Java Higher Order Functions
Jakob Jenkov |
A higher order function is a function that either takes a function (method) as parameter, or returns a function after its execution. In this higher order tutorial I will show a few examples of higher order functions in Java.
Sorting Collections
The first example of a higher order function is the Collections.sort() method which takes a
Comparator as parameter. Here is an example:
List<String> list = new ArrayList<>();
list.add("One");
list.add("Abc");
list.add("BCD");
Collections.sort(list, (String a, String b) -> {
return a.compareTo(b);
});
System.out.println(list);
The Collection.sort() takes two parameters. The first parameter is a List and
second parameter is a lambda (function). The lambda parameter is what makes Collections.sort()
a higher order function.
Sorting in Reverse Order
Here is another example of a higher order function. This time it is a function that returns another function as result. Here is the Java higher order function example:
Comparator<String> comparator = (String a, String b) -> {
return a.compareTo(b);
};
Comparator<String> comparatorReversed = comparator.reversed();
Collections.sort(list, comparatorReversed);
System.out.println(list);
This example first creates a Java lambda expression that implements the Comparator interface.
Second, the example calls the reversed() method on the Comparator lambda. The
reversed() method returns a new Comparator lambda, which reverse the result
returned by the first Comparator implementation. By "reversing" I mean that it simply
returns -1 * comparator.compare(a,b)
Because the reversed() method returns a lambda (function), the reversed() method
is considered a higher order function.
Third, the example sorts the List of Strings using the Collections.sort() method.
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Jakob Jenkov | |
