Functional programming has started (re)gaining prominence in recent years, and with good reason too. Functional programs lend an elegant solution to the concurrency problem, result in more modular systems, are more concise and are easier to test. While modern languages like Scala and Clojure have embraced the functional style whole-heartedly, Java has lagged a bit behind in its treatment of functions as first-class citizens. With the advent of Java 8 and its support for lambdas, however, Java programmers can finally start reaping the power of functional programs as well. Even without Java 8, it is possible to adopt a functional style with the aid of excellent libraries such as Guava.

1. Functional
Programming in Java
Prem Chandrasekaran & Jorge Lee
ThoughtWorks Inc.
Jan 29th 2014

2. What is Functional
Programming?

3. An Inventory
Management Example

4. Product

5. Item

6. Warehouse

7. Task 1 - Print names of
all items in inventory

8. In Java 6…

9. Task 2 - Print names of all
in-stock items of a
category

10. Implementation

11. Now We Have…

12. Issues?
•
Too much duplication
•
Not sustainable
•
Too brittle

13. Issues?
We need to externalize the search criteria

14. Refactor #1

15. Refactor #1

16. Refactor #1

17. Refactor #1

18. Refactor #2

19. Recap…

20. Now We Have…

21. Task 3 - Search for
Products by name

22. Create Interface?

23. Now We Have…

24. Can we Refactor?

25. And Now…

26. Still Too Verbose?

27. IDE Magic!

28. That’s It! For Java 6

29. Enter Java 8

30. We Have…

31. Refactor #3 - Lambdas

32. Refactor #4 - No Types Necessary

33. Refactor #5 - Method References

34. Java 8 Gyaan
•
Functional Interface
•
Interface with a single method
without an implementation
•
Can be enforced using the
@FunctionalInterface
annotation
•
There are several standard
functional interfaces defined in
the java.lang.function
package

35. So…

36. Becomes…

37. Standard Functional
Interfaces

38. Task 4 - Filter in-stock
items and print the Item not just its name

39. We Have…

40. Becomes…

41. We Can Also…

42. Task 5 - Print the number
of items in inventory

43. In Java 6…

44. What are we doing?
1. Providing an initial/default
value
2. Accumulating a result
3. Combining the result

45. In Functional Terms…

46. Alternatively…

47. Alternatively…

48. Conveniently

49. Task 6 - Print number of
items in all our
warehouses

50. In Java 6…

51. In Functional Terms..

52. Also We Can…

53. Task 7 - Print the item
with the least amount
across warehouses

54. In Java 6

55. In Functional Terms…

56. Conveniently

57. Even more Conveniently

58. Even More Conveniently

59. Why Functional?
•
A newer different way of thinking
•
More modular
•
Side-effect free programs
•
Immutable programs
•
•
Easier to write concurrent code
Easier to test

60. What to do in Java 6?
•
Look in the Guava or Apache Commons Lang3 library
•
But…
“As of Java 7, functional programming in Java can only be approximated
through awkward and verbose use of anonymous classes. This is expected to
change in Java 8, but Guava is currently aimed at users of Java 5 and above.!
Excessive use of Guava's functional programming idioms can lead to verbose,
confusing, unreadable, and inefficient code. These are by far the most easily
(and most commonly) abused parts of Guava, and when you go to
preposterous lengths to make your code "a one-liner," the Guava team weeps.”
— Guava Documentation

61. What to do in Java 6
•
First — Stop Whining
•
Prefer final variables
•
Minimize the use of void methods
•
Avoid use of mutable globals (statics)
•
Avoid returning nulls
•
Read Effective Java - And apply learnings!

62. Thank You!