How do you design functional interfaces that support polymorphism and extensibility?

3 How can you design your own functional interfaces?

Sometimes, you may need to design your own functional interfaces to suit your specific needs. For example, suppose you want to write a function that takes two integers and returns an integer. You can create a functional interface called IntBinaryOperator that has a single abstract method called applyAsInt that takes two int parameters and returns an int result. You can then use this interface to create lambda expressions, method references, or anonymous classes that implement the function. For example, you can create an IntBinaryOperator that adds two integers, another one that multiplies them, and another one that calculates their greatest common divisor. You can then use these functions in your code as arguments, results, or variables.

4 How can you make your functional interfaces extensible?

One way to make your functional interfaces extensible is to use default methods. Default methods are methods that have a default implementation in an interface, and can be overridden by the implementing classes. For example, suppose you have a functional interface called Comparator that compares two values and returns an int. You can add a default method called reversed that returns a new Comparator that reverses the order of comparison. This way, you can extend the functionality of the Comparator interface without changing its contract or breaking its existing implementations. You can then use the reversed method to create new Comparators that sort values in descending order.

5 How can you use functional interfaces with referential transparency?

Referential transparency is a property of functional programming that means that a function always returns the same output for the same input, and does not have any side effects. This makes the function predictable, testable, and easy to reason about. Functional interfaces can help you achieve referential transparency by enforcing the contract of the function and avoiding any stateful or mutable operations. For example, suppose you have a functional interface called Transformer that transforms a value and returns a new value. You can ensure that the Transformer is referentially transparent by making sure that it does not modify the input value, does not depend on any external state, and does not perform any IO or other side effects. You can then use the Transformer in your code with confidence that it will behave consistently and purely.

6 How can you benefit from functional interfaces in your functional programs?

Functional interfaces can help you write functional programs that are concise, elegant, and testable. By using functional interfaces, you can leverage the power of lambda expressions, method references, and anonymous classes to create and manipulate functions easily. You can also use functional interfaces to write generic and reusable code that can work with different types of values and functions. You can also use functional interfaces to extend and modify the functionality of your functions without breaking the code or violating the contract. Finally, you can use functional interfaces to ensure referential transparency and avoid side effects in your functions. These benefits can help you write functional programs that are modular, expressive, and reliable.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?