Lecture Slides on Interfaces - Introduction to Programming II | CS 112, Study notes of Computer Science

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Intro to Programming II^ Interfaces

Chris Brooks

Department of Computer ScienceUniversity of San Francisco

Department of Computer Science — University of San Francisco – p. 1/

15-2:^ Inheritance review^ •^ Inheritance allows us to reuse existing code.^ •^ Allows us to define a hierarchy of classes.^ •^ Base class

has the most general behavior

-^ Derived classes

have more specific behavior.

Department of Computer Science — University of San Francisco – p. 2/

15-4:^ Example public^ class

Professor

extends

Person

{^ public

String

officeNum; public^ void

teach()

public^ void

grade()

public^ void

forget()

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15-5:^ Constructors and Inheritance^ •^ Suppose that the Person constructor looks like this:^ public

Person(String

lname)

lastName

= lname; } • How can we still set the last name in derived classconstructors?

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15-7:^ Constructors and Inheritance^ •^ Instead, let’s just indicate that the parent class’ constructorshould be called.^ •^ We do this with super()^ public

Professor(String

lname,^

String^

office)^

super(lname);officeNum

= office; } • Now we don’t need to worry about what the base class’constructor does anymore.

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15-8:^ More on super()^ •^ We can also use super to explicitly call a superclass’ method.^ •^ This lets us

extend

a method, rather than overriding it.

-^ For example, let’s say that Person has the following method:^ /*in^

Person.java

public^ void

greet()

System.out.println(‘‘Nice

to^ meet

you’’);

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15-10:^ Exercise^ •^ Write a base class called Animal. Give it two instance variables(name and furColor). Give it a constructor that sets both ofthese.^ •^ Give Animal a printSelf method that prints out the following:^ ◦^

My name is (name). My fur is (furColor). • Now create a class called Cat that inherits from Animal. Catshould have one instance variable: age. • Write a constructor for Cat that takes three arguments: name,furColor, and age. It should set age itself, then call super withthe other two arguments. • Write a method in Cat called printSelf. It should print “I am acat”, then call the superclass’ printSelf method.

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15-11:^ Interfaces^ •^ Previously, we talked about abstract classes.^ •^ They allow a superclass to specify the methods a subclass willrespond to without providing an implementation.^ •^ Sometimes abstract classes can be awkward to deal with.^ •^ For example, let’s say we want to create a class called Bat thatinherits from Animal.^ •^ We also want to say that Bat is a FlyingThing, and thatFlyingThings respond to the fly() method.^ •^ But we already inherited from Animal!

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15-13:^ Interfaces^ •^ Interfaces let us specify which methods an object shouldrespond to, without specifying how they should respond.^ •^ This provides polymorphism - each object responds to amethod in the appropriate way.^ •^ A class can implement as many interfaces as it wants.

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15-14:^ Interfaces in the JDK^ •^ Comparable^ •^ Iterable^ •^ Iterator^ •^ Cloneable^ •^ Readable^ •^ and many, many more

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15-16:^ Comparable public^ class

Cat^ extends

Animal^

implements

Comparable

public^ int

compareTo(Object

other)

if^ (!(other

instanceof

Cat))^

System.out.println("Can’t

compare

these!");

return^ 0; } else^ { int^ age

=^ ((Cat)other).age; if^ (age^

< age2) return^ -1;else

if^ (age

age2)

return^ 1;elsereturn^ 0; } }

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15-17:^ Exercise^ •^ Rewrite Shape to be an Interface that declares an area()method.^ •^ Rewrite Circle and Rectangle to implement the Shape interface.^ •^ Now have Rectangle and Circle also implement theComparable interface.^ ◦^

They will need to implement the compareTo method. Havethem compare areas.

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