• Why is API Design Important?
• APIs can be among a company's greates assets.
  • Customers invest heavily: buying, writing, learning.
  • Cost to stop using an API can be prohibitive
  • Successful public APIs capture customers.
• Can also be among company's greatest liabilities
  • bad APIs result in unending stream of support calls.
• Public APIs are forever
  • One chance to get it right.
  • Why is API Design Important to You?
• If you program, you are an API Designer
  • Good code is modular - each module has an API
• Usefull modules tend to get reused
  • Once module has users, can't change API at will
    • You can add new ones,
    • But you should not delete or modify existing ones at will.
• Thinking in tersm of API improves code quality.

• Characteristics of a Good API
• Easy to learn.
• Easy to use, Hard to misuse
• Easy to read and maintian code that uses it
• Sufficiently powerful to satisfy requirements.
• Easy to extend
• Appropriate to audience.

• Gather requirements - with a Healthy Degree of Skepticism
  Often you'll get proposed solutions instead, but better solutions may existing, and your job is to extract true requirements from the use-cases.

  And, keep in mind that it can be easier and more rewarding to build somthing more general.

• Make spec short: single page is okey
• Write to Your API Early and Often
• Writing to SPI is Even More Important
  • Service Provider Interface (SPI)
  • Plugin-in interface enabling multiple implementations
  • Example: Java Cryptography Extension (JCE).
  • Write multiple plugins before release
  • The Rule of threes.
• Maintain Realistic Expectations

• API should do one thing and do it well
• Functionality should be easy to explain:
  • If it's hard to name, that generaty a bad sign.
  • Good names drive development.
  • Be amenable to spliting and merging modules.
• API should be as mall as possible, but no smaller
  • API should satisfy its requirements
  • When in doubt, leave it out!
    • You can always add, but you can never remove!
  • Conceptual weight more important than bulk.
  • Look for a good power-to-weight ratio
• Implementation should not impact API
• Minimize Accessibility of Everything
  • Make classes and members as private as possible.
  • Public classes should have no public fields.
  • Maximizes information hiding
  • Allow modules to be used, understood, built, tested, and debugged independently.
• Names Matter – API is a Little Language
  • Names shoudl be Largely Self-Explanatory
  • Be consistent – same word meas same thing
  • Be regular – strive for symmetry
  • Code should read like prose
• Documentation Matters
• Document Religiosly:
  All public APIs should have Documentation:
  • Classes: what an instance represents
  • Method: contract between method and its client
    • Preconditions, postconditions, side-effects.
  • Parameter: indicat units, form, ownership
  • Document state space very carefully

• Consider Performace Consequences of API Design Decisions
  • bad decisions can limit performance.
• API Must Coexist Peacefully with Platform

• Minimize Mutability
  Classes should be immutable unless there's a good reason to do otherwise. If mutable, keep state-space small, well-defined.
• Subcalss only where it makes sense
  Subclassing implies subsitituability,
  • Public classses should not subclasses other public classes.
• Design and Document for Inheritance or Else Prohibit it!

• Don't make the Client Do Anything the Module Could Do
• Don't Violate Principle of Least Astonishment
• Fail Fast – Report Errors as Sonn as Possible After They Occur
• Provide Programmatic Access to All Data Available in String Form
• Overload With Care

if you must provide ambiguous overloadings, ensure same behavior for same arguments.

• Use Appropriate Parameter and Return Types
  • Favor interface types over classes for input.
  • Use most specifi possible input parameter type
    

  Moves error from runtime to compile time.

  • Don't use string if a better type exists
  • Do't use floating point for monetary values:
    

  Binary floating point causes inexact result.

  • Use double (64 bits) rather than float(32 bits):
    

  Precision loss is real, performance loss negligible.

• Use Consistent parameter Ordering Across Methods

#include <string.h>
char   *strcpy(char   *dest, char   *src);
void  bcopy (void* src, void* dst, int n); // XXX: Bad example!

• Avoid Long Parameter Lists
  • Three or fewer parameters is ideal
  • Two techniques for shortening parameter lists:
    • Break up method
    • Create helper class to hold parameters
• Avoid Return Values that Demand Exceptional Processing

• Throw Exceptions to Indicate Exceptional Conditions
  • Don't force client to use exceptions for control flow.
  • Don't fail silently
• Favor Unchecked Exceptions
• Include Failure-Capture Information in Exceptions

Just some examples….