Chapter 4

Section 4.1

objects have a state of being and a behavior – these are their variables and methods
each object has its own state, but in general we can describe any object’s behavior
state often modifies its behavior
objects are defined by classes
class is model or blueprint of object – describes types of data object will hold and code for its methods
no data space is reserved in a class – we have to instantiate one or more objects of the class, then each object will have its own space for data

class has any number of data declarations and methods, called the members of the class
constructor has the same name as the class, and gets called with new is used to create a new instance of that class
listing 4.1, 4.2, p. 179, 180
scope of variable is defined by location where it is declared – if a variable is declared in a class, its scope includes all the methods of the class
data declared in a class is called instance data, since memory space is created for each instance of the class created
listing 4.3, p. 182
objects should be self-governing, so variables in an object should only be able to be modified within that object – only methods of that object should have access to them
this is called encapsulation – object should be encapsulated from rest of program
should write methods to allow any instance data modification – these define the interface between the object and the rest of the program
client shouldn’t be able to interact with instance data directly, but use methods to do it
visibility modifiers: public and private
public – can be directly referenced from outside the object
private – can only be reference inside class definition
if a method provides a service of the class, it should be public, but many instance variables will be declared private
constants are public many times, since they are final and can’t be changed anyway

every method is part of a class, and has method declaration
when method is called, flow of control transfers to method, then back to where call was made
header includes type of return value, method name, list of parameters, then statements of body are in { }
listing 4.4, 4.5, p. 188, 189
return type can be primitive type, class name or void
if method returns a value, must have return in method
constructors have no return value ever, so no return line
can ignore return value when call a method, if desired
parameter is value passed to a method
in method declaration, formal parameters are listed, whereas when method is called, actual parameters are passed
parameters always go in ( ) after method name, and if there are no parameters, have empty ( )
parameter list in invocation must match that of method declaration
constructors can also take parameters
constructor is called when object is created
it is same name as class, and never has return value
every class has default constructor, if you choose not to define one
scope of a variable is part of program where that variable can be referenced validly – part of the program where it is considered alive
variable can be declared in a method, making it local data as opposed to instance data – its scope is only that method
instance data has scope of entire class – any method in class can use it

some objects are made up of other objects, like a String object – this is has-a relationship
methods of class can take objects as parameters
listing 4.6, 4.7, p. 196, 197

can overload methods – give multiple methods the same name
some combination of type and number of parameters must be different to make them unique – must have unique method signature
println is overloaded many times
constructors are many times overloaded, too
listing 4.8. 4.9, p. 203, 204