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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

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Recitation 9

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  1. Recitation 9 October 28, 2011

  2. Today’s Goals: • Review abstract classes • Briefly review Java’s Scanner class • Get more practice with scanning

  3. 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”:

  4. Abstract classes: Like classes • Has exactly one superclass (Object if none specified) • Superclass can be either abstract OR non-abstract • Can implement zero or more interfaces • Can contain constructors; these constructors can call super constructors • Can contain variables, constants, and implement methods

  5. Abstract classes: Like classes

  6. 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:

  7. Abstract classes: Unlike classes or interfaces • Can define abstract methods (i.e. methods without implementations) and call them (!)

  8. 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

  9. 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

  10. Using abstraction

  11. Using abstraction

  12. Using abstraction

  13. 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()

  14. 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)

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