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lecture03-api-design-and-implementation.md

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Lecture 03 API Design and Implementation

Object-Oriented Programming basics

Object

  • An object is a bundle of state and behavior
  • State - fields
  • Behavior - methods

Classes

  • Every object has a class
  • Class defines both type and implementation
  • the methods of a class are its Application Programming Interface (API)
    • Defines how users interact with instances

Interfaces and implementations

  • Multiple implementations of API can coexist
  • In Java, an API is specified by interface or class
    • Interface - only API
    • Class - API and implementation
    • An interface defines but does not implement API
    • Interface decouples client from implementations
  • Why multiple implementations
    • Often performance and behavior both vary
    • Provides a functionality-perfomance tradeoff

Information hiding

  • Hides internal data and implementation details from other modules
  • Well-designed code hides all implementation details

Benifits of information hiding

  • Decouples the classes that comprise a system
    • Allows them to be developed, tested, optimized, used, understood, and modified in isolation
  • Speeds up system development
    • Classes can be developed in parallel
  • Eases burden of maintenance
  • Enables effective performance tuning
    • Hot classes can be optimized in isolation
  • Increases software reuse
    • Loosely-coupled classes often prove useful in other context

Information hiding with interfaces

  • Declare variables using interface types
  • Client can use only interface methods
  • Fields not accessible from client code

Mandatory Information Hiding

  • private - Accessible only from declaring class
  • package-private - Accessible from any class in same package
  • protected - Accessible from subclasses of declaring class (and within package)
  • public - Accessible from anywhere

Exceptions

  • Inform caller of problem by transfer of control
  • Semantics
    • Propagate up stack until main method is reached (terminate program), or exception is caught
  • Sources
    • Program can throw explicitly
    • Underlying virtual machine can generate

Benefits of Exceptions

  • You can't forget to handle common failure modes
    • Compare: using a flag or special return value
  • Provide high-level summary of error, and stack trace
    • Compare: core dump
  • Improve code structure
  • Ease task of writing robust code

Checked vs. Unchecked Exceptions

  • Checked exception

    • Must be caught or propagated, or program won't compile
    • Exceptional condition that programmer must deal with

  • Unchecked exception

    • No action is required for program to compile
    • Usuallt\y indicates a programming error

  • Error

    • Special unchecked exception thrown by JVM
    • Recovery is impossible

  • Java's exception hierarchy

    • object -> Throwable -> (Exception, Error)

  • Design choice: checked exceptions, unchecked exceptions, and error codes

    • Unchecked exception - Programming error, other unrecoverable failure
    • Checked exception - An error that every caller should be aware of and handle
    • Special value
      • Common but atypical result
      • Ex: null from Map.get
    • Do not return null for zero length result
    • Guidelines for using exceptions - refer to Effective Java
    • Try with resources - Automatically closes resource