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Intro to Programming II
Interfaces
Chris Brooks
Department of Computer Science
University of San Francisco
Department of Computer Science — University of San Francisco – p. 1/
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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/
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Example
public
class
Person
public
String
lastName;
public
String
id;
public
void
eat()
public
void
sleep()
Department of Computer Science — University of San Fra
Example
public
class
Professor
extends
Person
public
String
officeNum;
public
void
teach()
public
void
grade()
public
void
forget()
Department of Computer Science — University of San Francisco – p. 4/
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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?
Department of Computer Science — University of San Francisco – p. 5/
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Constructors and Inheritance
-^
We could do something like this: public
Professor(String
lname,
String
office)
lastName
lname;
officeNum
= office;
-^
Except:^ ◦
We just had to cut and paste code. (we hate that!) ◦ If the base class changes, all derived classes need tochange.
Department of Computer Science — University of San Fra
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.
Department of Computer Science — University of San Francisco – p. 7/
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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’’);
Department of Computer Science — University of San Francisco – p. 8/
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More on super()
-^
We’d like for professors to do this greeting, plus a little extra.(They’re long-winded.) So we can do this: /*
in
Professor.java
public
void
greet()
System.out.print(‘‘I
do
say,
old
chap,’’);
super.greet(); }
Department of Computer Science — University of San Fra
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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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!
Department of Computer Science — University of San Francisco – p. 11/
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Interfaces
-^
Interfaces
allow us to specify methods that an object is
guaranteed to respond to, without specifying animplementation.
-^
A class can implement as many interfaces as it wants. public
interface
FlyingThing
public
void
fly();
} public
class
Bat
extends
Animal
implements
FlyingThing
public
void
fly()
System.out.println(‘‘I’m
flying!’’);
Department of Computer Science — University of San Fran
