BOOK NOTES
CHAPTER 3

BLACK BOXES

- black box: device whose 
  - inner workings are hidden
  - interaction w/ outside world is well-defined
- encapsulation
  - hiding of unimportant details
  - achieved by taking away inessential features until only essence remains
- manage software complexity
  - higher level data types
  - object-oriented programming
    - design objects
    - use objects
    - may be done by same or different people
    - easy to do bad designs
- implementation: write the code that turns an idea into software

DESIGNING THE PUBLIC INTERFACE OF A CLASS

- abstraction: determine essential feature set (commonality)
- design so users aren't dependent on your implementation details
- express actions/operations with methods
- method syntax
- method example
- method header
  - access specifier
    - which methods can use this method
    - public
      - always use for now
      - means any method can use this method
  - return type
    - type of output value
    - keyword void if nothing
    - otherwise, a type (class or primitive)
  - method name
  - list parameters, each with a 
    - inputs to the method
    - type
    - name
- method body: statements that are executed when the method is called
- constructor
  - defines what the object looks like when it's initially set up
  - differences from ordinary methods
    - name must match class name
    - no return type
  - syntax
  - example
- class definition
  - contains constructor and method definitions
  - syntax
  - example
- public interface of a class
  - its public constructors and methods
  - operations any programmer can use to manipulate objects of that class

COMMENTING THE PUBLIC INTERFACE

- comment behavior of classes and methods as you create
- documentation comments
  - Javadoc program uses these to automatically generate HTML description pages
  - place before class/method
  - start with /**
  - describe purpose
  - for (public) methods, also
    - for each parameter
      - @param
      - parameter name
      - description
    - @return return value description
    - first sentence is included in summary
- provide these comments for everything
- update code and documentation together

INSTANCE FIELDS

- instance field: storage location present in every object of the class
- declaration
  - access specifier
    - usually private
    - encapsulation
      - only methods of the same class can access them
      - force access through methods
  - type
  - name
- every object of a class has it's own set of instance fields

IMPLEMENTING CONSTRUCTORS AND METHODS

- a method body contains a sequence of instructions
- constructor
  - purpose: initialize the fields of an object
- what happens when a constructor is called
  - new object of that class is created
  - decide which constructor to use by matching parameters
  - follows statements to initialize instance fields
  - return an object reference as the value of the new expression
  - store the object reference in the variable it's assigned to
- return statement
  - specify the value that a method returns
  - exit the method immediately
  - return value is the value of the method call expression
  - syntax
  - expression optional
- what happens when a regular method is called
  - parameters are copied
  - statements are executed
  - return value is the value of the method call expression

TESTING A CLASS

- no dev env for testing class interactively
- write test class instead
- test class
  - has main method
  - contains statements to test another class
  - steps (on class being tested)
    - construct object(s)
    - invoke method(s)
    - print result(s)
- must associate classes to use them together
  - make program subfolder
  - put each class in its own file
  - put both files in the subfolder 
  - compile both files
  - run the test program
- how to design and implement a class
  - determine object behavior
  - specify public interface
    - set of methods with parameters and return types
    - constructor(s)
  - document public interface
  - determine instance fields: object must have enough 
    internal mem to process method calls in any order
  - implement constructors and methods: troubles may mean 
    you should rethink your interface
  - test class

CATEGORIES OF VARIABLES

- categories
  - instance fields
  - local variables
  - parameter variables
- similarities: hold values of specific types
- differences
  - lifetime
    - instance
      - occurs for each copy of an object
      - created when object constructed
      - exists until no method uses the object
    - local and parameter
      - belong to a method
      - creation
        - parameter: when method runs
        - local variable: when declared in method
      - destroyed when the method completes
  - initialization
    - local: must do manually
    - parameter: automatically when method called
    - instance
      - default value supplied
        - 0 for numbers
        - null for object references
        - not caught as an error by the compiler
      - stylistically, should always do manually in 
        constructor
      
IMPLICIT AND EXPLICIT METHOD PARAMETERS

- reference to instance field inside method means that field 
  of the object on which the method was called
- use reserved word "this" to refer to implicit parameter
- compiler automatically inserts "this." before every 
  instance field used in your code
- implicit parameter of any method calls is the same as that 
  of the method containing the call
- call one constructor from another
  - use "this" as the method name, instead of the name of 
    the class
  - must be the first line of the constructor containing the 
    call