1. Lecture 7: Reusing Code & Interfaces
CSE 116/504 – Intro. To Computer Science for Majors II
Lecture 7: Reusing Code & Interfaces
Lecture included 15 minute presentation by Bloomberg & three TopHat questions gauging student confidence in OO concepts

2. Announcements Score posted to UBLearns so you can track grades
Calculated grades drop lowest 3 homeworks & 1 quiz
Homework for week #3 due Monday at 12:45PM
Tim Sweeney is the owner of Epic Games and was one of the major drivers of the Unreal Engine (among other things)

3. Inheritance Composition Sharing is Caring
Often need to write classes sharing actions & data
Could just cut-and-paste code between the classes
If When bug found, must remember to fix both copies
Cutting-and-pasting takes effort & have better things to
OO languages (Java) offer two possible solutions:
Inheritance
&
Composition

4. Composition Typically used for cases with a “has-a” relationship
Student has a full name
Full name has a first name
Car has an engine
Rectangle has an upper-right vertex

5. Use a field to compose classes
Composition
Data not really shared, but belong to specific class
Odd to discuss the x-coordinate of a rectangle
Discuss student's first name & despite FullName holding
Use a field to compose classes
So we would add name field to Student class
firstName field in FullName class
If must use data externally, add getters & setters

6. Composition Key Concept #1
When relationship between classes is Has A(n): Add fields

7. Composition Example
public class FullName { private String firstName; private String lastName; // Constructor, getters & setters also here } public class Student {

8. Composition Example
public class FullName { private String firstName; private String lastName; // Constructor, getters & setters also here } public class Student { private FullName name; // Brevity is the soul of wit- Shakespeare public FullName getName() { return name; }

9. Composition Example
public class FullName { private String firstName; private String lastName; // Constructor, getters & setters also here } public class Student { private FullName name; // Brevity is the soul of wit- Shakespeare public FullName getName() { return name; } // In some other code that exists to demonstrate this point Student student = /* Code to create and initialize a Student instance*/ FullName fName = student.getName(); fName.setLastName("Jingleheimerschmidt");

10. Composition Example
public class FullName { private String firstName; private String lastName; // Constructor, getters & setters also here } public class Student { private FullName name; // Brevity is the soul of wit- Shakespeare public FullName getName() { return name; } // In some other code that exists to demonstrate this point Student student = /* Code to create and initialize a Student instance*/ FullName fName = student.getName(); fName.setLastName("Jingleheimerschmidt");
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11. Shouldn't Student control changing names?
Composition Example
public class FullName { private String firstName; private String lastName; // Constructor, getters & setters also here } public class Student { private FullName name; // Brevity is the soul of wit- Shakespeare public FullName getName() { return name; } // In some other code that exists to demonstrate this point Student student = /* Code to create and initialize a Student instance*/ FullName fName = student.getName(); fName.setLastName("Jingleheimerschmidt");
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Composition + getters can make yucky code;
Shouldn't Student control changing names?

12. Delegation Delegation improves composition of classes
Goal creating classes & code easier to write, read, & use
Not universal solution, but useful additional tool
Important for larger projects & splitting work in groups
Replace getters & add methods for needed actions
Methods can act, but mostly delegate to field's methods
Adds (slightly) to code size, but performance unchanged
Trade-off worth it, since we write once but use frequently

13. Delegation Key Concept
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14. Replace yuck-inducing getter method…
Delegation Example
public class FullName { private String firstName; private String lastName; // Constructor, getters & setters also here } public class Student { private FullName name; // Brevity is the soul of wit- Shakespeare public FullName getName() { return name; } // In some other code that exists to demonstrate this point Student student = /* Code to create and initialize a Student instance*/ FullName fName = student.getName(); fName.setLastName("Jingleheimerschmidt");
Replace yuck-inducing getter method…

15. Delegation Example
public class FullName { private String firstName; private String lastName; // Constructor, getters & setters also here } public class Student { private FullName name; // Brevity is the soul of wit- Shakespeare public void setFirstName(String newName) { name.setFirstName(newName); } // In some other code that exists to demonstrate this point Student student = /* Code to create and initialize a Student instance*/ FullName fName = student.getName(); fName.setLastName("Jingleheimerschmidt");
Replace yuck-inducing getter method… … with method calling field's method

16. Reducing how much you must understand…
Delegation Example
public class FullName { private String firstName; private String lastName; // Constructor, getters & setters also here } public class Student { private FullName name; // Brevity is the soul of wit- Shakespeare public void setFirstName(String newName) { name.setFirstName(newName); } // In some other code that exists to demonstrate this point Student student = /* Code to create and initialize a Student instance*/ FullName fName = student.getName(); fName.setLastName("Jingleheimerschmidt");
Reducing how much you must understand…

17. Delegation Example
public class FullName { private String firstName; private String lastName; // Constructor, getters & setters also here } public class Student { private FullName name; // Brevity is the soul of wit- Shakespeare public void setFirstName(String newName) { name.setFirstName(newName); } // In some other code that exists to demonstrate this point Student student = /* Code to create and initialize a Student instance*/ student.setLastName("Jingleheimerschmidt");
Reducing how much you must understand… …so writing & updating code becomes easier!

18. Composition + Delegation =
public class FullName { private String firstName; private String lastName; // Constructor, getters & setters also here } public class Student { private FullName name; // Brevity is the soul of wit- Shakespeare public void setFirstName(String newName) { name.setFirstName(newName); } // In some other code that exists to demonstrate this point Student student = /* Code to create and initialize a Student instance*/ student.setLastName("Jingleheimerschmidt");

19. Composition Key Concept #1
When relationship between classes is Has a(n): Add fields &
Delegate actions

20. Inheritance “Is-a” relationships implemented via inheritance
Automobile is a Vehicle
Car is a Vehicle
Truck is an Automobile
Car is an Automobile
Subclass includes data and actions from superclass
Adds more data or actions to extend definition further
Some tasks in the superclass specialized in the subclass
Classes related to each other as fundamental ideas

21. Inheritance “Is-a” relationships implemented via inheritance
Automobile is a Vehicle
Car is a Vehicle
Truck is an Automobile
Car is an Automobile
Specify that subclass extends superclass
Automobile extends Vehicle
Truck extends Automobile
Car extends Automobile (& Vehicle?)

22. Inheritance Key Concept #1
When relationship between classes is Is A(n): Use extends

23. extends Example Vehicle Automobile Car Truck
public class Vehicle {…} public class Automobile extends Vehicle {…} public class Car extends Automobile {…} public class Truck extends Automobile {…}
Vehicle
Automobile
Car
Truck

24. Inheritance Key Concept #2
Every Java class extends exactly 1 class

25. extends Example extends explicitly specifies superclass for class
Can skip & class automatically extends Object
Object
Vehicle
Automobile
Car
Truck

26. extends Example
public class Vehicle {…} public class Automobile extends Vehicle {…} public class Car extends Automobile {…} public class Truck extends Automobile {…}
Object
Vehicle
Automobile
Car
Truck

27. extends Example No limit on subclasses which extend a specific class
Automobile superclass of both Car & Truck
Vehicle superclass of Automobile, Car, & Truck
Object
Vehicle
Automobile
Car
Truck

28. Superclass (& Subclass)
In Java, each and every class is subclass of:
superclass

29. Superclass (& Subclass)
In Java, each and every class is subclass of:
superclass
superclass’s superclass

30. Superclass (& Subclass)
In Java, each and every class is subclass of:
superclass
superclass’s superclass
superclass’s superclass’s superclass

31. Superclass (& Subclass)
In Java, each and every class is subclass of:
superclass
superclass’s superclass
superclass’s superclass’s superclass
superclass’s superclass’s superclass’s superclass, …
Object

32. Superclass (& Subclass)
In Java, each and every class is subclass of:
superclass
superclass’s superclass
superclass’s superclass’s superclass
superclass’s superclass’s superclass’s superclass, …
Object
Automobile is-a Vehicle
Since Truck is-an Automobile … …then Truck is-a Vehicle & … …also Truck is-an Object

33. Inheritance Key Concept #2
Every Java class extends 1 class directly and can inherit from many others via superclass

34. What Gets Inherited And How?
Class inherits all fields & methods in superclass
Does not matter if inherited from superclass's superclass
Without extra typing use fields & call methods directly*
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35. Inheritance Example
public class Sailor { String str = "Aye, aye"; public String goal() { return "Capt."; } } public class Pirate extends Sailor { public String getRealGoal() { return "Dread Pirate " + goal(); } public Pirate() { str = "Yarr"; } public static void main(String[] args) { Pirate wesley = new Pirate(); Sailor popeye = new Sailor(); System.out.println(wesley.goal()); System.out.println(popeye.goal()); System.out.println(wesley.getRealGoal()); System.out.println(popeye.getRealGoal()); System.out.println(wesley.str); System.out.println(popeye.str);

36. Inheritance Example
public class Sailor { String str = "Aye, aye"; public String goal() { return "Capt."; } } public class Pirate extends Sailor { public String getRealGoal() { return "Dread Pirate " + goal(); } public Pirate() { str = "Yarr"; } public static void main(String[] args) { Pirate wesley = new Pirate(); Sailor popeye = new Sailor(); System.out.println(wesley.goal()); System.out.println(popeye.goal()); System.out.println(wesley.getRealGoal()); System.out.println(popeye.getRealGoal()); System.out.println(wesley.str); System.out.println(popeye.str);

37. Inheritance Example Changes in subclass ONLY affect subclass;
public class Sailor { String str = "Aye, aye"; public String goal() { return "Capt."; } } public class Pirate extends Sailor { public String getRealGoal() { return "Dread Pirate " + goal(); } public Pirate() { str = "Yarr"; } public static void main(String[] args) { Pirate wesley = new Pirate(); Sailor popeye = new Sailor(); System.out.println(wesley.goal()); System.out.println(popeye.goal()); System.out.println(wesley.getRealGoal()); System.out.println(popeye.getRealGoal()); System.out.println(wesley.str); System.out.println(popeye.str);
Changes in subclass ONLY affect subclass;
Sailor does not gain getRealGoal()

38. How Does Inheritance Work?
Inheritance occurs automatically with extends
DO NOT RETYPE INHERITED MEMBERS
Redeclaring fields yields bad things that cannot be fixed
Some value in overriding methods by redeclaring them

39. Inheritance Key Concept #3
Automatically inherit fields & methods: Can override methods; NEVER redeclare fields

40. Overriding Methods In code, can redeclare inherited method in subclass
"Overloaded" if different signature used in subclass
Method said to be "overridden" if signatures identical
If subclass method different, then okay to override
Method changed only for subclass & its subclasses
Original used by superclass & other classes, however
Method depends on this - instance specifies definition

41. Overriding Methods Within override, can call method in superclass
Only while overriding method – cannot call elsewhere
Allows reusing inherited method as part of new method
super.methodName(…) to call method
Chaining illegal super.super.methodName(…)
Cannot make less accessible when overriding
However, method can make more accessible

42. Overriding Methods Within override, can call method in superclass
Only while overriding method – cannot call elsewhere
Allows reusing inherited method as part of new method
super.methodName(…) to call method
Chaining illegal super.super.methodName(…)
Cannot make less accessible when overriding
However, method can make more accessible

43. Delegation Key Concept
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44. Composition Key Concept #1
When relationship between classes is Has a(n): Add fields &
Delegate actions

45. Inheritance Key Concept #1
When relationship between classes is Is A(n): Use extends

46. Inheritance Key Concept #2
Every Java class extends exactly 1 class

47. Inheritance Key Concept #2
Every Java class extends 1 class directly and can inherit from many others via superclass

48. For Next Lecture Read 1.4 - 1.6 for Friday and be ready to discuss
What is polymorphism?
Why would chains be important in CSE?
Is there ever a reason to add a typecast?
As usual, the weekly assignment due next week
Due by 12:45PM Monday using AutoLab