The document discusses best practices for designing Java APIs. It emphasizes making APIs intuitive, consistent, flexible and evolvable. Key recommendations include writing meaningful documentation, using static factories instead of overloading methods, promoting fluent interfaces, using the weakest possible types, and supporting lambdas. The document notes API design is iterative and different perspectives should be considered to balance various characteristics and trade-offs.

1. Let's make a contract:
the art of designing a
Java API
by Mario Fusco
mario.fusco@gmail.com
@mariofusco

2. What is an API?

3. What is an API?

4. An API is what a
developer uses to
achieve some task
What is an API?

5. What is an API?
An API is a contract between
its implementors and its users

6. And why should I care?
We are all API designers
Our software doesn't work in
isolation, but becomes
useful only when it interacts
with other software written
by other developers

7. Basic Principles
●
Intuitive
●
Understandable
●
Learnable
●
Discoverable
●
Consistent
●
Self-defensive
●
Concise
●
Easy to use
●
Minimal
●
Orthogonal
●
Idiomatic
●
Flexible
●
Evolvable
●
Well documented
●
Right level of abstraction
●
Correct use of the type system
●
Limited number of entry-points
●
Respect the principle of least astonishment

8. Best Practices
&
Practical Hints

9. Write meaningful Javadocs

10. Write meaningful Javadocs

11. Write meaningful Javadocs

12. Write meaningful Javadocs

13. Convenience methods
Use overloading judiciously
and sparingly

14. Convenience methods
Use overloading judiciously
and sparingly
Primitives often cause
methods proliferation
{

15. Convenience methods
Use overloading judiciously
and sparingly
Are these all
necessary?
Primitives often cause
methods proliferation
{

16. Convenience methods
public interface StockOrder {
void sell(String symbol, double price, int quantity);
void buy(String symbol, int quantity, double price);
void buy(String symbol, int quantity, double price, double commission);
void buy(String symbol, int quantity, double minPrice, double maxPrice, double commission);
}
What’s wrong with this?

17. Convenience methods
public interface StockOrder {
void sell(String symbol, double price, int quantity);
void buy(String symbol, int quantity, double price);
void buy(String symbol, int quantity, double price, double commission);
void buy(String symbol, int quantity, double minPrice, double maxPrice, double commission);
}
Too many overloads
What’s wrong with this?

18. Convenience methods
public interface StockOrder {
void sell(String symbol, double price, int quantity);
void buy(String symbol, int quantity, double price);
void buy(String symbol, int quantity, double price, double commission);
void buy(String symbol, int quantity, double minPrice, double maxPrice, double commission);
}
Too many overloads
Inconsistent argument order
What’s wrong with this?

19. Convenience methods
Long arguments lists (especially of same type)
public interface StockOrder {
void sell(String symbol, double price, int quantity);
void buy(String symbol, int quantity, double price);
void buy(String symbol, int quantity, double price, double commission);
void buy(String symbol, int quantity, double minPrice, double maxPrice, double commission);
}
Too many overloads
Inconsistent argument order
What’s wrong with this?

20. Convenience methods
Long arguments lists (especially of same type)
public interface StockOrder {
void sell(String symbol, double price, int quantity);
void buy(String symbol, int quantity, double price);
void buy(String symbol, int quantity, double price, double commission);
void buy(String symbol, int quantity, double minPrice, double maxPrice, double commission);
}
public interface StockOrder {
void sell(String symbol, int quantity, Price price);
void buy(String symbol, int quantity, Price price);
}
Too many overloads
Inconsistent argument order
What’s wrong with this?
How to do better

21. Consider static
factories
public interface Price {
static Price price( double price ) {
if (price < 0) return Malformed.INSTANCE;
return new Fixed(price);
}
static Price price( double minPrice, double maxPrice ) {
if (minPrice > maxPrice) return Malformed.INSTANCE;
return new Range(minPrice, maxPrice);
}
class Fixed implements Price {
private final double price;
private Fixed( double price ) {
this.price = price;
}
}
class Range implements Price {
private final double minPrice;
private final double maxPrice;
private Range( double minPrice, double maxPrice ) {
this.minPrice = minPrice;
this.maxPrice = maxPrice;
}
}
enum Malformed implements Price { INSTANCE }
}
➢
nicer syntax for users
(no need of new
keyword)

22. Consider static
factories
public interface Price {
static Price price( double price ) {
if (price < 0) return Malformed.INSTANCE;
return new Fixed(price);
}
static Price price( double minPrice, double maxPrice ) {
if (minPrice > maxPrice) return Malformed.INSTANCE;
return new Range(minPrice, maxPrice);
}
class Fixed implements Price {
private final double price;
private Fixed( double price ) {
this.price = price;
}
}
class Range implements Price {
private final double minPrice;
private final double maxPrice;
private Range( double minPrice, double maxPrice ) {
this.minPrice = minPrice;
this.maxPrice = maxPrice;
}
}
enum Malformed implements Price { INSTANCE }
}
➢
nicer syntax for users
(no need of new
keyword)
➢
can return different
subclasses

23. Consider static
factories
public interface Price {
static Price price( double price ) {
if (price < 0) return Malformed.INSTANCE;
return new Fixed(price);
}
static Price price( double minPrice, double maxPrice ) {
if (minPrice > maxPrice) return Malformed.INSTANCE;
return new Range(minPrice, maxPrice);
}
class Fixed implements Price {
private final double price;
private Fixed( double price ) {
this.price = price;
}
}
class Range implements Price {
private final double minPrice;
private final double maxPrice;
private Range( double minPrice, double maxPrice ) {
this.minPrice = minPrice;
this.maxPrice = maxPrice;
}
}
enum Malformed implements Price { INSTANCE }
}
➢
nicer syntax for users
(no need of new
keyword)
➢
can return different
subclasses
➢
can check
preconditions and
edge cases returning
different
implementations
accordingly

24. Promote fluent API
public interface Price {
Price withCommission(double commission);
Price gross();
}
public interface Price {
void setCommission(double commission);
void setGross();
}

25. Promote fluent API
public interface Price {
Price withCommission(double commission);
Price gross();
}
stockOrder.buy( "IBM", 100, price(150.0).withCommission(0.7).gross() );
public interface Price {
void setCommission(double commission);
void setGross();
}
Price price = price(150.0);
price.setCommission(0.7);
price.setGross();
stockOrder.buy( "IBM", 100, price );

26. Promote fluent API
public interface Price {
Price withCommission(double commission);
Price gross();
}
stockOrder.buy( "IBM", 100, price(150.0).withCommission(0.7).gross() );
public interface Price {
void setCommission(double commission);
void setGross();
}
Price price = price(150.0);
price.setCommission(0.7);
price.setGross();
stockOrder.buy( "IBM", 100, price );
Concatenate multiple invocations
Use result directly

27. Promote fluent API
Streams are a very
nice and convenient
example of fluent API

28. Promote fluent API
Streams are a very
nice and convenient
example of fluent API

29. Promote fluent API
Name consistency???
Streams are a very
nice and convenient
example of fluent API

30. Use the weakest possible type
public String concatenate( ArrayList<String> strings ) {
StringBuilder sb = new StringBuilder();
for (String s : strings) {
sb.append( s );
}
return sb.toString();
}

31. Use the weakest possible type
public String concatenate( ArrayList<String> strings ) {
StringBuilder sb = new StringBuilder();
for (String s : strings) {
sb.append( s );
}
return sb.toString();
}
Do I care of the actual
List implementation?

32. Use the weakest possible type
public String concatenate( List<String> strings ) {
StringBuilder sb = new StringBuilder();
for (String s : strings) {
sb.append( s );
}
return sb.toString();
}

33. Use the weakest possible type
public String concatenate( List<String> strings ) {
StringBuilder sb = new StringBuilder();
for (String s : strings) {
sb.append( s );
}
return sb.toString();
}
Do I care of the
elements’ order?

34. Use the weakest possible type
public String concatenate( Collection<String> strings ) {
StringBuilder sb = new StringBuilder();
for (String s : strings) {
sb.append( s );
}
return sb.toString();
}

35. Use the weakest possible type
public String concatenate( Collection<String> strings ) {
StringBuilder sb = new StringBuilder();
for (String s : strings) {
sb.append( s );
}
return sb.toString();
}
Do I care of the
Collection’s size?

36. Use the weakest possible type
public String concatenate( Iterable<String> strings ) {
StringBuilder sb = new StringBuilder();
for (String s : strings) {
sb.append( s );
}
return sb.toString();
}

37. Using the weakest possible type...
public String concatenate( Iterable<String> strings ) {
StringBuilder sb = new StringBuilder();
for (String s : strings) {
sb.append( s );
}
return sb.toString();
}
… enlarges the applicability of your method, avoiding to restrict your client
to a particular implementation or forcing it to perform an unnecessary and
potentially expensive copy operation if the input data exists in other forms

38. Use the weakest possible type
also for returned value
public List<Address> getFamilyAddresses( Person person ) {
List<Address> addresses = new ArrayList<>();
addresses.add(person.getAddress());
for (Person sibling : person.getSiblings()) {
addresses.add(sibling.getAddress());
}
return addresses;
}

39. Use the weakest possible type
also for returned value
public List<Address> getFamilyAddresses( Person person ) {
List<Address> addresses = new ArrayList<>();
addresses.add(person.getAddress());
for (Person sibling : person.getSiblings()) {
addresses.add(sibling.getAddress());
}
return addresses;
}
Is the order of this List
meaningful for client?

40. Use the weakest possible type
also for returned value
public List<Address> getFamilyAddresses( Person person ) {
List<Address> addresses = new ArrayList<>();
addresses.add(person.getAddress());
for (Person sibling : person.getSiblings()) {
addresses.add(sibling.getAddress());
}
return addresses;
}
Is the order of this List
meaningful for client?
… and shouldn’t we maybe return only
the distinct addresses?
Yeah, that will be easy let’s do this!

41. Use the weakest possible type
also for returned value
public List<Address> getFamilyAddresses( Person person ) {
Set<Address> addresses = new HashSet<>();
addresses.add(person.getAddress());
for (Person sibling : person.getSiblings()) {
addresses.add(sibling.getAddress());
}
return addresses;
}
It should be enough to
change this List into a Set

42. Use the weakest possible type
also for returned value
public List<Address> getFamilyAddresses( Person person ) {
Set<Address> addresses = new HashSet<>();
addresses.add(person.getAddress());
for (Person sibling : person.getSiblings()) {
addresses.add(sibling.getAddress());
}
return addresses;
}
It should be enough to
change this List into a Set
But this doesn’t
compile :(

43. Use the weakest possible type
also for returned value
public List<Address> getFamilyAddresses( Person person ) {
Set<Address> addresses = new HashSet<>();
addresses.add(person.getAddress());
for (Person sibling : person.getSiblings()) {
addresses.add(sibling.getAddress());
}
return addresses;
}
It should be enough to
change this List into a Set
But this doesn’t
compile :(
and I cannot change the returned type to
avoid breaking backward compatibility :(((

44. Use the weakest possible type
also for returned value
public List<Address> getFamilyAddresses( Person person ) {
Set<Address> addresses = new HashSet<>();
addresses.add(person.getAddress());
for (Person sibling : person.getSiblings()) {
addresses.add(sibling.getAddress());
}
return new ArrayList<>( addresses );
}
I’m obliged to uselessly create an expensive
copy of data before returning them

45. Use the weakest possible type
also for returned value
public Collection<Address> getFamilyAddresses( Person person ) {
List<Address> addresses = new ArrayList<>();
addresses.add(person.getAddress());
for (Person sibling : person.getSiblings()) {
addresses.add(sibling.getAddress());
}
return addresses;
}
Returning a more generic type (if this is acceptable
for your client) provides better flexibility in future

46. Support lambdas
public interface Listener {
void beforeEvent(Event e);
void afterEvent(Event e);
}
class EventProducer {
public void registerListener(Listener listener) {
// register listener
}
}
public interface Listener {
void beforeEvent(Event e);
void afterEvent(Event e);
}
public interface Listener {
void beforeEvent(Event e);
void afterEvent(Event e);
}
EventProducer producer = new EventProducer();
producer.registerListener( new Listener() {
@Override
public void beforeEvent( Event e ) {
// ignore
}
@Override
public void afterEvent( Event e ) {
System.out.println(e);
}
} );

47. Support lambdas
class EventProducer {
public void registerBefore(BeforeListener before) {
// register listener
}
public void registerAfter(AfterListener after) {
// register listener
}
}
@FunctionalInterface
interface BeforeListener {
void beforeEvent( Event e );
}
@FunctionalInterface
interface AfterListener {
void afterEvent( Event e );
}
EventProducer producer = new EventProducer();
producer.registerAfter( System.out::println );
Taking functional interfaces as
argument of your API enables
clients to use lambdas

48. Support lambdas
class EventProducer {
public void registerBefore(Consumer<Event> before) {
// register listener
}
public void registerAfter(Consumer<Event> after) {
// register listener
}
}
@FunctionalInterface
interface BeforeListener {
void beforeEvent( Event e );
}
@FunctionalInterface
interface AfterListener {
void afterEvent( Event e );
}
EventProducer producer = new EventProducer();
producer.registerAfter( System.out::println );
Taking functional interfaces as
argument of your API enables
clients to use lambdas
In many cases you don’t need
to define your own functional
interfaces and use Java’s one

49. Optional – the mother of all bikeshedding

50. Optional – the mother of all bikeshedding
Principle of least
astonishment???
"If a necessary feature has a high
astonishment factor, it may be
necessary to redesign the feature."
- Cowlishaw, M. F. (1984). "The
design of the REXX language"

51. Optional – the mother of all bikeshedding
Principle of least
astonishment???
Wrong default

52. Optional – the mother of all bikeshedding
Principle of least
astonishment???
Wrong default
This could be removed if
the other was correctly
implemented

53. API design is an
iterative process
and there could
be different points
of view ...

54. … that could be
driven by the fact
that different
people may
weigh possible
use cases
differently...

55. … or even see
use cases to
which you didn’t
think at all

56. Also a good API
has many
different
characteristics ...

57. … and they
could be
conflicting so you
may need to
trade off one to
privilege another

58. What should
always drive the
final decision is
the intent of the
API … but even
there it could be
hard to find an
agreement

59. ●
Write lots of tests and examples against your API
●
Discuss it with colleagues and end users
●
Iterates multiple times to eliminate
➢
Unclear intentions
➢
Duplicated or redundant code
➢
Leaky abstraction
API design is an
iterative process

60. ●
Write lots of tests and examples against your API
●
Discuss it with colleagues and end users
●
Iterates multiple times to eliminate
➢
Unclear intentions
➢
Duplicated or redundant code
➢
Leaky abstraction
Practice Dogfeeding
API design is an
iterative process

61. And that’s all
what you were
getting wrong :)
… questions?
Mario Fusco
Red Hat – Principal Software Engineer
mario.fusco@gmail.com
twitter: @mariofusco