Transcript Recitation 9

Recitation 9
October 28, 2011
Today’s Goals:
 Review abstract classes
 Briefly review Java’s Scanner class
 Get more practice with scanning
Abstract classes
 An abstract class is a hybrid of a class and
an interface
 Abstract classes are defined by including
the word “abstract” before the word
“class”:
Abstract classes: Like classes
 Has exactly one superclass (Object if none
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specified)
Superclass can be either abstract OR nonabstract
Can implement zero or more interfaces
Can contain constructors; these
constructors can call super constructors
Can contain variables, constants, and
implement methods
Abstract classes: Like classes
Abstract classes: Like interfaces
 Can’t create an instance of an abstract
class (even if it contains constructors!)
 Illegal:
 Can implement interfaces but not provide
implementations for the methods in those
interfaces
 Legal:
Abstract classes: Unlike classes
or interfaces
 Can define abstract methods (i.e. methods
without implementations) and call them (!)
Non-abstract classes
 Also called “concrete” classes
 If a class is non-abstract, then it provides
implementations of every unimplemented
abstract method from all of its
superclasses, as well as all methods from
all interfaces implemented by all
superclasses
Benefits of abstract classes
 Similar to benefits of inheritance
 Avoid repeating code
 Allow common behaviors
 Defer implementation to subclasses
 Indicate behavior must be provided, but is
subclass-specific
 Prevents the creation of incomplete
objects
Using abstraction
Using abstraction
Using abstraction
Review: java.util.Scanner
 import java.util.Scanner;
 Create a scanner object to read input from the
console:
 Scanner scanner = new Scanner(System.in);
 Note: This reads console input, not program
argument input
 To get the next string entered by the user (up to
the first whitespace): scanner.next()
 To get the next integer entered by the user:
scanner.nextInt()
Recitation Specification
 Download Recitation9.zip from the Recitations page
 Repeatedly read the next string entered by the user
(scanner.next())
 If the user enters “size”, print the size of the stack to the console
 If the user enters “pop”, pop the top element off of the stack
 If the user enters “triangle”, read in the following 6 integers
(using scanner.nextInt()), create a CartesianTriangle object with
these values, and push it onto the stack
 Remember, you need to manually refresh Object Editor
to see these changes (View -> Refresh)
 Bonus (for candy): Allow the user to push as many of
these as possible: lines, circles, ovals, rectangles,
squares (all these classes are provided in graphics)