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Polymorphism Read Day 6 in OOP21
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More specifically, it is the ability to redefine methods for derived classes. ... Can specialize by doing stuff and still calling the superclass method. Coercion ... – PowerPoint PPT presentation
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Title: Polymorphism Read Day 6 in OOP21
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PolymorphismRead Day 6 in OOP21
- C Sc 335
- University of Arizona
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From Wepodia http//www.webopedia.com/TERM/p/polym
orphism.html
- polymorphism Generally, the ability to appear in
many forms. In object-oriented programming,
polymorphism refers to a programming language's
ability to process objects differently depending
on their data type or class. More specifically,
it is the ability to redefine methods for derived
classes. For example, given a base class shape,
polymorphism enables the programmer to define
different circumference methods for any number of
derived classes, such as circles, rectangles and
triangles. No matter what shape an object is,
applying the circumference method to it will
return the correct results. Polymorphism is
considered to be a requirement of any true
object-oriented programming language (OOPL).
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From Computing Fundamentals, Mercer02
- To understand polymorphism, take an example of
a typical workday in an office.Someone brought in
pastries and everyone stood around chatting. When
the food was mostly devoured, Jim, the president
of the company, invited everyone to Get back to
work. Sue went back to read a new section of a
book she was editing. Tom continued laying out
the new edition of a book. Stephanie went back to
figure out some setting in her word-processing
program. Ian finished the company catalog. - Jeni met with Jim to discuss a new project.
Chris began contacting professors to review a new
manuscript. And Krista continued her Web search
on whether colleges are using C or Java. Rick
went back to work on the index of his new book.
Eight different behaviors from the same message!
The message Get back to work is a polymorphic
messagea message that is understood by many
objects (or employees), but responded to with
different behaviors by different objects (or
employees).
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How to get polymorphism
- Polymorphism A single name can represent
different behaviors - withdraw deposit actionPerformed toString
- In Java, polymorphism is possible through
- inheritance
- Same message to Account results in 1 of 3 methods
- interfaces
- JButton and JTextField objects send
actionPerformed messages to the listeners
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actionPerformeda polymorphic message
- Run
- Polymorphic_actionPerformed,java
- ThreeListeners.java
- Four types of polymorphism
- Inclusion
- Parametric (can use templates)
- Overloading
- Overriding
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PolymorphismInclusion
- Allow for a substitutability relationship
- Can assign any subclass of Account to a reference
variable of type Account - Account bAccount new BankAccount(...)
- Account fAccount new FeeAccount(...)
- Account sAccount new SafeAccount(...)
- Can write one method for different types
- Can treat different types of objects generically
- Sort any List
- Send withdraw messages to any Account
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ParametersInclusion Polymorphism
- Can pass any subclass argument to a superclass
parameter - add(Account anySubclass)
- add(new BankAccount(...))
- add(new FeeAccount(...)
- add(new SafeAccount(...))
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Parameters
- Can pass instances of a class that implement an
interface to that interface type parameter - addActionListener(ActionListener anyListener)
- addActionListener(new ButtonListener())
- addActionListener(new TextFieldListener())
- Both classes must implement the ActionListener
interface - This design prefers compile time errors rather
than the runtime errors that occur later when the
button sends a message the object does not
understand
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Parametric Polymorphism
- Generic methods and Generic Types
- Templates in C since 97, may come to Java
- Construct a generic vector using templates
- vectorltintgt testScore(100, -1)
- // From Computing Fundamentals with C, Mercer
94 - templateltclass VectorElementTypegt
- class vector
- public
- vector(int initCapacity, VectorElementType
initVal) - vector(const vectorltVectorElementTypegt
source) - vector()
- void resize(int newCapacity)
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Overloadingad-hoc polymorphism
- a.k.a. ad-hoc polymoprhism
- same method names
- different number and/or type of arguments
- Three constructors (Java does this often)
- vectorltintgt tests
- vectorltdoublegt x(100)
- vectorltstringgt names (100, "Default")
- templateltclass VectorElementTypegt
- class vector
- public
- vector() // default capacity and values
- vector(int initCapacity) // use the types
default values - vector(int initCapacity, VectorElementType
initVal)
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Overloading
- Some languages allow operator overloading
- // Allow x1 0.0
- VectorElementType operator (int subscript)
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Method overloading in Java
- Constructors in the PrintWriter class
- PrintWriter(OutputStream out)
- PrintWriter(OutputStream out, boolean autoFlush)
- PrintWriter(Writer out)
- PrintWriter(Writer out, boolean autoFlush)
- Methods in the PrintWriter class
- void print(boolean b)
- void print(char c)
- void print(char s)
- void print(double d)
- void print(float f)
- void print(int i)
- void print(long l)
- void print(Object obj)
- void print(String s)
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Overriding
- Subclasses give new behavior to methods
- Famous override toString
- Can specialize by doing stuff and still calling
the superclass method
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Coercion
- Makes an int look like a double
- add(double x, double y)
- add( 1, 2)
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