Improving structure with
inheritance
(Chapters 8 and 9)
Main concepts to be covered
•
•
•
•
Inheritance
Subtyping
Substitution
Polymorphic variables
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2
The DoME example
"Database of Multimedia Entertainment"
• stores details about CDs and DVDs
– CD: title, artist, # tracks, playing time, gotit, comment
– DVD: title, director, playing time, got-it,
comment
• allows (later) to search for
information or print lists
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3
DoME objects
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DoME classes
top half
shows fields
bottom half
shows methods
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DoME object model
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Class diagram
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CD
source
code
[
public class CD
{
private String title;
private String artist;
private String comment;
public CD (String theTitle, String theArtist)
{
title = theTitle;
artist = theArtist;
comment = "<no comment>";
}
public void setComment (String newComment)
{ ... }
]
incomplete
(comments!)
public String getComment ()
{ ... }
public void print ()
{ ... }
...
other methods
}
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8
DVD
source
code
[
public class DVD
{
private String title;
private String director;
private String comment;
public DVD (String theTitle, String theDirector)
{
title = theTitle;
director = theDirector;
comment = "<no comment>";
}
public void setComment (String newComment)
{ ... }
]
incomplete
(comments!)
public String getComment ()
{ ... }
public void print ()
{ ... }
...
other methods
}
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Database source code
class Database {
private ArrayList<CD> cds;
private ArrayList<DVD> dvds;
...
addCD, addDVD
public void list ()
{
for(CD cd : cds) {
cd.print ();
System.out.println ();
}
for(DVD dvd : dvds) {
dvd.print ();
System.out.println ();
}
// empty line between items
// empty line between items
}
}
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Critique of DoME
• code duplication
– CD and DVD classes very similar (large
part are identical)
– makes maintenance difficult/more work
– introduces danger of bugs through
incorrect maintenance
• code duplication also in Database
class
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Using inheritance
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Using inheritance
• define one superclass : Item
• define subclasses for Video and CD
• the superclass defines common
attributes
• the subclasses inherit the superclass
attributes
• the subclasses add own attributes
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Inheritance hierarchies
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Inheritance in Java
public class Item
{
...
}
public class CD extends Item
{
...
}
no change here
change here
public class DVD extends Item
{
...
}
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Superclass
public class Item
{
private
private
private
private
...
String title;
int playingTime;
boolean gotIt;
String comment;
constructors and methods
}
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Subclasses
public class CD extends Item
{
private String artist;
private int numberOfTracks;
...
constructors and methods
}
public class DVD extends Item
{
private String director;
...
constructors and methods
}
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17
Inheritance and
constructors
public class Item
{
private String title;
private int playingTime;
private boolean gotIt;
private String comment;
/**
* Initialise the fields of the item.
*/
public Item (String theTitle, int time)
{
title = theTitle;
playingTime = time;
gotIt = false;
comment = "";
}
...
methods
}
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Inheritance and
constructors
public class CD extends Item
{
private String artist;
private int numberOfTracks;
/**
* Constructor for objects of class CD
*/
public CD (String theTitle, String theArtist,
int tracks, int time)
{
super (theTitle, time);
artist = theArtist;
numberOfTracks = tracks;
}
...
methods
}
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Superclass constructor call
• Subclass constructors must always
contain a 'super' call.
• If none is written, the compiler
inserts one (without parameters)
– this works only, if the superclass has a
constructor without parameters
• The 'super' call must be the first
statement in the subclass
constructor.
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Adding more item types
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Deeper hierarchies
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Review (so far)
Inheritance (so far) helps with:
• Avoiding code duplication
• Code reuse
• Easier maintenance
• Extendibility
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public class Database
{
private ArrayList<Item> items;
/**
* Construct an empty Database.
*/
public Database ()
{
items = new ArrayList<Item> ();
}
/**
* Add an item to the database.
*/
public void addItem (Item theItem)
{
items.add (theItem);
}
...
New
Database
source
code
avoids code
duplication in
client!
list
}
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24
New Database source code
/**
* Print a list of all currently stored CDs and
* DVDs to the text terminal.
*/
public void list()
{
for(Item item : items) {
item.print();
// Print an empty line between items
System.out.println();
}
}
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Subtyping
First, we had:
public void addCD (CD theCD)
public void addVideo (DVD theDVD)
Now, we have:
public void addItem (Item theItem)
We call this method with:
DVD myDVD = new DVD (...);
database.addItem (myDVD);
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Subclasses and subtyping
• Classes define types.
• Subclasses define subtypes.
• Objects of subclasses can be used
where objects of supertypes are
required.
(This is called substitution.)
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Subtyping and assignment
subclass objects
may be assigned
to superclass
variables
Vehicle v1 = new Vehicle();
Vehicle v2 = new Car();
Vehicle v3 = new Bicycle();
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Subtyping and parameter
passing
public class Database
{
public void addItem(Item theItem)
{
...
}
}
DVD dvd = new DVD(...);
CD cd = new CD(...);
subclass objects
may be passed to
superclass
parameters
database.addItem(dvd);
database.addItem(cd);
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Object diagram
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Class diagram
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Polymorphic variables
• Object variables in Java are
polymorphic.
(They can hold objects of more than one type.)
• They can hold objects of the
declared type, or of subtypes of the
declared type.
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Casting
• Can assign subtype to supertype.
• Cannot assign supertype to subtype!
Vehicle v;
Car c = new Car();
v = c; // correct;
c = v; // will not compile
• Casting fixes this:
c = (Car) v; // compiles OK
(run-time error if v is not a Car!)
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Casting
• An object type in parentheses.
• Used to overcome 'type loss'.
• The object is not changed in any
way.
• A run-time check is made to ensure
the object really is of that type:
– ClassCastException if it isn't!
• Use it sparingly.
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The Object class
All classes inherit
from Object.
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Polymorphic collections
• All collections are polymorphic.
• We can have collections of <Object>.
public void add (Object element)
public Object get (int index)
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Collections and primitive
types
• All objects can be entered into such
collections ...
• ... because such collections accept
elements of type Object...
• ... and all classes are subtypes of
Object.
• Great! But what about simple types?
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Wrapper classes
• Primitive types (int, char, etc) are not
objects. To collect them, they must be
wrapped into an object!
• Wrapper classes exist for all simple types:
simple type
int
float
char
...
wrapper class
Integer
Float
Character
...
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Wrapper classes
int i = 18;
Integer iwrap = new Integer(i);
...
int value = iwrap.intValue();
wrap the value
unwrap it
In practice, autoboxing and
unboxing mean we don't
often have to do this.
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Autoboxing and unboxing
private ArrayList<Integer> markList;
…
public void storeMark (int mark)
{
markList.add (mark);
autoboxing
}
int firstMark = markList.remove (0);
unboxing
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Autoboxing and unboxing
private ArrayList<Integer> markList;
…
public void storeMark (int mark)
{
markList.add (new Integer (mark));
}
so we don’t have to write this
int firstMark = markList.remove(0).intValue();
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Review so far ...
• Inheritance allows the definition of classes
as extensions of other classes.
• Inheritance
–
–
–
–
avoids code duplication
allows code reuse
simplifies the code
simplifies maintenance and extending
• Variables can hold subtype objects.
• Subtypes can be used wherever supertype
objects are expected (substitution).
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More about inheritance
Exploring polymorphism
(Chapter 9)
Main concepts to be covered
•
•
•
•
•
method polymorphism
static and dynamic type
overriding
dynamic method lookup
protected access
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The inheritance hierarchy
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Conflicting output
What we want
CD: A Swingin' Affair (64 mins)*
Frank Sinatra
tracks: 16
my favourite Sinatra album
DVD: O Brother, Where Art Thou? (106 mins)
Joel & Ethan Coen
The Coen brothers’ best movie!
What we have
title: A Swingin' Affair (64 mins)*
my favourite Sinatra album
title: O Brother, Where Art Thou? (106 mins)
The Coen brothers’ best movie!
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The problem
• The print method in Item only
prints the common fields.
• Inheritance is a one-way street:
– A subclass inherits the superclass fields.
– The superclass knows nothing about its
subclass’s fields.
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Attempting to solve the
problem
• Place print where it
has access to the
information it needs.
• Each subclass has its
own version.
• But Item’s fields are
private.
• Database cannot find
a print method in
Item.
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Static type and dynamic type
• A more complex type hierarchy needs
further concepts to describe it.
• Some new terminology:
– static type
– dynamic type
– method dispatch/lookup
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Static and dynamic type
What is the type of c1?
What is the type of v1?
Car c1 = new Car();
Vehicle v1 = new Car();
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Static and dynamic type
• The declared type of a variable is its
static type.
• The type of the object a variable
refers to is its dynamic type.
• The compiler’s job is to check for
static-type violations.
for(Item item : items) {
item.print();
// Compile-time error.
}
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Overriding: the solution
print method
in both superand subclasses.
Satisfies both
static and
dynamic type
checking.
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Overriding
• Superclass and subclass define
methods with the same signature.
• Each has access to the fields of its
class.
• Superclass satisfies static type check.
• Subclass method is called at runtime
– it overrides the superclass version.
• What becomes of the superclass
version?
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Method lookup
No inheritance or polymorphism.
The obvious method is selected.
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Method lookup
Inheritance but no
overriding. The inheritance
hierarchy is ascended,
searching for a match.
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Method lookup
Polymorphism and
overriding. The ‘first’
version found is used.
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Method lookup summary
The variable is accessed.
The object stored in the variable is found.
The class of the object is found.
The class is searched for a method match.
If no match is found, the superclass is
searched.
• This is repeated until a match is found, or
the class hierarchy is exhausted.
• Overriding methods take precedence.
•
•
•
•
•
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Super call in methods
• Overridden methods are hidden ...
• ... but we often still want to be able
to call them.
• An overridden method can be called
from the method that overrides it.
– super.print (...)
– Compare with the use of super in
constructors.
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Calling an overridden method
public class CD extends Item
{
...
public void print ()
{
super.print ();
System.out.println ("
" + artist);
System.out.println ("
tracks: " +
numberOfTracks);
}
...
}
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Method polymorphism
• We have been discussing polymorphic
method dispatch.
• A polymorphic variable can store
objects of varying types.
• Method calls are polymorphic.
– The actual method called depends on
the dynamic object type.
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The Object class’s methods
• Methods in Object are inherited by
all classes.
• Any of these may be overridden.
• The toString method (of Object)
is commonly overridden:
– public String toString ()
– Returns a string representation of the
object.
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Overriding toString
public class Item
{
...
public String toString ()
{
String line1 = title +
" (" + playingTime + " mins)");
if (gotIt) {
return line1 + "*\n" + "
" +
comment + "\n");
} else {
return line1 + "\n" + "
" +
comment + "\n");
}
}
...
}
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Overriding toString
• Explicit print methods can often be
omitted from a class:
System.out.println (item.toString());
• Calls to println with just an object
automatically result in toString
being called:
System.out.println (item);
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Protected access
• Private access in the superclass may be too
restrictive for a subclass.
• The closer inheritance relationship is
supported by protected access: protected
things (fields, constructors, methods, etc.)
may be used by sub-classes.
• Protected access is more restricted than
public access.
• We still recommend keeping fields private.
– Define protected accessors and mutators for
sub-classes to use.
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Access levels
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Review
• The declared type of a variable is its static
type:
– Compilers check static types.
• The type of an object is its dynamic type:
– Dynamic types are used at runtime.
• Methods may be overridden in a subclass.
• Method lookup starts with the dynamic
type.
• Protected access reflects inheritance.
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