The nightmare begins'''

1
Chapter 2

• The nightmare begins...

2
Warning!

• This lecture will be, by far, the most difficult
  lecture of the class. There is a lot of material
  here that youve never seen, so it may take you
  two or three times looking at it before you
  understand it!
• Peace,
• -Jeff-

3
Hierarchy

• Problem Company has different types of
  employees - Full-time, Retired, Others
• Create class hierarchy for this

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One Solution
Employee
Full-Time
Retired
Other
This is bad, because an employee can change
status while at the company! Youll have to
change the object that the employee is!
5
Better Solution
Employee id and status
Status
Full-Time
Retired
Other
6
Class Template

• keywords class MyClass extends Superclass
  implements Interface, Interface, Interface
• // class variables
• // instance variable
• // static initilization
• // constructors
• // class methods
• // instance methods

7
Method Template

• keywords return-type name pars
  exceptions
• //local vars
• // implementation

8
Packages

• To work with a bunch of class all at once, JAVA
  allows you to group into packages.
• Example java.awt is a package
• Can define stricter access control
• Easier to reuse common names
• Collect classes so they can be shared more easily

9
Access with Packages

• All classes that you create are put into a
  default package (whether you like it or not)
• If not specified, all classes, methods, and
  attributes have default access
• This means classes in the same package can access
  each others variables/method except private

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Packages

• Classes outside the package can only see public
  classes, and only see public or protected
  members/methods
• Keywords public and protected do not affect
  access to classes in the same package, but
  private does.
• There is no such thing as a private class or
  protected class its either public or default

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In the SAME package

• Public class Default class
• public member yes yes
• protected member yes yes
• private member no no
• default member yes yes

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Outside the package

• Public class Default class
• public member yes no
• protected member yes no
• private member no no
• default member no no
• if the accessing class is a subclass

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Packages

• Can reuse name if in different packages
• Specify which class you want by full name
• java.awt.Button
• jeff.myPackage.Button
• Use import statement to bring in a package

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Names

• You can do something like this
• java.awt.Button myButton
• myButton new java.awt.Button (text)

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Creating a Package

• Use the package keyword
• MUST BE THE FIRST THING - Even before import
  statements!!!!
• Example package jeff.myPackage
• This would create a directory jeff with a
  subdirectory myPackage.
• The class would be put in /jeff/myPackage/
• Import with import jeff.myPackage.

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Example

• package jeff// This MUST be the first thing
• import java.awt.Button // Any number of
  //import statements
• import java.util.Vector
• class X

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Keywords

• public - been there, done that
• private - that too
• protected - yeah, yeah
• synchronized - well see this later with Threads
• native - means method is defined in something
  like C/C. Only used for methods

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Other keywords

• final - has three different meanings
• members they cannot change after it is defined.
  final float PI 3.14159
• method the method cannot be overriden by
  subclasses. final void bark ( )
• class if class is final, it cannot be subclassed

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Other Keywords

• static - two different meanings
• if variable is declared static, then there is
  only one variable for all instances! All
  instances share one variable (before everyone had
  their own copy)
• if method is static, then you dont need to
  create an instance to use that method! Example
  Math.sin (45)

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Other Keywords

• transient - for variables only. Object cannot be
  written to a file or over the internet. For
  sensitive data, like credit card numbers.
• transient vars cant be final or static!
• volatile - not on exam, but used for variables
  that are in multiprocessor environments (can
  change asynchronously)

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Other keywords

• abstract - used on methods/classes. If a method
  is abstract, it does not implement the code, but
  lets the subclass do it!
• If method is abstract, class must be abstract as
  well
• Only used in inheritance
• example Number (superclass of Byte, Short,
  Float, Double, Integer, etc)

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Abstract Class Example

• abstract class Bob
• final int myInt 17
• abstract void bark ( )
• Note You cannot create one of these! It can
  only be subclassed!

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Last Difficult Thing

• We can cast from one data type to another
• int myInt float myFloat 6.4
• myInt (int) myFloat
• // myInt will now contain the value 6
• We can do the same thing with classes, up and
  down the class hierarchy

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Assume the following

• class Parent
• class Child1 extends Parent
• class Child2 extends Parent
• class Main
• public static void main (String args )
• Child1 c1 new Child1 ( )
• Child2 c2 new Child2 ( )
• Parent p new Parent ( )
• // Below, remember that the data type on the
  right of the assignment has to be the same as
• // the left!
• p c1 // Allowed, because c1 is of type
  Parent through inheritance
• c1 p // Not allowed p is not of type
  Child1
• c1 (Child1) p // Allowed - type cast (will
  compile) but wont run
• c2 c1 // Not allowed - only cast up and
  down the hierarchy
• c2 (Child2)c1 // Still not allowed - wont
  even compile

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Danger!

• Just because something compiles when casted
  doesnt mean it will work when it runs.
• Working example
• Parent p new Child1 ( )
• c1 (Child1)p

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Interfaces

• An interface defines a set of abstract constants
  and methods.
• Does not define the implementation (i.e. code)
• Very much like an abstract class
• Overcomes single inheritance

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Interface example

• public interface WildAndFunky
• public final int myConst 42
• public boolean pass ( )
• public int doSomething ( )

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How to Clone an Object

• If you need to make a copy of an object, you must
  invoke its clone ( ) method.
• Not all classes can be cloned, only ones that
  implement Cloneable interface
• Doesnt call the objects constructor!
• Button b1, b2 b1 new Button (Smile)
• b2 b1.clone( ) // Copies all of b1 info into b2

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Static Initialization

• Very Powerful for showing status of loading the
  applet/application
• These things execute while the class is loading
  (before anything else)
• Use keyword static and curly braces
• static
• System.out.println (Class is loading)

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Example

• class Bob
• Bob ( )
• System.out.println (Yo)
• static
• System.out.println (Dude!)
• public static void main (String args )
• Bob b1 new Bob ( )