CS1502: Introduction to Programming II

1
Classes and Objects
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Classes and Objects
Class describes the form of an object, a
template or blueprint or mold specifies data
representation, behavior, and inheritance (via
variables, methods and parents)
Object an instance of a class has unique copy
of every non-static variable (i.e., the instance
variables but not the class variables).
Difference between a class and an object of
that class is analogous to the difference
between a type and a variable of that type.
Naming Conventions Classes Identifiers begin
with cap letters for each word in
the Identifier, e.g., class NeuralNetwork Objects
Identifiers begins with lower case letter,
then caps for other words in identifier,
e.g., thisObjectIdentifier
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Specification of a Class
class Box int iLength int iWidth int
iHeight public void setLength
(int iNewLength) iLength iNewLength
// of setLength public int getLength ( )
return (iLength) // of getLength
public void setWidth (int
iNewWidth) iWidth iNewWidth // of
setWidth public int getWidth ( )
return (iWidth) // of getWidth
public void setHeight (int
iNewHeight) iHeight iNewHeight
// of setHeight public int getHeight ( )
return (iHeight) // of getHeight
public int getVolume ( ) return (
getLength( ) getWidth( )
getHeight( ) )
// of getVolume // of class Box
Use accessors and modifiers!
4
Usage tip

• Box sampleBox new Box()
• sampleBox.iLength 10
• Legal?
• Preferred
• sampleBox.setLength(10)
• Why?

5
Constructors
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Constructors

• Default provided unless...
• Must have same name as class
• Must not have return type (if you want a
  constructor)
• Multiple constructors typical
• Each must have unique signature
• number, type, order of parameters

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Example

• class Box
• int iLength
• int iWidth
• int iHeight
• public Box()
• // Default constructor
• public Box(int iL, int iW, int iH)
• iLength iL
• iWidth iW
• iHeight iH
• // Box Constructor
• // class definition continues...

Comments omitted for clarity?
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Example

• class Box
• int iLength
• int iWidth
• int iHeight
• public Box()
• this(0, 0, 0)
• // I can even do this...
• public Box(int iL, int iW, int iH)
• iLength iL
• iWidth iW
• iHeight iH
• // Box Constructor
• // class definition continues...

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Creating Objects or Instances
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Creating Instances of Classes
Involves three things 1. Creating the
reference Box thisBox 2.
Instantiating the object thisBox new
Box( ) OR do first two steps at once, e.g., Box
thisBox new Box( ) 3. Having constructor(s)
set initial values public Box (int
iNewLength, int iNewWidth, int iNewHeight)
setLength (iNewLength) setWidth
(iNewWidth) setHeight (iNewHeight) // of
constructor With an appropriate constructor, we
can do all three at once Box thisBox new Box
(10, 5, 25)
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If you just understand one thing...

• ...it should be what we mean by a reference.
• Every object created in Java is created in the
  dynamic area of memory (heap).
• The creation operation involves the return (to
  you) of the location of the object. This is known
  as a reference. It is the same idea as a pointer.
• Box someBox // This creates a reference
• someBox new Box(10, 20, 30)
• // We create the new box and stick its
• // reference in someBox

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Objects and References
Distinguish between primitives and
objects. Assignment with Primitives
Code Memory
int x
x
y
x
int y
5
x
y
x 5
y x
5
5
x
y
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Objects and References
Assignment with References to Objects
Code
Memory
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Questions?
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Parameters
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Our Next Real Challenge

• Java only has in parameters
• public void swap(int a, int b)
• int temp
• temp a
• a b
• b temp
• We can return values from functions but this
  wont be enough.
• When a parameter is a reference to an object
• We can access the objects methods
• We can make persistent changes

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Pop Quiz!

• class Pop
• public static void swap(int x, int y)
• int t
• t x
• x y
• y t
• // swap
• public static void main(String args)
• int a 7
• int b 99
• swap(a, b)
• System.out.println("a " a " b " b)
• // main
• // Pop

What prints?
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The pseudocode for our Example

• algorithm Compute_Grade
• // Calculates the course grade based
• // on the students assignments
• prog_avg, quiz_avg, lab_avg, exam_score,
• grade_avg isoftype Num
• Get_Data(prog_avg, quiz_avg,
• lab_avg, exam_score)
• grade_avg lt- Average(prog_avg, quiz_avg,
• lab_avg, exam_score)
• Output_Grade(grade_avg) // Outputs letter grade
• endalgorithm

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The pseudocode for Get_Data

• procedure Get_Data (prog, quiz, lab,
• exam isoftype out Num)
• // Purpose Prompts user for data returns them
• // Pre-condition none
• // Post-condition passes values back to point of
  call
• print(Enter your program average)
• read (prog)
• print (Enter your quiz average)
• read (quiz)
• print (Enter your lab average)
• read (lab)
• print (Enter your exam score)
• read (exam)
• endprocedure //Get_Data

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How do we do this in Java?

• Make a Grade class which contains a single grade
  value and some accessor/modifier methods.
• Figure out how to do Get_Data
• Pass in references to grade objects.
• Get necessary values from user
• Use grade objects methods to set values

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Java code for Grades Class
public class Grade String name double
value public Grade(String newName)
name newName // Constructor public void
setValue(double v) value v public
double getValue() return value public
void setName(String n) name n public
String getName() return name // Grade
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Java code for Main Algorithm
class Compute_Grade ltother stuffgt /
Calc course grade based on the assignments / pub
lic static void main(String args)
double grade_avg Grade prog new
Grade(Program) Grade quiz new
Grade(Quiz) Grade lab new
Grade(Lab) Grade exam new Grade(Exam)
get_Data(prog, quiz, lab, exam)
grade_avg average( prog, quiz, lab, exam )
output_Grade( grade_avg ) // end of
class Compute_Grade
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Java code for Get_Data
// Still inside the class Compute_Grade /
Purpose Prompts user for data returns them
Pre-condition none Post-condition passes
values back to point of call / public static
void get_Data(Grade g1, Grade g2, Grade g3, Grade
g4) g1.setValue( IOGadget.readDouble("Enter
g1.getName() ) g2.setValue(
IOGadget.readDouble("Enter g2.getName() )
g3.setValue( IOGadget.readDouble("Enter
g3.getName() ) g4.setValue(
IOGadget.readDouble("Enter g4.getName()
) //Get_Data
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Questions?
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Generic Classes
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All Container Classes you build should be Generic

• Generic classes are almost trivial to build
• Every class is derived from the class Object
• Make this the type of the contained data
• An object of any type can then be put into the
  container
• You have to cast the object on output to its
  specific class

• The almost
• Primitive data types int, char, double dont
  work like this
• your first, trivial example programs need a
  little TLC
• each atomic data element has a corresponding
  wrapper class for this purpose
• e.g. the Integer class is an Object-type
  container for an int.

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The Object Class provides Basic Methods

• clone()
• Creates a clone of the object.
• Used to make a deep copy
• equals(Object)
• Compares two Objects for equality.
• getClass()
• Returns the Class of this Object.
• hashCode()
• Returns a hashcode for this Object.
• Used for indexing this object with others
• toString()
• Returns a String that represents the value of
  this Object.
• Used implicitly by print() to stringify the
  object
• And some others well deal with later, perhaps.

Why?
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Overloading Objects Methods

• Each of these methods does something generic.
• However, with the exception of getClass(), they
  should all be overloaded when you write a
  specific class
• consider them as virtual methods
• should be implemented specifically in derived
  classes
• generic behavior is not usually what you want
• e.g. hashCode() will be unique to every instance
  of class Object some representation of its
  memory address.
• However, you probably want objects with identical
  content to return the same hashCode.

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Example of a Linked-List Node
Pseudocode LL_Node isoftype record data
isoftype ltgeneric typegt next isoftype Ptr toa
LL_Node endrecord // LL_Node
data
next
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First a Node
public class Node private Object
data public Node( Object d ) setData( d
) // Constructor public void setData( Object
d ) data d public Object getData()
return data public String toString()
return ( data.toString() ) public Object
clone() return new Node( data ) public
int hashCode() return data.hashCode()
Ok?
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Subtle Bug
// public class Node (continued) public static
void main(String args) Node n new
Node("Hello!") System.out.println("Should be
Hello " n) n new Node(null) System.out.
println("Should be null " n) // main //
Node
Should be Hello Hello! Exception in thread
"main" java.lang.NullPointerException at
Node.toString(Node.java10) at
java.lang.String.valueOf(String.java1911)
at java.lang.StringBuffer.append(StringBuffer.jav
a365) at Node.main(Node.java25)
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First a Node
public class Node private Object data public
Node( Object d ) setData( d ) //
Constructor public void setData( Object d )
data d public Object getData() return
data public String toString() if(data
null) return "null" else return
data.toString() public Object clone()
return new Node( data ) public int
hashCode() // need to deal with potential
null here too return data.hashCode()
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Now the Linked-List Node
class LLNode extends Node private LLNode
next public LLNode( Object d, LLNode n )
super( d ) set_next( n ) public void
setNext( LLNode n ) next n public LLNode
getNext() return next public String
toString() return (" data "
super.toString() " next " next "
" ) // LLNode
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Questions?
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