Java Summary

1
Java Summary

• OOP
• classes, data and methods
• definition and use
• public and private
• inheritance - extends keyword
• method overloading
• method overriding
• threads and dynamic memory
• libraries

2
Objects

• OOP - object oriented programming
• code built from objects
• Java these are called classes
• Each class definition is coded in a separate
  .java file
• Name of the object must match the class/object
  name

3
Classes

• eg. Fruit.java contains the lines
• class Fruit
• int grams
• int cals_per_gram

4
Methods ...

• Class Fruit
• int grams
• int cals_per_gram
• int total_calories()
• return(gramscals_per_gram)
• brackets () signify a method rather than data

5
Using objects

• Here, code in one class creates an instance of
  another class and does something with it
• Fruit plumnew Fruit()
• int cals
• cals plum.total_calories()
• Dot operator allows you to access (public)
  data/methods inside Fruit class

6
Public/private

• Methods/data may be declared public or private
  meaning they may or may not be accessed by code
  in other classes
• Good practice
• keep data private
• keep most methods private
• well-defined interface between classes - helps to
  eliminate errors

7
Creating objects

• Following code creates an instance of the Fruit
  class
• Fruit plum
• defines the plum object
• plum new Fruit()
• creates it in memory
• the content of the Fruit class must be defined in
  another file Fruit.java

8
Constructors

• The line
• plum new Fruit()
• invokes a constructor method with which you can
  set the initial data of an object
• You may choose several different type of
  constructor with different argument lists
• eg Fruit(), Fruit(a) ...

9
Overloading

• Can have several versions of a method in class
  with different types/numbers of arguments
• Fruit()grams50
• Fruit(a,b)
• gramsacals_per_gramb
• By looking at arguments Java decides which
  version to use

10
Inheritance ...

• Important feature of OOP - new classes can be
  based on existing classes eg. Could define a
  specialized type of Fruit class called Citrus
• Has all methods of Fruit plus possibly some new
  ones eg
• class Citrus extends Fruit
• void squeeze().

11
Inheritance II

• How to use
• eg.
• Citrus lemon new Citrus()
• lemon.squeeze()
• lemon.total_calories()
• old methods exist alongside new methods

12
Overriding

• Even more powerful concept of OOP
• can override the functionality of one method in a
  descendant class
• eg. Add method peel() to Fruit class. Since
  Citrus extends Fruit this method will also be
  available to an instance of Citrus
• But can redefine content of peel() inside of
  Citrus - the new definition hides the earlier ...
  

13
Threads ...

• Java allows code in some objects to be run like a
  separate program - it is given its own thread of
  execution
• eg. Runner new Thread(sim)
• sim is name of class containing a run() method
  which is responsible for eg a simulation
• class sim implements Runnable ..run()

14
Libraries

• Java comes with libraries for creating GUIs and
  network applications and for embedding in Web
  pages- java.applet.Applet
• eg import java.awt.
• compile to byte code - can be run by any system
  with a Java interpreter - portable!
• Relatively robust and secure