COMP 2013 Software Maintenance
Workshop 01 (with answers)
OO and Java Refresher (1/2)
Peer-Olaf Siebers
Please note that the slides published AFTER the lectures and workshops are the official slides and are the ones that should be used for revision.
Topics
• Lecture 1
– What is Software Maintenance?
– Information about module organisation
– Some examples of software maintenance challenges
• Lab 1
– Eclipse and IntelliJ
– Practicing Java basics
– Working with existing code
• Workshop 1
– OO and Java Programming Refresher
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Case Study: Zoo Management
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Case Study: Zoo Management
• Come up with a draft class diagram
– Note that this is only a small choice of relevant classes!
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Case Study: Zoo Management
As we focus on Java basics today we want to keep it simple …
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Basic OO Concepts
• Object-oriented programming is founded on these ideas:
– Abstraction: Simple things like objects represent more complex underlying code and data
• A class is a blueprint for a category of objects
• An object is an entity that combines data with behavior that acts on that data
– Encapsulation (information hiding): The ability to protect some components of the object
from external access
• e.g. keeping fields within a class private, then providing access to them via public methods
– Inheritance: The ability for a class (“subclass”) to extend or override functionality of another
class (“superclass”)
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Basic OO Concepts
• Object-oriented programming is founded on these ideas:
– Polymorphism: The provision of a single interface to entities of different types
• Compile time (static) polymorphism through…
– Method overloading: Create multiple methods with same name but different signatures
• Run time polymorphism through…
– Method overriding: Create method in derived class with same name and signature than in base class
– Sub classing: reference of base class is able to reference, instantiate and destroy objects of derived class
– Interface: A specification of method signatures (without implementations) as a mechanism
for enabling polymorphism in a declarative way.
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What’s coming up …
• Public vs. Private
• Accessors and Modifiers
• Encapsulation
• The “this” keyword
• Constructors
• Passing parameters
• Static fields and methods
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Public vs. Private
• What are the general rules for constructors, methods, helper methods, fields,
and static constants?
– Constructors and methods
• Usually declared public (they constitute the interface of a class)
– Helper methods that are needed only inside the class
• Usually declared private
– Fields
• Usually declared private (to support encapsulation)
– Static constants
• Usually declared public
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Accessors and Modifiers
• Accessors (also called Getters):
– Methods that return values of private fields
– Name often starts with get
• Modifiers (also called Mutators or Setters):
– Methods that set values of private fields
– Name often starts with set
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Encapsulation
• Hiding the implementation details of a class (making all fields and helper
methods private) is called encapsulation
• Encapsulation helps in program maintenance: a change in one class does not
affect other classes
• A client of a class interacts with the class only through well-documented public
constructors and methods; this facilitates team development
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The Keyword “this”
• “this” refers to the implicit parameter inside your class
– A variable that stores the object on which a method is called
– Refer to a field
• this.field
– Call a method
• this.method(parameters);
– One constructor can call another
• this(parameters);
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Constructors
• What are constructors used for?
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Constructors
• A constructor is a procedure for creating objects of the class
– A constructor often initialises an object’s fields
– Constructors do not have a return type
– All constructors in a class have the same name (the name of the class)
– Constructors may take parameters
– If a class has more than one constructor, they must have different numbers and/or types of
parameters (constructor overloading)
• Important!
– Java provides a default constructor for a specific class
– If you define a constructor for a class, Java does not provide the default constructor anymore
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Constructors
• Constructors of a class can call each other using the keyword “this” (referred to as
constructor chaining) – a good way to avoid duplicating code
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Invoking Constructors
• Constructors are invoked using the operator new.
– Declare a reference variable of the required type and then invoke the constructor method
after the “new” keyword
• Parameters passed to “new” must match the number, types, and order of
parameters expected by one of the constructors.
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Invoking Constructors
• What does the output look like?
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Passing Parameters
• In Java, parameters sent to methods are passed by value
– Just to clarify some terminology
• The “type” of data that a method can receive is referred to as a “parameter”
• What is passed “to” a method is referred to as an “argument”
• Meaning of “pass-by-value”
– In this case actual parameter is evaluated and its value is copied into memory (stack) used by
the parameters of the method.
• Common misconception: “In Java primitives are passed by value and objects are
passed by reference”
– Objects are not passed by reference but object references (pointers) are passed by value
– You can test this by using the “Litmus” test (writing a simple swap() function)
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http://www.javadude.com/articles/passbyvalue.htm
http://www.javadude.com/articles/passbyvalue.htm
http://www.javadude.com/articles/passbyvalue.htm
Passing Parameters
• Inside a method, “this” refers to the object for which the method was called.
“this” can be passed to other constructors and methods as a parameter.
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Return Statement
• A void method can use a return statement to quit the method early
• There is no need for a return at the end
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Overloaded Methods
• Methods of the same class that have the same name but different numbers or
types of parameters are called overloaded methods
• The compiler treats overloaded methods as completely different methods
• The compiler knows which one to call based on the number and the types of the
parameters passed to the method
• The return type alone is not sufficient for making a distinction between
overloaded methods
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Static Fields
• A static field (class field or class variable) is shared by all objects of the class
• A non-static field (instance field or instance variable) belongs to an individual
object
• Static fields are stored with the class code, separately from instance variables that
describe an individual object
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Static Fields
• Public static fields, usually global constants, are referred to in other classes using
dot notation
– ClassName.constName
• Usually static fields are NOT initialized in constructors (they are initialized either
in declarations or in public static methods).
• If a class has only static fields, there is no point in creating objects of that class (all
of them would be identical).
– Math and System are examples of the above (they have no public constructors and cannot be
instantiated)
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Static Methods
• Static methods can access and manipulate class’s static fields. They belong to the
class – not an instance of it.
• Static methods cannot access instance fields or call instance methods of the class;
instance methods can access all fields and call all methods of their class – both
static and non-static
• Static methods will usually take input from the parameters, perform actions on it,
then return some result.
• Static methods are called using dot notation
– ClassName.statMethod(…)
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Static Fields and Methods
• What does the output look like?
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Static Fields and Methods
• Does this compile?
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And finally …
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References
• Sommerville (1992) ‘Software Engineering’ 4e, Pearson.
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Acknowledgement
• Slides based on material from
– Bill Leahy’s lecture slides
• http://www.cc.gatech.edu/~bleahy/xjava/cs1311xjava05_poly.ppt
– Maria Litvin’s & Gary Litvin’s book slides
• http://skylit.com/javamethods/ppt/Ch10.ppt
– Marty Stepp’s lecture slides
• http://www.cs.washington.edu/331/
– And others …
But I also contributed some stuff myself 🙂
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http://www.cc.gatech.edu/~bleahy/xjava/cs1311xjava05_poly.ppt
http://www.cc.gatech.edu/~bleahy/xjava/cs1311xjava05_poly.ppt
http://skylit.com/javamethods/ppt/Ch10.ppt
http://skylit.com/javamethods/ppt/Ch10.ppt
http://skylit.com/javamethods/ppt/Ch10.ppt
http://www.cs.washington.edu/331/
http://www.cs.washington.edu/331/