COMP 121

1. COMP 121 Week 04

2. Solutions • Review Homework 1 Solutions

3. Outcomes • Distinguish between abstract and concrete classes and methods. • Describe the uses and effects of the keywords final, public, protected, and private as they apply to classes, methods, and instance fields. • Supply appropriate equals, toString, and clone methods for base, derived, and composed classes. • Apply appropriate access modifiers within class design. • Recognize and apply the Template Method design pattern to solve a given problem.

4. Abstract Classes • Can’t be instantiated • May contain abstract methods that derived classes will be required to implement • May contain concrete methods with implementations as well as member variables • An interface can be thought of as a pure abstract class

5. Abstract Classes abstract class Dog { private boolean hungry = true; public abstract void bark(); public void eat() { hungry = false; } } class Husky extends Dog { public void bark() { System.out.println(“Woof!”); } } class Chihuahua extends Dog { public void bark() { System.out.println(“Yip!”); } }

6. Guided Learning Activity Solutions for Week 04

7. Learning Activities Activity 4-1 Outcome: Distinguish between abstract and concrete classes and methods.

8. Access Modifiers • public – can be accessed from any class • private – can only be accessed from inside the declaring class • default – can only be accessed from inside the same package • protected – can be accessed from the same package, or a derived class

9. Access Modifiers class A { int a; public int b; protected int c; private int d; } // In same package class B extends A { void foo() { a = 1; b = 1; c = 1; d = 1; // Error } }

10. Access Modifiers class A { int a; public int b; protected int c; private int d; } // In different package class C extends A { void foo() { a = 1; // Error b = 1; c = 1; d = 1; // Error } }

11. Learning Activities Activity 4-2 Outcome: Describe the uses and effects of the keywords final, public, protected, and private as they apply to classes, methods, and instance fields.

12. The Object Class • equals() – compares objects using == • clone() – makes a shallow copy • toString()– prints class name and hash code (Point@65fab3e)

13. Implementing equals() public class Student { private String firstName; private String lastName; public boolean equals(Object o) { Student s2 = (Student)o; return firstName.equals(s2.firstName) && lastName.equals(s2.lastName); } }

14. Implementing toString() public class Student { private String firstName; private String lastName; public String toString() { return firstName + “ “ + lastName; } public static void main(String [] args) { Student s = new Student(“John”, “Doe”); System.out.println(s); } }

15. Implementing clone() public class Circle implements Cloneable { private int r; private Point center; public Object clone() { Circle cloned = super.clone(); cloned.center = new Point(center.getX(), center.getY()); return cloned; } public static void main(String [] args) { Circle c = new Circle(5, new Point(3, 3)); Circle c2 = c.clone(); c.getCenter().setX(2); System.out.println(c + “ “ + c2); } }

16. Learning Activities Activity 4-3 Outcome: Supply appropriate equals, toString, and clone methods for base, derived, and composed classes. Activity 4-4 Outcome: Apply appropriate access modifiers within class design.

17. Template Method Pattern • Creates a fixed structure for an algorithm • Lets the details of each step change

18. Template Method Pattern public class PizzaShop { public final void makePizza() { makeDough(); tossDough(); addToppings(); cook(); slice(); } public void makeDough() {…} public void tossDough() {…} public void addToppings() {…} public void cook() {…} public void slice() {…} }

19. Learning Activities Activity 4-5 Outcome: Recognize and apply the Template Method design pattern to solve a given problem.

20. Homework Assignments • Due this week • Lab 1 • Homework 2 • Reflection Paper Draft • Next week • Exam 1

21. Question and Answer Session