- display a window on the screen
- pixel: picture element
  - std unit of screen resolution
- OO programs use objects
- programmer-defined classes: we make
- standard classes: come with Java package
- dependency relationship
  - class that sends msgs to another is dependent on the obj it sends msgs to
  - UML diagram: dotted arrow from dependent 
- use object
  1) declare
  2) create
  3) send msgs
- obj declaration
  - every obj used must be declared
  - syntax: <class name> <object names>;
  - class name must already be defined
  - object name can be comma-delim list of new objs of the class
  - use sys classes only for now
  - identifier
    - use to name class, obj, method, etc.
    - letters, digits, underscores, dollar signs
    - first must be letter
    - case-sensitive
    - no spaces
  - naming convention
    - uppercase first letter of classes
    - lowercase first letter of obj
    - multiple words: capitalize all subsequent words
- obj creation
  - declaration does not create obj
  - syntax: <obj name> - new <class name>(<args>);
  - state of memory diagram not true UML
  - may use same identifier to refer to dif objs of same class @ dif times
  - deallocation
    - when no more references to an obj, eventually automatically erased
    - garbage collection: mechanism of deallocating unused space where automatically erased
- msg sending
  - only after obj created
  - syntax: <obj name>.<method name>(<args>);
  - calling a method synonym for sending a message
  - 2 logical values
    - true, false
    - reserved words
      - identifiers used for certain purposes
      - cannot be used for any other purpose (e.g. obj name)

Quick Check:
1. b (no spaces allowed), c (must begin with a letter), g (no commas or spaces allowed)
2. It should not use parentheses in the declaration, and it should insert a line declaring the JFrame:
   myWindow = new JFrame();
3. The naming conventions are reversed in the first: usually MainWindow would be a class name, and mainWindow would be an instance name.
   In the second, you can only declare one type per statement.  You should instead do:
      Account account;
      Customer customer;
4. b is valid; a uses the wrong type: String instead of boolean