Chapter 6 Class Hierarchies, Inheritance, and Interfaces

This is a very good chapter for us at this point in time.

We have been programming hard this semester and know how to use most java programming commands.

Now we will take a look at inheritance. Some areas that I have been making you

address in your programs and now we will look at these things again.

I want to thank you for all your hard work thus far this semester.

Chapter 6

Object Oriented design Abstraction Encapsulation Polymorphism Inheritance

Modular programming Inheritance, polymorphism, and

interfaces

6.1 Class Hierarchies and Inheritance

Extending existing classes New class is called subclass

can add data fields and methods can override existing methods

Original class is called Super Class See example page 360 Hierarchical

in biology

is a versus has a

a car is a vehicle car is subclass of vehicle

a car has a wheel attribute of car

Not all vehicles have wheels Snowmobile

public class Car extends Vehicle {Wheels[ ] w = new Wheels[4]

Keyword extends makes car subclass of vehicle

Case study: Hierarchy employee class

Class called NewEmployee stores basic employee data name SSN job title address phone number age start date total pay to date

Methods

Besides standard methods (accessor, modifiers….

Also methods to compute numbers of years with

company years to retirement update total pay

What if you need to differentiate between hourly and salaried

Analysis and Design

See tables page 362 Now what do we need to do to add

SalaryEmployee HourlyEmployee

We inherit from NewEmployee then add necessary methods and

attributes See tables on pages 363 and 364

Implementation

See pages 364-366 for definition of base class See attributes page 364 Notice use of this. page 365 Notice 3 constructors page 365

one default one only basic information one complete

Look at equals method bottom page 366

This used heavily

Used this to differentiate names this.name this.age

Used to call methods not typically used

New use bottom page 367 use to call constructor this(name, social);

Class SalaryEmployee

public class SalaryEmployee extends NewEmployee {

SalaryEmployee takes on all methods and attributes of superClass

So implementation on page 369 only includes the necessary additional data fields and methods

Implementation SalaryEmployee

See data field only need annualSalary

Notice constructor calls super(name, social); same for larger constructor calls the constructor of the super class

Can user super in other methods see toString

Implementation HourlyEmployee

See Pages 372 – 373

Pages 373 – 374 test app, simply tests the classes

See running code in JBuilder

Operations in Class Hierarchy Section 6.1

Object

NewEmployee

HourlyEmployeeSalaryEmployee

Design

See tables pages 377-378

Method Overloading

Each class has 3 constructor methods

Known as Overloading Java knows which to call based on

the method signature. If none found get method not found

error

Method Overriding

Each class has a toString method Which one does it call The one in the subclass overrides the

one in the superclass If none in sub class will call one in

super class allows you to override functionality of

methods in superclass if don’t like default functionality

Protected Visibility

Data fields defined as private in a super class can not be accessed in the subclass directly.

Data fields defined as protected in a super class can be accessed directly in a subclass.

Shadowing data fields

If local variable (in method) has same name as class data field. can not access class data field local field shadows it can use prefix this to access it

Same applies to data fields in super class can use prefix super to access it

Assignment in class Hierarchy

See example 6.2 page 381 See declarations first Assignments obj = anEmp

can only access methods defined in obj toString is one But will call toString of NewEmployee

Assignment in class Hierarchy

Similarly on page 382 – 383 anEmp = hourEmp anEmp was originally instantiated as

NewEmployee therefore will only have methods available

that are a part of NewEmployee When we assign within a class hierarchy

A variable of type super class can reference an object of sub class converse is not true

Casting in Class hierarchy

We can remedy not being able to access methods that are not a part of super class when made to reference sub class via casting

((HourlyEmployee) anEmp).setHours(30.0);

Need to be careful with this, if not correctly can create run time errors.

Passing Objects as arguments

When passing arguments of different class types same rules apply as for assignment

Remember that objects passed by reference

Therefore change the state of the object in a method it is changed in the calling program

instanceof Operator

object instanceof ClassName test to see whether object (instance

variable) is and instance of className (class type)

See bottom of page 387 clerk instanceof HourlyEmployee When might you use this?

Section 6.3 Polymorphism

Another aspect of Polymorphism would allow us to have an array of employee of any type.

Create an array of NewEmployee objects. can then store any object type

See methods bottom of page 389 See code page 391

notice use of instanceof in computePayroll

Dynamic binding

Notice bottom page 391 toString loop inside we have result +

employees[I].toString() java does not know which toString

method to call until run time because does not know what type of

object employees[I] refers to until run time

known as dynamic binding

6.4 Interfaces

An interface is very similar to a class It is used to specify the requirements

for a class If a class implements an interface it

must have a certain base functionality See simple example bottom page 395 Classes that implement this interface

must provide a method: public double calcWeeklyPay();

Abstract method

The class in this interface: public double calcWeeklyPay()

is an abstract method since it has no body

it is not defined in the interface, only the header this does define the signature

interfaces can only have: abstract methods constant definitions (static final)

Implements

A class can only extend on class, but can implement several interfaces

If our classes implement Payable it would simplify the use of this class

See code middle of page 396 Method to call (which calcWeeklyPay)

determined at runtime

Steps for Payable

Write Payable interface (file Payable.java)

Add implements Payable to all classes that implement it. Verify that these classes contain complete definitions for method calcWeeklyPay()

Use casting to ensure that all calls to method calcWeeklyPay() are applied only to type Payable references

Comparable interface

Java has a number of useful built in interfaces

Comparable requires those that implement interface

to provide compareTo method See table page 398 Allows us to sort Comparable objects

Sorting

Code page 399 allows sorting If employee types implements

Comparable must provide compareTo bottom page 402 allows sort by SSN

Abstract Classes 6.5

Inheritance is used to make it easier to re-use code.

Additionally inheritance is used to provide structure to groups of related classes.

A car is a vehicle and a car has a wheel A car is a subclass of vehicle A car has a data field of wheel

Case Study Page 405

Need to find the total area of a collection of geometric figures.

For instance a painter looking to find exterior area of house to purchase paint.

Would like to create an array of geometric shapes. Array must be of like data types right?

Abstract class

An abstract class provides an outline of a class. Leaves the complete definition to those

classes that extend it. Contains classes that are defined by those

classes that extend it. You can not create an instance of an

abstract class. Abstract classes put the common members

as high up in the hierarchy as possible

GeoFigure case study

Page 408 Rectangle extends GeoFigure

Defines the methods computeArea computePerimeter

Page 410 Circle extends GeoFigure Defines the methods

computeArea computePerimeter

GeoFigure case study

Page 411 Triangle extends GeoFigure

Defines the methods computeArea computePerimeter

GeoFigure case study

See implementation Page 412 Can now create an array of GeoFigure

and put in it Triangle Circle Rectangle

Since they are all of the same type

GeoFigure case study

Can put in loop and use .toString to print each type. Since each shape class properly overrode

the toString we each prints in it’s own proper format

Can put in loop and call the computeArea method to add up all the areas. Abstract class makes sure that all the

method names are the same.

Drawing figures using an Abstract class and an Interface 6.6

GeoFigure

TriangleCircle

Drawable

DrawableTriangleDrawableRectangle DarwableCircle

Rectangle

New Geofigure

Add new data fields and methods to Geofigure

See table bottom of page 415

Drawable interface

See Drawable interface top of page 416 drawMe draws the figures on the

screen

Implementation

See implementation pages 418-421

Multiple inheritance

Java does not support multiple inheritance

Multiple inheritance in when a class extends more than one superclass.

The use of multiple inheritance is and it’s appropriateness has been debated by object oriented developers.

Typical example

Toy Elephant

ToyElephant

Interfaces

Interfaces also allow for a limited form of multiple inheritance

An interface has all abstract methods

It’s data fields can only be final class data fields

A class does not extend an interface, rather it implements it.

Packages 6.7

The package to which a class belongs is declared at the top of the class. Reserved word package followed by

package name and semicolon All classes in the same package are

stored in the same directory or folder

Packages

Classes that are not a part of the package can only access the public members. Member is a data field or method defined in a

class The complete name of a class is

packageName.className See middle 425 Only need this until variable is “bound” to

the instance. Once this occurs java knows which class to use

Packages and default visibility

There is a fourth kind of visibility we have not looked at yet. default visibility also called package visibility

Visibility

Private Only by class members

Protected Only by child classes

Public Anyone

Default (none) By anyone in the package

Section 6.8 Testing a Program System

Must learn to test in stages as the program develops

Top down Testing and stubs

Use a stub when testing for all methods that are not yet complete

A stub implements the interface but the body performs some minimal basic function

Typically also displays when in stub See example middle page 428

Bottom up testing and drivers

As method is completed it is substituted for it’s stub

Before putting new method in class we typically preliminarily test it with a driver.

Simple program to test a class has small main that instantiates object then calls the methods

Bottom up testing and drivers

These drivers are typically inserted directly in the class That is the class includes a main

method that is only used when testing the class

called bottom-up testing these can then be used to test the class

whenever changes are made allows for keeping previous tests made

Testing techniques

Black box versus White box testing will cover this heavily in software

engineering class black-box

tester has no knowledge of the code being tested

verifies that system meets the specifications white-box (or glass-box)

tester has knowledge of the code develops tests to test all the code

for instance all branches in an if statement

Common Programming Errors

Be VERY conservative with visibility of data fields all remain private except is very good

reason not to Can use this and super to call

constructors