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