Lecture 9

Fall 2018

Narges Norouzi

what's a class made of?

  • Members of a class:

    • Attributes or Fields (instance variables, data)

      • For each instance of the class (object), values of attributes can vary, hence they are instance variables

    • Methods (instance methods)

  • ​Person class

    • Attributes: name, address, phone number

    • Methods: change address, change phone number

  • Alice object

    • Name is Alice, address is ...

 

Constructors

  • Objects must be initialized before use.

    • We must specify what is the initial state of the object before we can use it.

  • Space for holding the object state data is only allocated when the object is constructed.
  • We specify the way an object is initialized using a constructor, which is a special method invoked every time we create a new object.
  • The name of the constructor is always identical to the class name

 

clock constructor

"new" operator

  • "new" creates a new object from

    specified type:

    • ​new String();

    • new Book();

Invoking clock constructor

Passing parameters

  • When a parameter is passed, a copy of the value is made and assigned to the formal parameter:

Memory management in heap

5 minutes break

LINK

Instance methods

secondelapsed() method

method context

  • The getHours() and secondElapsed() methods are instance methods, which means they act on a particular instance of the class

  • They cannot be invoked “out of the blue”. They must act on a particular object

 

 

  • An instance method is executed in the context of the object it acts upon.

method settime()

Testing clock class

variable scope

  • Variables may be declared in:

    • ​Class – state variables

    • Method/constructor – local variables (and parameters)

    • Inner block of a method – also local variables

  • A variable is recognized throughout the block in which it was defined.
  • Local variables are allocated when the method is entered and freed when the method exits.
  • The same name may be used in different scopes, and refer to totally different things.
  • If the same name is used in an outer and inner scope, then the inner scope definition “hides” the outer one.

 

the "this" reference

  • When appearing inside an instance method, the “this” keyword denotes a reference to the object that the method is acting upon.

  • The following are equivalent:

Using same names for parameters & fields

  • It is usually a bad practice to use the same name for a state variable and a local variable or parameter.

  • Exception: parameters that correspond to fields

  • Zybooks chapters we covered

  • Chapter 7: Objects and Classes

    • section 7.1, 7.2, 7.3, 7.4, 7.6, 7.9, 7.10, 7.11 

  • Zybooks chapters we'll cover next

  • Chapter 10: Inheritance

    • Sections 10.1, 10.2, 10.3, 10.4

Assignment 3 Review