Spring Semester 2003

Let's start by mentioning a student 's observations, one past semester (it was Florentina Tone, if I remember correctly). It refers to how we calculate intersections of rectangles with the Java API. These are very perceptive observations. She said:

• The intersection method in class Rectangle does not return a valid (that is, meaningful) answer when the Rectangles don't overlap. (This is what the text of the problem might say too, but one needs to read even the text of the problem with care!)

• In other words, you can call intersection and you will always obtain an answer. You can't tell by just looking at this answer if the rectangles overlap or not. So what do we do when they do not overlap?

• The solution is to
  1. check (see method intersects) if they overlap
  2. before you calculate the intersection
  3. (which you should do only if they overlap!).

Let's go through the complete and annotated development of a solution to a problem.

(This is problem no. 5, below).

1. Let's implement a class Student.

frilled.cs.indiana.edu%pico Student.java
frilled.cs.indiana.edu%cat Student.java
public class Student {

} 
frilled.cs.indiana.edu%javac Student.java
frilled.cs.indiana.edu%ls -ld Student*
-rw-------   1 dgerman       188 Feb  1 08:18 Student.class
-rw-------   1 dgerman        27 Feb  1 08:17 Student.java
frilled.cs.indiana.edu%

3. We need a tester class with a main method.

frilled.cs.indiana.edu%pico StudentTest.java
frilled.cs.indiana.edu%cat StudentTest.java
public class StudentTest {
  public static void main(String[] args) {
    Student a = new Student(); 
  } 
} 
frilled.cs.indiana.edu%javac StudentTest.java
frilled.cs.indiana.edu%ls -ld Student*
-rw-------   1 dgerman       188 Feb  1 08:18 Student.class
-rw-------   1 dgerman        27 Feb  1 08:17 Student.java
-rw-------   1 dgerman       297 Feb  1 08:21 StudentTest.class
-rw-------   1 dgerman       110 Feb  1 08:21 StudentTest.java
frilled.cs.indiana.edu%

5. We can run StudentTest but we get no output.

frilled.cs.indiana.edu%java StudentTest
frilled.cs.indiana.edu%

7. Do we know what happens inside?

8. The Student class is empty. Student objects are amorphous.

9. I see... Let's make it such that each Student has (at least) a name, then.

frilled.cs.indiana.edu%pico Student.java
frilled.cs.indiana.edu%cat Student.java
public class Student {
  private String name; 

} 
frilled.cs.indiana.edu%

11. It means that to know the name of a Student you need to ask the Student what its name is.

12. I don't feel very comfortable using he or she for a Student object.

13. Fine. How do you inquire about a Student's name?

14. We need to add this functionality to class Student first, then make use of it.

15. Here's a more comprehensive blueprint of Student objects.

frilled.cs.indiana.edu%pico Student.java
frilled.cs.indiana.edu%cat Student.java
public class Student {
  private String name; 

} 
frilled.cs.indiana.edu%pico Student.java
frilled.cs.indiana.edu%cat Student.java
public class Student {
  private String name; 

  public String whatsYourName () {
    return name; 
  }

} 
frilled.cs.indiana.edu%

17. It means you can ask a Student "What's your name?"

18. What if we make it private?

19. Then we can never ask.

20. How do we create a Student?

21. Just invoke new the way we did in the tester's main.

22. And if we invoke it, how do things get created, and initialized.

23. Well, a default no-arg constructor is present, but we don't see it.

24. I think we should add it, so that we not forget that it's there.

frilled.cs.indiana.edu%pico Student.java
frilled.cs.indiana.edu%cat Student.java
public class Student {
  private String name; 

  public String whatsYourName () {
    return name; 
  }

  Student() {

  } 
} 
frilled.cs.indiana.edu%

26. Can we create a Student with an initial name?

27. Only if we provide that type of constructor.

28. To be able to create a Student with an initial name we need to

• be able to initialize the name of any Student

  (at creation time)

• with an actual name (to be specified when we create the Student).

new Student("Larry Johnson")

frilled.cs.indiana.edu%pico Student.java
frilled.cs.indiana.edu%cat Student.java
public class Student {
  private String name; 
  public String whatsYourName () {
    return name; 
  }
  Student() { } 
  Student(String givenName) {
    name = givenName;     
  }
} 
frilled.cs.indiana.edu%

frilled.cs.indiana.edu%pico StudentTest.java
frilled.cs.indiana.edu%cat StudentTest.java
public class StudentTest {
  public static void main(String[] args) {
    Student a = new Student("Larry");
    Student b = new Student("Michael");
    String answer; 
    System.out.print("Printing the name of the first student: "); 
    answer = a.whatsYourName(); 
    System.out.println(answer); 
    System.out.print("Printing the name of the second student: "); 
    answer = b.whatsYourName(); 
    System.out.println(answer); 
  } 
} 
frilled.cs.indiana.edu%javac StudentTest.java
frilled.cs.indiana.edu%java StudentTest
Printing the name of the first student: Larry
Printing the name of the second student: Michael
frilled.cs.indiana.edu%

33. Let's enable the Students to keep track of their scores.

frilled.cs.indiana.edu%pico Student.java
frilled.cs.indiana.edu%cat Student.java
public class Student {
  private String name; 
  public String whatsYourName () {
    return name; 
  }
  Student() { } 
  Student(String givenName) {
    name = givenName;     
  }
  void addQuizScore(int newScore) {
    totalScore = totalScore + newScore; 
  } 
  private int totalScore; 
} 
frilled.cs.indiana.edu%

35. Yes, so you need to define an instance variable totalScore (which will keep the cumulative score for the Student) and use it as if it were a balance.

36. This way a Student is like a BankAccount with a name.

37. Let's write getBalance, then.

frilled.cs.indiana.edu%pico Student.java
frilled.cs.indiana.edu%cat Student.java
public class Student {
  private String name; 
  public String whatsYourName () {
    return name; 
  }
  Student() { } 
  Student(String givenName) {
    name = givenName;     
  }
  void addQuizScore(int newScore) {
    totalScore = totalScore + newScore; 
  } 
  private int totalScore; 
  int whatsYourTotalScore() {
    return totalScore; 
  }
} 
frilled.cs.indiana.edu%

frilled.cs.indiana.edu%pico StudentTest.java
frilled.cs.indiana.edu%cat StudentTest.java
public class StudentTest {
  public static void main(String[] args) {
    Student a = new Student("Larry");
    Student b = new Student("Michael");
    String answer; 
    System.out.print("Printing the name of the first student: "); 
    answer = a.whatsYourName(); 
    System.out.println(answer); 
    System.out.print("Printing the name of the second student: "); 
    answer = b.whatsYourName(); 
    System.out.println(answer); 

    a.addQuizScore(100); 
    a.addQuizScore(90); 
    a.addQuizScore(100); 
    System.out.println("Student " + a.whatsYourName() + "reports: "); 
    System.out.println("   cumulative score: " + a.whatsYourTotalScore());

  } 
} 
frilled.cs.indiana.edu%javac StudentTest.java
frilled.cs.indiana.edu%java StudentTest
Printing the name of the first student: Larry
Printing the name of the second student: Michael
Student Larryreports: 
   cumulative score: 290
frilled.cs.indiana.edu%

40. I'll let you fix that. But overall we've come a long way, don't you think?

41. I sure do think so. What if I want the Students to be able to report the average score in addition to the cumulative score? I don't think this is possible at the moment, because they don't remember how many quizzes they have taken.

42. Indeed, they only keep the cumulative score.

43. To remember how many quizzes they have taken they would need to keep a counter, to be updated (incremented by 1) every time a new score is added to the totalScore.

44. If we kept the number updated we could easily report the average at any time, as follows.

frilled.cs.indiana.edu%pico Student.java
frilled.cs.indiana.edu%cat Student.java
public class Student {
  private String name; 
  public String whatsYourName () {
    return name; 
  }
  Student() { } 
  Student(String givenName) {
    name = givenName;     
  }
  void addQuizScore(int newScore) {
    totalScore = totalScore + newScore; 
  } 
  private int totalScore; 
  int whatsYourTotalScore() {
    return totalScore; 
  }
  private int numberOfScores; 
  double reportAverage() {
    return (double)totalScore / numberOfScores; 
  }   
} 
frilled.cs.indiana.edu%

46. Ooops!...

frilled.cs.indiana.edu%pico Student.java
frilled.cs.indiana.edu%cat Student.java
public class Student {
  private String name; 
  public String whatsYourName () {
    return name; 
  }
  Student() { } 
  Student(String givenName) {
    name = givenName;     
  }
  void addQuizScore(int newScore) {
    totalScore = totalScore + newScore; 
    numberOfScores = numberOfScores + 1; 
  } 
  private int totalScore; 
  int whatsYourTotalScore() {
    return totalScore; 
  }
  private int numberOfScores; 
  double reportAverage() {
    return (double)totalScore / numberOfScores; 
  }   
} 
frilled.cs.indiana.edu%

48. Can you test that?

49. Sure, how about this:

frilled.cs.indiana.edu%pico StudentTest.java
frilled.cs.indiana.edu%cat StudentTest.java
public class StudentTest {
  public static void main(String[] args) {
    Student a = new Student("Larry");
    Student b = new Student("Michael");
    String answer; 
    System.out.print("Printing the name of the first student: "); 
    answer = a.whatsYourName(); 
    System.out.println(answer); 
    System.out.print("Printing the name of the second student: "); 
    answer = b.whatsYourName(); 
    System.out.println(answer); 

    a.addQuizScore(100); 
    a.addQuizScore(90); 
    a.addQuizScore(100); 
    System.out.println("Student " + a.whatsYourName() + "reports: "); 
    System.out.println("   cumulative score: " + a.whatsYourTotalScore());
    System.out.println("   average score:    " + a.reportAverage());

  } 
} 

51. Indeed. And here's the actual test:

frilled.cs.indiana.edu%javac StudentTest.java
frilled.cs.indiana.edu%java StudentTest
Printing the name of the first student: Larry
Printing the name of the second student: Michael
Student Larryreports: 
   cumulative score: 290
   average score:    96.66666666666667
frilled.cs.indiana.edu%

53. Yes. Isn't it time for a break?

54. I sure think so.

55. See you next week!

Until then, here's a brief summary of chapter 4:

Here are some problems for you to practice with (in preparation of your next week's lab assignment).

Try to solve these problems to practice some of the things you learned this week. In the examples that follow, your program's answers are always in blue, to distinguish them from what you would type as a user. Remember: the resulting programs are elementary, and the problems are interesting.

1.

Write a program that asks for an initial balance amount. Create a BankAccount object with that amount. Then ask for a deposit amount and a withdrawal amount. Carry out the deposit and withdrawal, then print the remaining balance. Use ConsoleReader from Lab Notes Two, and please place your main method in the class BankAccount.

Here's a sample run of such a program: frilled.cs.indiana.edu%javac BankAccount.java frilled.cs.indiana.edu%java BankAccount Hello, and welcome to JavaOne Bank. An account will be created for you. What will the initial balance be? Type it now: -40.2 The current balance in your account is: -40.2 You now want to make a deposit. How much? Type the amount here: 120.3 The current balance in your account is: 80.1 You now want to make a withdrawal. How much? Type it now: 34 The current balance in your account is: 46.099999999999994 Thanks for using class BankAccount. Good-bye! frilled.cs.indiana.edu%

Good luck and don't forget: the harder they seem the more you can learn from them.

But these problems are not difficult. They're just that: good practice.