CS 46B - Lecture 2

Cay S. Horstmann

Pre-class reading

• Sections 9.5 - 9.6, Worked Example 1
• Did you do the quiz?

Inner Class

• Class defined inside another class
• Can be outside or inside method

  class Outer
  {
     class Inner { ... }
     void someMethod() 
     {
        Inner fred = new Inner();
        ...
     }
  }

  class Outer
  {
     void someMethod()
     {
        class Inner { ... }
        Inner fred = new Inner();
        ...
     }
  }

• Useful for small helper classes

Example: Comparator

• Want to sort an array of strings by length
• Want to use Arrays.sort
• But that sorts strings by dictionary order (why?)
• Use Comparator instead
• Need a class that implements Comparator

  public static void main(String[] args)
  {
     String[] names = { "Fred", "Wilma", "Barney" };
     class LengthComparator implements Comparator
     {
        public int compare(Object o1, Object o2)
        {
           return ((String) o1).length() - ((String) o2).length();
        }
     }
     Arrays.sort(names, new LengthComparator());
  }

Anonymous Classes

• Look at

  Arrays.sort(names, new LengthComparator());

• The red object is anonymous
• Could have given it a name

  Comparator fred = new LengthComparator();
  Arrays.sort(names, fred);

• Can make classes anonymous too:

  Arrays.sort(names, new Comparator() 
     {
        public int compare(Object o1, Object o2)
        {
           return ((String) o1).length() - ((String) o2).length();
        }
     });

• We won't use it yet, but you'll see it a lot in CS151

Lecture 2 Clicker Question 1

Let's make Item an inner class of ArrayListAddressBook (from the lab):

public class ArrayListAddressBook 
{
   private class Item 
   {
      String name;
      String key;
      String value;
   }
   ...
}

Which of the following is true?

1. All methods of the ArrayListAddressBook class can construct and access Item objects
2. Any code outside the ArrayListAddressBook class refers to the inner class as ArrayListAddressBook.Item
3. The Item class is poorly designed because the instance variables of Item should have been declared as private
4. The Item class should have been added to the AddressBook interface instead of the ArrayListAddressBook class

Mock Objects

• Problem: How can you test code if you don't have all the pieces yet
• Example: Want to write and test user interface for AddressBook
• Define a class that only implements the services that you absolutely need right now

  public class MockAddressBook implements AddressBook
  {
     public void load(String sourceName) {}
     public void save() {}
     public void get(String name, String key) { return "Fred"; }
     public void put(String name, String key, String value) {}
     public void remove(String name, String key) {}
  }

• Great for team work. You can make progress while someone else writes the real address book
• Later, change

  AddressBook book = new MockAddressBook()

  to

  AddressBook book = new ArrayListAddressBook()

Lecture 2 Clicker Question 2

Why is it necessary that the real class and the mock address book class implement the same interface?

1. In order to make use of already-written methods in Java classes such as Arrays and Collections
2. In order to minimize the amount of code that needs to be updated when the mock class is replaced with the real class
3. In order to allow two programmers to work independently on each class
4. In order to reuse the address book class in other projects

Interfaces in the Wild

• Our CS160 students wrote a mobile app for the Cinequest film festival
• Works on BlackBerry, Android, iPhone
• Really tough to debug cell phone app
• Solution: Debug as much as possible on the desktop
• Solution: Share as much as code as possible among BlackBerry, Android
• Interfaces to the rescue
• Platform interface provides services for getting data from the web, storing/loading data on the device, running tasks in parallel, etc.
• Three implementations AndroidPlatform, BlackberryPlatform, DesktopPlatform

Worked Example: Sequences

• Worked examples are at book companion site
• Read them!
• Gets frequencies of last digits

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

• Different classes implement this interface

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

• Fun exercise P9.11—first digits are not random

Lecture 2 Clicker Question 3

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

Lab

• You will work with a buddy
• Buddy list is in D2L
• No buddy? Come to the front of the room
• One of you (driver) fusses with the code, the other (scribe) types up answers
• Switch roles each class meeting
• Submit lab work to Desire2Learn