CS 46B - Lecture 3

Cay S. Horstmann

Pre-class reading

• Sections 10.1 - 10.2, Worked Example 10.1
• Did you do the quiz?

Plagiarism and Cheating

• When you submit work, it must be your intellectual creation
• Can you copy and paste?
  • Always ok to copy—pasting is the problem ☺
  • Ok to use code from the problem statement or the textbook without attribution
  • Otherwise, you must attribute the source
• You can discuss your work with others
• You can post snippets (but not a solution) on Piazza for critique
  • But you cannot copy from Piazza
• Don't email working code—you don't know what the recipient will do with it
  • Sending someone a solution is cheating
• I periodically run a plagiarism checker
• If I find cheating, I don't play Sherlock Holmes to find who wrote it—I report both
• Anyone who gets caught cheating twice in this class gets an automatic F in the course

Interface

• Set of methods that must be implemented

  public interface Customer
  {
     void eat();
     boolean pay();
  }

  
• Class implements them

  public class Cat implements Customer
  {
     public void eat() { ... }
     public boolean pay() { ... }
  }
  

• What's the point? Reusable code.

  public void serve(Customer c) // Can be a Cat or another implementing class
  {
     c.eat();
     if (!c.pay()) doDishes(c);
  }

Lecture 3 Clicker Question 1

Which objects can be passed to the pay method in the preceding slide?

Click all that apply

1. Instances of Customer
2. Instances of Cat
3. Instances of Person
4. Instances of Object

Lecture 3 Clicker Question 2

Which service does this stand mixer provide to any attachment that conforms to the “interface” for plugging in attachments?

1. Mixing ingredients
2. Whipping a liquid such as cream or mayonnaise
3. Rotating the attachment
4. All of the above

Measurable Interface

• public interface Measurable
  {
     double getMeasure();
  }

• Not a standard interface—created for this course
• What's the point? Reusable code.

  public static double average(Measurable[] objects)
     // Can be a BankAccount, Country, etc. 
  {
     double sum = 0;
     for (Measurable obj : objects)
        sum = sum + obj.getMeasure();
     if (objects.length > 0) return sum / objects.length;
     else return 0;
  }

Decoupling

Lecture 3 Clicker Question 3

In Eclipse, make a project out of the ch10/section_1 directory of the Book Code. Add the class below, and add the print statement to the main method of the tester.

public class Word implements Measurable
{
   private String text;
   public Word(String text) { this.text = text; }
   public double getMeasure() { return text.length(); }
}
...
System.out.println(Data.average(new Word[] {
   new Word("Hello"), new Word("World!") }));

How should the tester be completed?

1. System.out.println("Expected: 5");
2. System.out.println("Expected: 5.5");
3. System.out.println("Expected: 6");
4. System.out.println("Expected: Hello, World!");

Interface Variables

• There are no objects of type Measurable
• An interface is not a class
  • Cannot be instantiated
• But you can declare variables of type Measurable

  Measurable obj = new BankAccount();

• Must be initialized to an object of a class that implements the interface
• Method invocation is polymorphic:

  double m = obj.getMeasure(); // method depends on class of obj

Casts

• public static Measurable larger(Measurable obj1, Measurable obj2)
  {
     if (obj1.getMeasure() > obj2.getMeasure()) return obj1;
     else return obj2;
  }

• Return type is Measurable

  Country uruguay = new Country("Uruguay", 176220);
  Country thailand = new Country("Thailand", 513120);
  Country max = larger(uruguay, thailand); // ERROR

• But the return type Measurable isn't so useful:

  Measurable max = larger(uruguay, thailand);
  System.out.println(max.getName()); // ERROR
  

• Remedy: Cast

  Country max = (Country) larger(uruguay, thailand);

Worked Example: Number Sequences

• Worked examples on Book Site
• Investigate properties of sequences

  public interface Sequence
  {
     int next();
  }

• Example: Distribution of last digits

  public void process(Sequence seq, int valuesToProcess)
  {
     for (int i = 1; i <= valuesToProcess; i++)
     {
        int value = seq.next();
        int lastDigit = value % 10;
        counters[lastDigit]++;
     }
  }

• Reusable: Works for any sequence

Sequence Classes

• public class SquareSequence implements Sequence
  {
     private int n;
     public int next()
     {
        n++;
        return n * n;
     }
  }

• public class RandomSequence implements Sequence
  {
     private Random generator = new Random();
     public int next()
     {
        return generator.nextInt();
     }
  }

Lecture 3 Clicker Question 5

Make an Eclipse project from the ch10/worked_example_1 directory in the Book Code

What are the last digits of the sequence of the cubes? (1³, 2³, 3³, etc.?)

Implement the sequence class and modify the SequenceDemo program.

What result do you get?

1. 100 100 100 100 100 100 100 100 100 100
2. 100 200 0 0 200 100 200 0 0 200
3. 105 94 81 112 89 103 103 100 108 105
4. Something else

Lecture 3 Clicker Question 6

public class Mystery implements Sequence
{
   private Sequence sequence;
   public Mystery(Sequence sequence) { this.sequence = sequence; }
   public int next() { return sequence.next() % 10; }
}

Which of the following is true?

1. It is not legal for a class to implement an interface and have an instance variable of the same interface.
2. If one makes a Mystery object and calls next on it, then an infinite recursion occurs, i.e. the next method keeps calling itself
3. The process method can be rewritten as

   public void process(Sequence seq, int valuesToProcess)
   {
      Sequence digits = new Mystery(seq);
      for (int i = 1; i <= valuesToProcess; i++) { counters[digits.next()]++; }
   }

4. The loop

   Sequence seq = new Mystery(new SquareSequence());
   for (int i = 1; i <= 5; i++) System.out.print(seq.next());

   prints 1491625

Before Next Class

• Read sections 10.3 - 10.5, Special Topic 14.4 (!)
• Make sure to do the quiz
• Turn in homework 2 draft
• Mac users: Install Emacs before Lab 2. Or use the VM during Lab 2.