as follows:

// Sort using first name, then last name in reversed order
Comparator firstLastReverseComparator = firstLastComparator.reversed();
Arrays.sort(persons, firstLastReverseComparator);

Comparators do not handle null values well. Typically, they throw a NullPointerException . Java 8 added two
useful, null-friendly, convenience static methods to the Comparator interface.

static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator)

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static <T> Comparator<T> nullsLast(Comparator<? super T> comparator) •