Inheritance

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Lecture 1

• Inheritance
• Files
• Exceptions
• Collections

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Class and Inheritance

• Aristotle was probably the first to begin a
  careful study of the concept of type he spoke of
  the class of fishes and the class of birds.
• The idea is all objects, while being unique,
  are also part of a class and have characteristics
  (data)and behaviors (methods) in common
• So, what we do in object-oriented programming is
  create new data types.
• But object-oriented programming languages use the
  class keyword. When you see the word type
  think class and vice versa

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In the top of java class hierarhy
Classes can be derived from classes that are
derived from classes that are derived from
classes, and so on
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An object have an interface

• Once a class is established, you can make as many
  objects of that class as you like, and then
  manipulate those objects to solve the problem.

Light lt new Light() lt.on() lt.dim()
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• An other way to reuse a class is to place an
  object of that class inside a new class. We call
  this creating a member object.
• public class Lamp
• Light lt
• Color color
• // other

UML
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Or by Inheritance

• A type does more than describe the constraints on
  a set of objects it also has a relationship with
  other types.
• Two types can have characteristics and behaviors
  in common, but one type may contain more
  characteristics than another

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Abstract classes

• Often you want the base class to present only an
  interface for its derived classes. You dont want
  anyone to actually create an object of the base
  class, only to upcast to it so that its interface
  can be used.
• This is accomplished by making that class
  abstract using the abstract keyword.
• If anyone tries to make an object of an abstract
  class, the compiler prevents them. This is a tool
  to enforce a particular design.
• To override a function, you simply create a new
  definition for the function in the derived class.
  Youre saying, Im using the same interface
  function here, but I want it to do something
  different for my new type.

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Interface

• In an interface is a reference type, similar to a
  class, that can contain only constants, method
  signature. !
• ! There are no method bodies.
• ! Interfaces cannot be instantiatedthey can
  only be implemented by classes or extended by
  other interface
• public inteface Moveable
• public void moveX()
• public void moveY()
• public void jump()

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Implement interfaces

• public class Boll implements Movable
• // variables and methods to define a boll-object
  but also
• public void moveX() your code to move boll X
  
• public void moveY() your code to move boll Y
• public void jump() your code to jump boll
• public class Mask implements Movable
• // variables and methods to define a mask-object
  and
• public void moveX() your code to move mask X
  
• public void moveY() your code to move maskY
• public void jump() your code to jump mask

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I/O Streams , java.io.

• Byte Streams handle I/O of raw binary data.
• Character Streams handle I/O of character data,
  automatically handling translation to and from
  the local character set.
• Buffered Streams optimize input and output by
  reducing the number of calls to the native API.
• Scanning and Formatting allows a program to read
  and write formatted text.
• Data Streams handle binary I/O of primitive data
  type and String values.
• Object Streams handle binary I/O of objects.

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Byte Streams (returns -1 to in

• Returns -1 to indicate the end of the stream
  (file)
• Always Close Streams
• Closing a stream when it's no longer needed
  is very important so important that CopyBytes
  uses a finally block to guarantee that both
  streams will be closed even if an error occurs.
  
• When Not to Use Byte Streams CopyBytes seems like
  a normal program, but it actually represents a
  kind of low-level I/O that you should avoid.
  Since data.txt contains character data, the best
  approach is to use character streams. There are
  also streams for more complicated data types.
  Byte streams should only be used for the most
  primitive I/O. So why talk about byte streams?
  Because all other stream types are built on byte
  streams.

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Character streams

• FileReader inputStream null
• FileWriter outputStream null
• try
• inputStream new FileReader(data.txt")
• outputStream new FileWriter(chardata.txt")
  
• Returns -1 to indicate the end of the stream
  (file)

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Line I/O use character streams

• BufferedReader inputStream null
• PrintWriter outputStream null
• try inputStream new BufferedReader( new
  FileReader(filnamn"))
• outputStream new PrintWriter(new
  FileWriter(filnamn"))
• Returns null to indicate the end of the file (EOF)

Se examples http//javaalmanac.com/egs/java.io/pk
g.html
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Write och read object

• If the class implements the interface
  Serializable.
• class Account implements Serializable
  Person holder double balance
• ObjectOutputStream oos new ObjectOutputStream(
  new FileOutputStream("Objects.dat
  "))
• Account general new Account( fred, 110.0) //
  create obj.
• oos.writeObject( general ) // write the obj.

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What is exception ?

• Exceptions provide the means to separate the
  details of what to do when something out of the
  ordinary happens from the main logic of a
  program.
• In traditional programming, error detection,
  reporting, and handling often lead to confusing
  spaghetti code.
• Example pseudocode method here that reads an
  entire file into memory.

readFile open the file determine its size
allocate that much memory read the file into
memory close the file
What happens if the file can't be opened? What
happens if the length of the file can't be
determined? What happens if enough memory can't
be allocated? What happens if the read fails?
What happens if the file can't be closed?
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What happend ?

• When an error occurs, the method creates an
  object, an exception object and throw it to the
  runtime system.
• The runtime searches the call stack for a method
  that contains a block of code that can handle the
  exception

Method call stack
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What happend?

• If the runtime system exhaustively searches all
  the methods on the call stack without finding an
  appropriate exception handler, catch statement,
  the runtime system (and, consequently, the
  program) terminates in a wrong way.

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In practice

• public void printArray(int lista)
• if (lista.length 0)
• throw new EmptyArrayException() ????
• else
• do print

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Tre types of exceptions

• unchecked Runtime exception
• NullPointerException, ArithemticException,
  NumberFormatException
• checked must be handled by the programmer
• errors a programmer can not anticipate this
  type of exceptions

Se http//java.sun.com/docs/books/tutorial/essent
ial/exceptions/advantages.html
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In practice

• try
• to do this or
• catch (ExceptionType1 name)
• handle this type on ex.
• catch (ExceptionType2 name)
• handle this type on ex.
• finally
• The runtime system always executes the
  statements within the finally block regardless of
  what happens within the try block

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Creating your own exceptions
public class EmptyArrayException
extends Exception public
EmptyArrayException (String s) super(
s)
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Propagate exceptions

• public void method ( ) throws IOExceptions,
  AnyExceptionType
• // do not handle exception just propagete it