Java程序辅导

1Object Oriented
Programming
1
Lecture 2
Agenda
„ Object Oriented Programming in Java
„ Lecture & Lab
„ Do the OO Selftest in WebCT!
„ Don’t forget to read Chapter 1 and 2!
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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What is Java?
„ An object-oriented programming language
„ Designed by SUN in the 1990s
„ Inventor James Gosling 
„ Idea: Programming language for consumer electronics
„ Design
„ Write once run everywhere (WORA) i e platform
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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 ,   , . .,  
independence 
„ Like C/C++ but much simpler
„ Fully object oriented and has strict type checking
„ Built-in and enforced exception handling
„ No Pointers!
„ Success Story:
„ Became internet programming language 
(applets, servelets, etc.)
2How does Java run
„ Java runs within a virtual machine (JVM)
„ Virtual machine needs to be provided for the 
platform (e.g., Windows XP, Linux, OS X,..) 
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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myClass.java myClass.class JVM
javac java
Example program
„ Print “Hello World!”
public class Example {
public static void main(String[] args){
System.out.println(“Hello World!”);
}
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„ Names are case sensitive
„ main() method is entry point of application
„ Compile program with javac Example
„ Run program with java Example
}
Classes and objects
„ Class defines the structure and behavior of objects
„ Objects are instances of a class
„ stored in memory and hold values
„ methods can be invoked for an object    
bli l {
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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pu c c ass Car 
string Type; 
void moveTo(string Loc);
}
Name: “KIA”Name: “VW”
Class:
Objects:
3Class
„ A class defines a new type
„ Declaring instance variables defines structure
„ Declaring instance methods defines behavior
public class Car { St t
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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string Type; 
voit setType(string type);
void moveTo(string Loc);
}
Behavior
ruc ure
Creating Objects
„ A new instance of a class is created with 
new (params,…);
returns a new instance of class .
„ Often called instantiation.
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„ Example
„ new Car();
„ new Car(“KIA”);
Constructor
„ Constructor is a special method in a class
„ Using new invokes class’ constructor
„ It must be the same name as class
„ No return type specified 
„ Generally used to initialize member variables
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„ A default constructor is supplied
public class Car {
public Car() {…}
public Car(String type) {…}
}
4Instance Methods
„ A methods of a class
„ Instance of the class can perform the method
„ Method can only be performed by an instance 
„ Methods
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„ provide functionality to the user (other code)
„ can directly access instance variables and 
invoke other instance methods
„ Rule of thumb: user should not access 
variables directly.
Example: 
Instantiation & Method Call 
// create instance of class Car 
Car myCar = new Car();
// invoke method
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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myCar.moveTo(“Cairns”);
this
„ Methods can refer to the object itself with this
„ Can be used to disambiguate and clarify
„ Example:
class Car { 
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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…
public boolean faster(Car other) {
return (this.maxSpeed > other.maxSpeed);
}
…
}
5null
„ Object reference null
„ reference value that does not point to an 
object
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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Car obj;
…
// if obj is null, instantiate new object
if (obj == null) {
obj = new Car();
}
Object Destruction
„ Programmer does not need to de-allocate objects
„ Java uses garbage collection
„ Garbage collector finds unreferenced objects and 
reuses memory
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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// create instance of class Car 
Car myCar = new Car();
// invoke method 
myCar.moveTo(“Cairns”);
// indicate GC that myCar is not used
myCar = null;
Inheritance
„ Sometimes one entity type includes some objects 
which belong to a more restricted entity type as well
„ subclass object “IS-A” superclass object
„ superclass generalizes subclass
„ A class can inherit feature of a superclass
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„ The original class is called superclass
„ The extended class is called subclass
„ Inheritance is a fundamental concept in OO
„ Existing code can be reused
„ New classes can be defined by their differences to 
existing classes
6Examples
„ Examples
„ Student “is-a” Person
„ ChequeAccount “is-a” Account
„ Fridge “is-a” ElectricalAppliance
class Student extends Person {
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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…
}
class ChequeAccount extends Account { 
…
}
Inheritance (cont’d)
„ A class can only extend one parent
„ Implicitly extend Object if nothing else is 
declared
„ Subclass has all instance variables and methods of 
th t
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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e paren
„ Plus new instance variables
„ Plus new methods
„ Methods of the superclass can be overwritten!
extends example
class ElectricalAppliance 
{
protected int energyRating;
// other fields
public void switchOn() 
{
class Fridge extends 
ElectricalAppliance
{
protected int 
freezerCapacity;
// other added fields
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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// code body here
}
// more methods
}
public void defrost()
{
// code body here
}
// other added methods
}
Fridge implicitly has 
protected int energyRating
public void switchOn()
7UML diagrams
„ UML notation for conceptual 
models (or for OO code 
structure)
„ Draw each class as a 
rectangle
„ Inheritance is arrow with
ElectricalAppliance
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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triangle head
„ From subclass to 
superclass
Fridge WashingMachine
Access modifiers
„ The subclass object has inherited fields and methods
„ Can they be used?
„ use is controlled by access modifiers
„ private
„ public
protected
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„
„ (default) [no modifier stated]
„ To be used in added code in the subclass, make fields 
and methods in superclass either protected or public
Access modifiers
„ public: can be used from anywhere provided a 
reference exists to an object of the class (or to an 
object of subclass)
„ private: can only be used in the code of the class 
itself
„ But that could be in executing a method of
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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another object instance of the class!
„ protected: can only be used in code in the class, or 
in a subclass
8Subclass Constructor
„ The first statement in any constructor of the subclass 
should be a call to some constructor of the 
superclass
„ Syntax is super(…);
U ll l t t i iti li th
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„ sua y, superc ass cons ruc or n a ses e 
inherited instance variables using a subset of 
the parameters in the subclass constructor
„ Then continue, initialising added instance variables
Points to note for Inheritance
„ Superclass can itself extend another class
„ Example: ElectricalAppliance extends Object
„ Subclass implicitly gets fields and methods in 
superclass including those the superclass inherited
„ Explicit code in added subclass methods can modify
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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or use accessible fields that are inherited, and can 
call accessible methods that are inherited
Hierarchy
„ A class can directly extend only one other 
class 
„ “extends Object” is implicit if nothing else 
is said
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„ Indirectly inherit from superclasses of the 
direct superclass
„ So the classes form a tree based on extends 
relationship
9Warnings I
„ Inheritance should only be used for 
generalisation
„ when subclass object “is-a” superclass object
„ Otherwise use delegation to get code re-use 
ith t li ti
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
25
w ou  genera sa on
„ Example: SportsTimekeeper with 
getCurrentTime() does not inherit from 
Clock
Warnings II
„ Inheritance isn’t appropriate for all generalisation situations
„ when subsets have the same structure of variables, and 
same code,
„ or when object can change between subsets, 
„ or when object can belong to several subsets at once
„ then use one class
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„ with instance variable to indicate subset
„ eg instance variable rank in class Soldier instead of 
separate subclasses for General, Colonel, Private
Subtype definition
„ One type is a subtype of another through a chain of extends
relationships
„ Example: Person, Student
class Person
class Student extends Person
Student is sub-type of Person.
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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Person p;
Student s = new Student(“Fred”);
// assignment possible because Student 
// is subclass of student (and a subtype)
p=s;
// not possible: s=p;  !!
10
Static and Dynamic Type
„ Static Type: 
„ The type with which the variable is declared.
„ Known at compile time.
„ The same for all uses of a variable
„ Dynamic Type:
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„ The type which was used when constructing the object 
that the variable is actually referencing
„ Known only at run time. 
„ Can be different each time a particular line is reached
„ Class or primitive, only
Late binding
„ Given a variable on which a method is called
„ The static type of the variable determines if 
the call is legal
„ But which code is actually executed is 
d t i d b th d i t f th
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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e erm ne  y e ynam c ype o  e 
variable (at runtime, when the call occurs)
Another Example of
Static and Dynamic Type
ElectricalAppliance myEquipment;
myEquipment = new Fridge( ); //(A)
… … … …
ElectricalAppliance onLoan = new WashingMachine();
myEquipment = onLoan; //(B)
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„ myEquipment at (A)
Š Static Type: ElectricalAppliance
Š Dynamic Type: Fridge
„ myEquipment at (B)
Š Static Type: still ElectricalAppliance
Š Dynamic Type: WashingMachine
11
Polymorphism and subtyping
„ Assignment x = v; is syntactically correct provided
static type of v is subtype of static type of x
„ Dynamic type of x after the assignment is dynamic type 
of v
„ Call x.method(); is syntactically correct provided
static type of x has method() in its definition
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„ Directly, or inherited through extends
„ Which code gets executed is determined by dynamic 
type of x
Substitutability
„ The dynamic type of a variable or expression 
must always be either the same as, or a 
subtype of, the static type
„ This ensures that the dynamic type has code 
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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for any method that will be called
„ the compiler checked that the method was 
appropriate for the static type!
Interface
„ Define a type which captures what we can do with 
objects of similar classes
„ It is given as an interface in Java
„ Indicates method signatures (name, argument types, 
return type) but not method body
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„ Note: no constructor!
„ In client code, a variable can be declared with an 
interface as type
12
interface Repairable {
boolean isWorking();
Define an interface
Note: no method body
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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void repair();
}
„ The interface is in a file Repairable.java, just like a 
class
„ Note: interface names often end in “able” since they 
describe what the type can do
Interface as type
„ Declare an instance variable in client code
„ can have interface as its type
„ Example: Repairable r = …;
„ Also use interface for type of local variable, or 
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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method argument 
„ But: 
„ cannot create object of type Repairable
Class implements interface
„ To use a class in contexts where interface is 
the type, you must set up the class to 
implement the interface
„ This puts obligation on code of class
it t h th d i th i t f ith
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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„  mus  ave every me o  n e n er ace w  
exactly the correct signature and with public 
access
„ it can also have more methods and instance 
variables
13
Example
class WashingMachine implements Repairable {
public boolean isWorking() {
// code for this method
}
bli id i () {
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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pu c vo  repa r  
// code for this method
}
// other methods
// instance variables
}
Using an interface
„ Assign to a variable whose type is some interface
„ any object whose type implements the interface
„ note: you can’t construct from the interface, only from 
classes!
„ Call a method on the variable whose type is some
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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interface
„ as long as the method is part of the interface
Example
allowed assignment
WashingMachine w = new WashingMachine();
Repairable r = w;
if (!r.isWorking()) 
{
i ()
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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methods in the Repairable interface
r.repa r ;
}
14
Casting
„ Sometimes there is an variable whose static type is 
an interface, but as programmer you know what the 
dynamic type is 
„ eg because you know which text constructed it
„ You can’t simply call a method of the variable that is
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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missing from the static type interface
„ Instead, cast the variable into its static type
„ then call the method
Casting example
Fridge f = new Fridge();
…
Repairable r = f;
// can’t call r.defrost();
// as defrost() not in Repairable interface
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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Fridge f1 = (Fridge) r;
f1.defrost(); // this is legal
// compressed syntax ((Fridge) r).defrost();
an expression whose static type is Fridge
„ If dynamic type is not what you try to cast it to, you 
get an Exception at run-time
„ Avoid this by checking first
Instanceof() Query
if (r instanceof Fridge) 
{
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
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f = (Fridge) r;
// do things with f
}
else
{
// do something instead
}
15
Thanks!
COMP5028 Object-Oriented Analysis and Design 
© Y. Zhou, and B. Scholz, School of IT, The University of Sydney 
43