CSC 142/Chapter 8

Chapter 8: Classes

Sections:

8.1 Object Oriented Programming

8.2 Object state/behavior

8.3 Object initialization: constructors

8.4 Encapsulation

8.5 Case study: Stock class

Chapters 1-7 focused on procedural programming. Now, we'll dive into classes, core concept in Java.

Start with terminology - like prior chapter covering objects, lots of terminology.

Discuss abstraction and encapsulation.

Discuss ideas for design of new classes, and programs to use them.

Section 8.1: Object Oriented Programming

Definitions:

• Object oriented programming
• Object
• State
• Behavior
• Client
• Driver

Material

Core concept:

• state and behavior
• state: values (internal data) stored by oejct
• behavior: set of actions
• Ultimately, it is iall just data - numbers, strings, other objects, but methods are also like data - a set of instructions, a recipe

Driver program:

• The program that sets things in motion (confusing because this is also an object in Java, but think of it as the non-object object
• Objects != programs: objects == components of programs

Encapsulation

• Represneting details
• Taking care of details
• Bank analogy again - bank acct object defines some data, like name and addr, balance, acct no., some can be changed, some cannot

Classes:

• Classes: template, definition, blueprint
• Objects: actual implementation, actual instance

Class:

• object
• object
• object

house blueprint

• house
• house
• house

Point objects: (x,y)

• Provied by awt
• has methods defined
• Usage of an object, understanding how to call its methods
• History reference: DOS (cmd line) --> Mac (GUI)
• Verb noun --> noun verb

Section 8.2: State and Behavior

Definitions

Definitions:

• Mutator
• Accessor

Material

Working through construction of Point class

Components:

• Fields (data stored in each object)
• Methods (behavior)
• Constructor (init code)
• Encapsulation (data protection)

Object state: fields

• x and y are two Point fields
• Separate init pair x and y for each instance (blueprint)
• Creation of client code to create 2 points, compute distance

Object behavior: methods

• Behavior --> instance methods
• Translating point: example of abstraction
• Just translate the point - we don't care how you do it!
• Alt is pkublic static method, lots of detail exposed, not optimal
• IF we change point class to be 3D, now we have to change the class, and change stuff that is outside the class

Implicit parameter:

• Code for an instance method has an implied knowledge of the object it operates on (ITSELF)
• Implicit parameter - the object that is referenced during an instance method call
• Python: we have to say self.x, self.y
• Java we say this.x, this.y, but we can also just say x, y

Methods for accessing changing data:

• Objects, Java in general, obsessed with concept of data protection
• Strange fixation, at beginning
• Let's explore this.
• Bank acct example: code needs to protect data, provide accessors to appropriate code, not unfettered access to all data
• Healthcare record system in Java: served to large hospital network, access control REALLY important

To deal with data protections:

• All data is private by default
• Accessor: getParam() method
• Mutator: setParam() method
• Le the object change the data itself, provide a high-level method for accessing that functionality

toString() method:

• Strings are the central object in Java
• Concatenation works with all objects and primitive types
• Printing objects by default prints address in memory
• Can replace this default behavior by overriding toString() method

Section 8.3: Object Init and Constructors

Definitions

Definitions:

• this

Material

Objects are collections of data

How do we initialize that data?

Don't always want user to initialize (e.g., initialize a new bank account with 1E16 dollars).