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

Definitions:

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

Material

Core concept:

• state and behavior
• state: values (internal data) stored by object
• behavior: set of actions
• Ultimately, it is all 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 are not program!
• Objects are components of programs!

Encapsulation

• Representing details
• Taking care of details
• Bank analogy again - bank acct object defines some data, like name and address, 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)

• Provided 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
• Cohesion

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:

• Let's consider a method (an instance method - meaning a non-static method)
• The method has implied knowledge of the object it operates on
• That is, if a method is defined for an object, the method knows about that object
• This object (itself) is called the "implicit parameter" because it doesn't need to be explicitly declared
• 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).

We initialize data using constructor

• What constructor looks like
• Syntax for constructors

Example of Point class:

public class Point {
  int x;
  int y;
  public Point( int x0, int y0) {
    x = x0; // or, this.x = x0;
    y = y0; // or, this.y = y0;
  }
}

Other methods:

• distance from origin
• to string
• translate

Constructors:

• Default constructor
• Constructor with points

Common constructor syntax error:

• Redeclaring fields (variables)
• Using void with constructor
• Constructors DO NOT have return type

This keyword:

• Less confusing than using implicit parameters
• Be explicit

Point p1 = new Point(10,5);
Point p2 = new Point(4,5);

Point 1 and point 2 are referencing particular parts of memory.

• this.x less ambiguous
• prevents later squashing of variable x, prevents ambiguity (argument named "x", private member named "x" - which one?)
• Mixing LOCAL method variables, LOCAL method arguments, and CLASS WIDE variables is a no-no!

Multiple constructors:

• Different ways to construct objects
• That is, can have default arguments, Point() vs Point(1,1)

Section 8.4: Encapsulation

Definitions

Definitions:

• Encapsulation
• Abstraction
• Class invariant

Material

Radio:

• Example of system with many internal complexities that the user doesn't care about, components with different values
• Partly about keeping everything simple and easy for the user - don't want the user to have to understand radio circuits
• Partly about protecting things - the user may pick BAD values of components, and fry the radio

Private fields:

• We can't just trust the user not to touch the things they shouldn't
• Bank account example: need protections on data
• This is the purpose behind get/set methods
• If we don't protect data, users only need to know the name of the variable
• Bank example:

public BankAcct { 
  public BankAcct(name, addr) { 
    int this.account = 1234;
    int this.balance = 0;
  }
}

With the above example, the user just needs to know the variable "balance," and they can set their balance to whatever they want. Not secure!

Syntax for private data:

private int num;
private String label = "CreativeNames";

If you lock up the class data, be nice to the users - provide them a way to access the data!

Example:

• Encapsulated Point class and PointDriver class

Class invariants:

• Class to represent time intervals
• Private fields for hours/minutes
• What if restrictions on values?
• In constructor, pass in values of hours/minutes
• Deal with minutes > 60 in constructor, with a percentage
• When we add minutes - same problem (IDENTIFY A PATTERN - ENCAPSULATE THE CODE)
• Class invariants are assertions about class state (data) that MUST be true
• Wrap up the "add" functionality, and call an internal add() method in constructor

Printing:

• Redefine toString() method to customize printing behavior
• Example TimeSpan class illustrates why encapsulation is essential to enforcing conditions about the class
• GOOD DESIGN = RIGOROUS CODE

Another example:

• Polar coordinate points (r, theta)

Chapter 8 Summary

Deliverables

Classes: basics

• Blueprint
• Class vs object
• State and behavior
• Encapsulation and abstraction
• Basic class syntax

State and behavior:

• Class fields
• Class methods
• Class constructors
• Class encapsulation
• Fields: implicit parameters
• Fields: getParam()/setParam()
• toString()

Object initialization:

• Objects are collections are data
• Syntax of how to initialize
• Protecting data from user
• Default constructor
• Common syntax errors
• This keyword
• Multiple constructors
• Memory and reference
• How this/constructors fit into this all

Encapsulation:

• Hide details, protect data
• Private fields - really protect data
• BankAcct example - these are not just made up problems, they are REAL!
• Syntax for private variables
• Class invariants and rigorously enforcing conditions
• Rigor requires encapsulation
• Good code = trustworthy code

Chapter 8 Code

Worksheet Code

Polynomial worksheet

• Implement a polynomial object, use an array
• Implement an add, evaluate, and toString method

Chapter 8 Homework

HW Questions

(Recommended) Self-check problems: #7, #10, #15, #18, #21, #26

(Required) Exercises: #3, #4, #5, #6, #14, #15, #16

(Required) Projects: (none)

HW Details

Self-check:

• 7 - create Name class, first/middle/last
• 10 - distance method to Point object
• 15 - tSoString method to Name class
• 18 - constructor problems
• 21 - constructor to Point class accepting Point
• 26 - encapsulate Name class

Exercises:

• 3 - add manhattanDistance() method to Point class
• 4 - add isVertical() method to Point class
• 5 - add slope() method to Point class
• 6 - add isCollinear() method to Point class
• 14 - Line class between two points, implement specified constructors and methods
• 15 - add getSlope() method to Line class
• 16 - add Line constructor

Projects:

• none

Chapter 8 Goodies

Puzzle 8

Puzzles/Fall 2016/Puzzle 8

Flags

http://design.tutsplus.com/articles/40-dark-and-futuristic-photoshop-effects--psd-291