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Module Administration
• 10 lectures (2 hours each): Tuesday 09:00 (LTB 7)• 10 labs (2 hours each) Tuesday/Wednesday (Lab 1) • 2 assignments:
– Assignment 1 (10%) due 4th November (week 6)– Assignment 2 (20%) due 7th December (week 11)
• Test (10%) on 10th November (week 7)• Two hour exam in May/June (60% of the module credit)• GLAs: Ludvig Kihlman (lzkihl) / Youngjae Song (syoungb)• Module area: http://orb.essex.ac.uk/CE/CE203/
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Acknowledgements
• A lot of the material was prepared by Mike Sanderson• Some parts were prepared by Martin Waite• Additional material by me
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Recommended Reading
There are many books that cover the material in this course (see last year’s recommendations in CE152) – I will refer to
Java How To Program (9th ed.), H.M. Deitel & P.J.Deitel (Prentice Hall, 2012)Introduction to Java Programming (8th ed.), Y.D. Liang (Pearson, 2011)
If you choose to buy a different text-book you should check if it uses at least Java 5.0 and covers the material listed in the syllabus in the course catalogue entry for CE203.(In Lab 1 you will find Java 8 installed now).
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Recommended Reading 2
Other good books:
• Learning Java (4th ed.), P. Niemeyer and D. Leuck (O’Reilly, 2013)
• Java in a Nutshell (5th ed.), D. Flanagan (O’Reilly, 2005)(reference book)
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Learning Outcomes
1. Demonstrate a knowledge of core Java application libraries
2. Explain the event-driven model underlying Java GUIs3. Write Java programs with interactive graphical user
interfaces (GUIs) 4. Write Java programs that interact with databases5. Write Java programs that make efficient use of the Java
collections package.6. Work with the Java Security Manager to create secure
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Brief Outline
• Review of object-oriented programming with Java• Applets• Exceptions• Collections• JDBC• Security• Guest lecture(s)
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Motivating Examples
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Today: A Bit of Revision
• What you should know by now• Abstract from the language-specific issues to get a more
conceptual picture of object-oriented programming• Understanding the principles helps seeing the overall
picture
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Java Characteristics• Simple• Object Oriented• Interpreted• Portable• Architecture-neutral• High-performance• Distributed• Robust• Secure• Multi-threaded• Dynamic
( Source: LIANG, Y.D., “Introduction to Java Programming”, 6th Edition, Prentice-Hall, 2007)
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Object Orientation – Classes & Objects
An object is an instance of a class many objects of the same class can exist in the same
program
A class can exist in a program even if no instance of it exists some classes (abstract classes) cannot be instantiated at
all
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Object Orientation - Attributes
• Both class and object can have attributes• Attributes in Java are simply called variables• Class variables are shared by all instances of the class• Instance variables belong to a particular instance (or
object)• The state of an object is given by the values of its instance
variables
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Object Orientation - Methods
• Methods can also be defined at either class or instance level
• There are basically three kinds of methods access methods (get and set attribute values) service methods (to offer services to other objects) housekeeping methods (for internal use)
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Object Orientation – Information Hiding
• External access to an object’s attributes and methods is controlled by means of the scope modifiers private, public and protected
• Direct access to attributes is usually prohibited • Access is usually gained through secure public methods
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Object Orientation: Composition & Specialisation
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Parameterisation and References
• In Java, objects are “reference types” (unlike primitive data types!) the value passed when the actual parameter is an object is a reference.
• Parameters of primitive type are always passed using call by value the value associated with the actual parameter cannot be changed
• Parameters that are objects are always passed using call by reference the value(s) associated with the actual parameter can be changed
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Method Overloading
We can supply several definitions for a method sum within the same class definition, as long as each definition has a different set of input types.
int sum (int i, int j) // version 1 {return i + j;} double sum (double e, double f) // version 2 {return e + f;} int sum (int i, int j, int k) // version 3 {return i + j + k;}
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Method Overloading 2
sum(4, 5) will call version 1 on the previous slidesum(4.5, 5.5) will call version 2 on the previous slidesum(4, 5, 6) will call version 3 on the previous slide This process is known as static binding, because the decision about which version of the method to apply is made at the compilation stage.
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Class Definitions
Class definitions provide a unit of scope: they encapsulate data and operations on that data they can specify the attributes and methods of an entire class they can specify the attributes and methods of instances of the
classClass definitions are hierarchical:
they can be written as extensions of existing class definitions they can inherit or redefine parts of the superclass definitions
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Class Definitions 2
The class definition is (in part) a specification for an objectAn object is an instance of a class just as the number 3
is an instance of the type int Objects are created dynamically (at run time) using the
keyword new and come into existence when memory is allocated for them on the heap
Objects cease to exist when they are no longer required (the space they occupied is automatically reclaimed by the garbage collector)
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Class Definitions 3
The class definition is (in part) a specification for the entire class
Any member (variable or method) introduced with thekeyword static becomes part of the static context,and exists throughout program execution
the class can exist independently of objects of the class static members can be used without creating objects
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Class Definitions 4
• A static variable is known as a class variable(one set of class variables is shared by all objects of the class)
• A non-static variable is known as an instance variable(each object of the class has its own set of instance variables)
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Inheritance
The class definition inherits method and variable definitions• methods and variables defined in the superclass are
automatically included in the definitions of its subclasses• the inheritance is transitive, so classes inherit the attributes
of the superclass of their superclass, and so on.• since every class is derived originally from the class Object,
all objects inherit Object’s methods, e.g. toString• inherited members of the superclass can be redefined in the
subclass (variable shadowing, method overriding)
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Accessibility
The public and private modifiers may be omitted from class definitions and variable and method declarations. By default, their scope becomes the enclosing package.
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Accessibility Rules
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1. With no modifier, access is permitted from anywhere in the enclosing package.
2. The public modifier permits access from beyond the enclosing package.
3. The private modifier prevents access from outside the enclosing class.
4. The most restrictive modification applies
Accessibility Examples
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• An unmodified member cannot be accessed from outside the package even if it is contained in a public class.
• A public member cannot be accessed from outside the package if it is contained in an unmodified class.
Classes and Instances
As soon as the program starts, the class descriptions are activated:
• space for class variables is allocated at a location within static memory
• the space remains allocated to those variables until the program stops
• classes exist throughout program execution• class members (variables and methods) can always be
used
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Classes and Instances 2
When a new instance (object) is created with new• space for the instance variables is allocated on the heap
(new returns a reference to that space), the reference may be assigned to more than one variable
• if, at any point, there are no “live” references, the space is reclaimed
• objects only exist while there are existing references to them
• instance members (variables and methods) can only be used when an instance exists
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Specialisation
Specialisation hierarchies and Interfaces are closely related topics
They both provide support for polymorphism(the ability to define methods that can operate on many different types) which is implemented by means of dynamic binding (which means that the decision about which version of the method should be applied is made while the program is executing)
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Specialisation 2
Redefinition of methods is called method overriding.
A superclass method can be completely redefined, or added to. When adding extra statements to the superclass method, the superclass method is called first using the super reference, e.g. if a superclass method meth is added to in a subclass, then in the subclass the first statement in the meth definition is the call super.meth(), and then further statements are added for the additional subclass code
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Specialisation – Dynamic Binding
When a method has been redefined (possibly more than once) in a hierarchy of classes, and that method is called on an object, then the closest definition (going up the hierarchy from the object’s class) is applied, for example:
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Specialisation – Dynamic Binding 2
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Example (continued):
Specialisation – Dynamic Binding 3
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Now consider the following situation:
It will not be known until runtime whether the user will choose “cat” or “dog”, i.e.the decision about which version of noise to use cannot be made by the compiler.
Specialisation – Dynamic Binding 4
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The decision about which version of the method to use is made when the method is called at run-time, and is therefore referred to as dynamic binding.
Dynamic binding is possible in Java because the internal representation of an object includes information about its class definition.Dynamic binding supports the following kind of polymorphism.
Specialisation – Dynamic Binding 5
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Animal[] animalArray = new Animal[4];…animalArray[0] = felix;animalArray[1] = rover;animalArray[2] = daisy;animalArray[3] = nanny;…for (int i = 0; i < 4; i++) System.out.println (animalArray[i].noise());
Abstract Classes
Abstract classes are partial specifications:• they (typically) contain at least one abstract method
(a prototype method definition with no body)• and they cannot be instantiated.
Subclasses of the abstract class can be instantiated if they provide full definitions for the abstract methods. The top most levels of a large hierarchy may consist of several abstract classes.
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