Polymorphism

1
Polymorphism

• Robert Salkin
• CSI 205 OOP using Java
• Summer 2005, 6W1

2
Odds and Ends

• Primitive boolean
• Ternary Operator
• Other Prints
• String Manipulation
• References as Return Types
• Coding Strategies
• Precondition/Postcondition

3
Primitive boolean

• In addition to primitives that can hold numerical
  values, there is a boolean datatype to hold truth
  values.
• Can only hold true or false.
• boolean doneYetfalse
• while(!doneYet)
• //do something until
• doneYettrue

4
Ternary Operator

• The ternary operator is a condensed if/else,
  usually used to select one of two values.
• (condition)?(true_value)(false_value)
• int alwaysNonNegativeX(xlt0)?(0)(x)
• String x(doneYet)?(true)(false)

5
Other Prints

• These are (or should be) going out of style, but
  will remain a part of Java forever.
• System.out.print(hi\n)
• Prints a message like printf, but cannot take
  format specifiers.
• System.out.println(hi)
• Prints a message like print, but always inserts
  an extra newline at the end of the print
  operation.

6
String Manipulation

• Strings can be concatenated together using the
  plus sign ().
• When two strings are pasted together, they are
  said to be concatenated together.
• The first String is placed before the second
  String, and so on.
• String shello world!\n
• Primitives and objects may be added to strings,
  as long as the string comes first toString() is
  automatically run on the object.
• int x42
• String sThe number is x

7
References as Return Types

• Object references can be used as return types for
  methods just like primitives.
• When a primitive is returned, the value of the
  primitive is returned.
• When an object reference is returned, a reference
  to the object is returned not a copy of the
  object.

8
Coding Strategies

• When coding, break the specifications up into
  data, blocks, methods, or other logical
  components.
• Compile often to see if youre on the right track
  do NOT wait until the end to compile, only to
  find out you wrote code that doesnt do anything
  close to what you wanted it to.
• Do not assume code does something for you when it
  really doesnt.
• Code is dumb, the coder is smart the code
  doesnt know when the coder intends to do
  something not explicitly written.
• Example

9
Precondition/Postcondition

• A precondition is a specific state the program
  must be in in order for the method to run
  appropriately.
• A postcodition is the state of the program after
  the method is run.
• These descriptions need only cover the
  non-trivial, related state concerns.

10
Polymorphism

• Polymorphism allows the same code to be run with
  different, more specific objects.
• Implementations may be more abstract and general,
  relying on polymorphism to make the right action
  happen.
• Polymorphism may be used with object references
  as well as methods.

11
Polymorphic References

• An object may be referenced by a more general
  reference variable.
• Car myCarnew Car()
• BMW myBMWnew BMW()
• myCarmyBMW //ok.
• myBMWmyCar //NOT ok.

12
Polymorphic Methods

• Polymorphism allows for methods to be written
  that are specific to a particular subclass.
• Animal anew Frog()
• a.move() //a quick hop.
• anew Turtle()
• a.move() //a slow walk.

13
Overloading vs. Overriding

• Overloading a method means that methods of the
  same name have been defined with different
  arguments.
• Overriding involves writing a method in a
  subclass with the same name and same arguments in
  order to mask the functionality of the
  superclasss method in favor of the new method.

14
Abstract Class

• An abstract class is unfinished.
• An Animal may require that a move() method be
  defined.
• Animal is too general without a subclass filling
  in the blanks, like move().
• The abstract keyword is used on the class as well
  as the unfinished methods.
• Example