Wednesday, January 31, 2007

Lecture 11

Inheritance

  • Objects that inherit features of a parent class can be thought to contain an object of the parent type
  • all methods and fields of the parent are "inherited"

Best Practices With Inheritance
  • discard packaging
  • make all instance fields private
    • this means instance fields are not visible to child methods
    • if you need to access instance fields of the parent from the child use the public interface

Why Do This?
  • child inherits parent's methods and therefore inherits the public interface
    • this simplifies access
    • and prevents privacy problems via inheritance

public class Parent
{
protected mutable class x;
}

public class Child extends Parent
{public mutable class leakyMethod()
{return m;
}
}

Constructors
  • form part of the public interface

public class Child extends Parent
{
public child ( ...parameters for child / parent )
//call parent constructor
super(...parameters for parent)
//assigns child parameter
}


Overriding Methods

  • Child classes inherit all methods from the parent
  • child has the option to re-implement these methods
    • called overriding

Example
  • All objects extend type Object by default

public class MyClass [extends Object]
[
  • type object contains several default methods
    • equals(), toString(), clone()
]

  • declaration of an overridden method in the child must be exactly the same as the parent
    • same return type
    • same parameter type
    • same number of parameters

Parent

public int myMethod(double a, int b)

Child

public int myMethod(double a, int b)

  • if myMethod is overridden in a child class, use 'super.myMethod' to gain access to the parent method
  • to prevent overriding use 'final'


Abstract Classes

  • a student is either graduate or undergraduate
    • there is no 'plain' student
  • so it doesn't make sense to instantiate this class
  • all students have letter grades
  • child classes of student should be forced to implement getLetterGrade()

Solution

  • declare Student an abstract class
    • can't instantiate an object of type student
  • declare getLetterGrade() an abstract method within abstract class student
  • children must override getLetterGrade() or they will be abstract classes themselves

public abstract class Student
{
.
.
.
public abstract String getLetterGrade();

}

14 comments:

George said...

Thank you
I appreciate your blog

Anonymous said...

Can simeone explain Copy Constructor and Deep COpy??
How are they used?
ThanX

Jake deVos said...

http://jacobdevos.blogspot.com/2007/01/lecture-8.html
lecture 8 explains it all ...
have you written the test yet?
if not.. that's a big part of it

Anonymous said...

ThanX Jake,
no, i'll write the test on Friday.

Anonymous said...

Do you know which examples from Andrew's blog are related to Deep Copy and Copy Constructor?

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