More on Objects

1
More on Objects
More, more, more on Objects

• CS 102

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Where Do You Want To Go Today?

• Building new objects
• Accessors
• this and That

3
Caution, Objects Under Construction!

• Objects have structure and structure has to be
  built
  
• new means create a new instance of an object
• How are objects actually built?
• Objects know how to do things, including how to
  build themselves

4
Constructor Methods

• Constructors are special methods that build new
  objects
  
• Declaration looks just like a method declaration
  that has no result type
  
• public class Circle
• double x_center, y_center, radius
• public Circle(double x, double y, double r)
• x_center x
• y_center y
• radius r

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A Few Notes on Constructors I

• Don't have to declare a constructor
• Java creates a default constructor
• public class Circle
• public double x_center, y_center, radius
• // Uses the default constructor
• public double Area()
• return 3.141592654 radius radius

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A Few Notes on Constructors II

• Constructors have the same name as the class
• No return type (why not?)
• Constructor declarations are not members
• Never inherited, so no hiding or overriding (More
  on this later)

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Multiple Constructors

• public Circle(double x, double y, double r)
• x_center x
• y_center y
• radius r
• public Circle()
• x_center 0
• y_center 0
• radius 1
• public Circle(double x, double y)
• x_center x
• y_center y
• radius 1

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Handling Multiple Constructors

• How does Java tell the constructors apart?
• Method signatures
• Signatures include
• Name of the method
• Number formal parameters
• Types of formal parameters
• Doesn't include the name of the parameter

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Method Signatures

• For this method
• public Circle(double x, double y, double r)
• Signature is
• Circle(double, double, double)
• Different methods, same signature
• public Circle(double x, double y)
• public Circle(double x, double r)

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Back to Multiple Constructors

• Multiple constructors are okay
• Each constructor has to have a unique signature
• (All of a class's methods have to have unique
  signatures, not just constructors)
  
• Method overloading multiple versions of the same
  method (but with different signatures)

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The Linda Tripp Problem

• How do we keep private data private?
• Avoid having object1 mess with object2's data
• Example Student objects know student's grades,
  but one student shouldn't modify another
  student's grades.
  
• Storing invalid data
• Stuffing 12-hr time into a 24-hr time object

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If It's Public...

• Start with
• public class Circle
• public double x_center, y_center
• public double radius, area
• public Circle(double x, double y)
• x_center x
• y_center y
• radius 1
• area 3.141592654

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other objects can mess with it

• Create a circle
• Circle duPont new Circle(100, 100)
• // Change the area, but leave r the same
• duPont.Area 11
• // Now a circle with radius 1 has area 11

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Using private

• Declare variables to be private
• public class Circle
• private double x_center, y_center
• private double radius, area
• Now only Circle can modify the variables
  (x_center, y_center, ...)

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Granting Access

• Other classes might need to read or write the
  values stored in private members
  
• Create accessor functions
• Set (or settor)
• Get (or gettor)
• Control access through get and set

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Setting and Getting

• What's the difference between using get and set
  methods, and just making everything public?
  
• Two benefits
• Protect instance variables from outside meddling
• Insulates class users from changes in variables

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Example from Fig 6.5
// blah, blah, blah. Set invalid values to zero.
public void setTime( int h, int m, int s )
setHour( h ) // set the hour
setMinute( m ) // set the minute
setSecond( s ) // set the second
// set the hour public void setHou
r( int h ) hour ( ( h 0 h 0 ) // set the minute public void
setMinute( int m ) minute ( ( m 0 m 60 ) ? m 0 ) // set the second
public void setSecond( int s )
second ( ( s 0 s 18
Me, a Name I Call Myself

• How can objects refer to themselves?
• Every object has a built-in reference to itself,
  called this
  
• this can be used only in
• Body of a method or constructor
• Initializer of an instance variable
• Anywhere else is a compile-time error

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This is Bad,

• public class Circle
• public double x, y
• public double r, area
• public Circle(double x, double y)
• x x // This doesn't do much, because we're
• y y // just assigning parameters to
  themselves
  
• r 1
• area 3.141592654

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But this is okay

• public class Circle
• public double x, y
• public double r, area
• public Circle(double x, double y)
• this.x x // Now we're assigning the instance
• this.y y // variables to the parameters
• r 1
• area 3.141592654

Better to avoid this mess altogether by using
different names for data members and method
parameters