Chapter 2 Class Definitions

1
Chapter 2Class Definitions

• Project naive TicketMachine
• Test
• Fields, Constructors, Methods.

2
Fields, Constructors, Methods

• Fields
• Define data to be stored in an object, i.e. the
  state of the object
• Specified as private
• Define data to be stored in an object, i.e. the
  state of the object
• Usually come at the beginning of the class
  definition
• NOT defined inside a method.

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Fields, Constructors, Methods

• Constructors
• Same name as the class
• No return type, not even void
• Automatically called when an object of the class
  is created
• Usually used to initialize values of the fields
• Usually come after the field definitions, but
  before the methods.

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Fields, Constructors, Methods

• Methods
• Define the object's behavior
• May specify a return type, or void
• May have parameters, separated by commas
• Must have the parentheses, even if there are no
  parameters
• May be public or private.

5
Passing Data Via Parameters

• Parameters
• Are used to send data into a called method
• Parameters in the calling method are actual
  parameters
• Parameters in the method being called are formal
  parameters
• Actual parameters may be expressions of the
  correct type 3.5 p p/3.0
• Formal parameters must be simple variables p
• Formal parameters die when the method terminates.

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Passing Data Via Parameters

• When a method is called from another method
• The actual parameters are evaluated
• The value of each actual parameter is copied
  into the corresponding formal parameter
• With primitive types (int, float, char, boolean)
  this is straightforward
• With object types (String, Color, ...) a
  reference to the data is copied
• Actual parameters and formal parameters must
  agree in number and type.

7
Assignment

• Syntax
• variable expression
• Examples
• interest balance 0.05
• price ticketCost
• age 18
• age age 1
• x y
• Semantics
• The expression is evaluated
• Its value is stored into the variable
• The value assigned is the result!

8
Assignment

• Object types are different
• Circle sun
• Circle sun2
• sun new Circle()
• sun2 sun
• sun.changeColor(red)
• // sun2 is now red!

9
Expressions

• An expression may be
• A variable
• A constant
• expression expression
• expression - expression
• expression expression
• expression / expression
• ( expression )