Download J4: OOP and

Java Objects and Classes
Overview
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Creating objects that belong to the
classes in the standard Java library
Creating your own classes
Definitions
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Object-oriented Programming
– involves 3 main concepts
– Encapsulation
– Inheritance
– Polymorphism
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Class vs Object (instance of class)
More definitions
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State - current set of values of the
object
Methods - operate on objects; may
change state; state may affect behavior
Inheritance in Java
– a class ‘extends’ another class
– has all the properties and methods of class
extended and new methods and data fields
that apply to new class
Using Existing Classes
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Example: Math
– only encapsulates methods, no data
– do NOT construct a instance of class Math!
– Call methods by using class name
Math.sqrt (x);
Using Existing Classes
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Example: Date
– construct them specifying initial state using
new, then apply methods
Date birthday = new Date();
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Difference between object and object
variable!!!
– birthday refers to an object variable
Date deadline; //object variable
String s = deadline.toString(); //NO!
Need between
deadline =new Date();
or
deadline = birthday
Semantics
Date deadline = new Date();
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Expression new Date() makes an object
of type Date and its value is a reference
to that newly created object. The
reference is then stored in the deadline
object variable.
Can also set deadline = null;
but don’t call any methods for it!
Java and C++ Differences
JAVA
Date birthday;
Date birthday =
new Date();
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Objects live on heaps,
access bad reference,
get error
Auto garbage
collection
Clone to get a copy
C++
Date * birthday;
Date * birthday =
new Date();
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Access bad pointer,
get another memory
location
Destructors necessary
Effort for copy and
assignment
Mutator and Accessor Methods
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Mutator methods change state of object
Accessor methods do NOT.
– In C++ should use const suffix for these,
most of you probably don’t
– no distinction in Java
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Convention:
– mutators prefix method name with set
– accessors use prefix get
Building your own classes
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A complete Java program requires one
class with a main method.
No other classes have a main method
Simplest form
class NameOfClass
{
constructor1
constructor2
Methods
Fields
}
//no ;
Example: ComplexNumber.java
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In main code:
ComplexNumber c = new ComplexNumber ();
ComplexNumber b = new ComplexNumber (4.5,5.5);
ComplexNumber [] cnums = new
cnums[0] = new ComplexNumber
cnums[1] = new ComplexNumber
cnums[2] = new ComplexNumber
ComplexNumber[3];
(5.4, 3.2);
();
(3.0, 4.1);
Using Multiple Source Files
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Put ComplexNumber class in
ComplexNumber.java file
Put ManipCN class (main class) in
ManipCN.java file
Make ComplexNumber class public!
To compile:
javac ManipCN.java
Notes:
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Constructor has same name as class
classes can have more than one
constructor
A constructor may take zero, one or
more parameters
A constructor has no return value
A constructor is called with the new
operator.
//different from C++
Fields
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Final instance fields
– must be set before end of constructor and is
never changed again
private final String name;
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Static fields (class fields)
– only one such field per class, shared among
all instances of the class
private static int nextId = 1;