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Interfaces

Allows differernt objects to make use of the same service

• a lightbulb socket
• a USB port
• a handle for a set of tools

Concept - Java interfaces

• We can also have software interfaces. In Java we use the keyword interface
• Syntax

  public interface interfaceName
  {
          //one or more methods that describe what the implementing class has to provide
          //public by default
          // no body. They are abstract
  }

• Example

  public interface Measurable
  {
     double getMeasure();
  }

• You can write your own interfaces as Dr Horstmann does in your book
• Most often at this stage you will use interfaces that have already been written

Concept - Implementing an interface

There are two parts

• Indicate that the class uses the interface with the implements keyword

  public class MyClass implements interfaceName

  For example

  public class Product implements Measurable

• Provide the body for the methods declared in the intefrace definition
  For example:
  T o complete the implementation of the Measuable interface in the Product class

  public double getMeasure()
  {
       return price;
  }
      

Concept - Simplified Comparable interface

• Defined in the Java library
• Has one method: compareTo

• public interface Comparable
  {
     int compareTo(Object otherObject);
  }

• You do not write the interface. Use the one in the Java library
• Call the campareTo method like this:

  object1.compareTo(object2)

• The compareTo method returns
  • a negative number if object1 should come before object2,
  • zero if object1 and object2 are the same
  • a positive number if object2 should come before object1.

Activity - Comparable interface in the String class

Record your participation in Piazza clicker question Lesson18 Q1

• String class implements Comparable. How could you find out for yourself if it implemented Comparable?
• What do these comparisons return?
  • "hello".compareTo("help")
  • "help".compareTo("hello")
  • "help".compareTo("help")
  • "Help".compareTo("apple")
  • "999".compareTo("apple")

Why implement the Comparable interface in your classes

• Allows you to use services provided by utility classes in the Java library
• Arrays.sort for arrays or Collections.sort for ArrayLists

Concept - Implementing the Comparable interface

• State that the class will implement the Comparable interface

  public class MyClass implements Comparable

• Implement the compareTo method - with this exact signature

  public int compareTo(Object otherObject)

• First you will need to cast otherObject to a MyClass to get a MyClass reference

  MyClass other = (MyClass)otherObject;

• Then you will compare the instance variables of the two obects in some way returning
  • a negative number if this object should come before other,
  • zero if this object and other are the same
  • a positive number if other should come before this object.

Activity - Try to sort BankAccounts

Record your participation in Piazza clicker question Lesson18 Q2

• Download comparable_interface.zip and make a project
• Make the project without a bunch of copy/paste
• Look at ComparableTester.
• We want to sort the array of BankAccounts using Arrays.sort.
• What happens when you compile ComparableTester?
• What happens when execute the main method?
• Why?

Activity - Implementing Comparable interface in BankAccount

Record your participation in Piazza clicker question Lesson18 Q3

Fix the problem by implementing the Comparable interface in BankAccount.

• The class header needs to specify that we are implementing Comparable interface
• BankAccount already implements one interface
• To add another add a comma and the new interface name

   , Comparable

• Won't compile
• Add the required method from the interface, compareTo(Object otherObject), as a stub. Compile
• Now implement the body of the compareTo method
  • You will need to cast otherObject to a BankAccount
  • order by balance
  • If the balances are equal, order by accountId

Activity - A trick for comparing doubles

Record your participation in Piazza clicker question Lesson18 Q4

• To compare doubles

  Double.compare(double d1, double d2)

  • returns a negative integer if d1 is smaller
  • returns 0 if d1 and d2 are equal
  • returns a positive integer if d1 is larger
• This is a static method. How can you tell?
• Modify BankAccount compareTo method to use Double.compare

Activity - Implementing Comparable interface in Student

Record your participation in Piazza clicker question Lesson18 Q5

Add the Comparable interface to the Student class. Students are ordered by GPA. If the GPA's are the same, they are ordered by name. Uncomment the code in ComparableTester that creates the ArrayList.

1. Got it
2. Got it but it was hard
3. Didn't get it

Activity - Casting

Record your participation in Piazza clicker question Lesson18 Q6

In ComparableTester, Student objects are in a Comparable array.

• Look at the code.
  • This is valid. Student is-a Comparable
• uncomment the code for testing and run
• What happens? Why?
• Fix the problem by casting

Activity - Creating Comparable objects

Record your participation in Piazza clicker question Lesson18 Q7

What will happen when you try to compile these statement

• Comparable c = new Comparable();
• Comparable c = new Student("Alice", 3.2);
• Comparable c1 = new Rectangle(10, 20, 30, 40); (Look in the Rectangle API)

Activity - Limits of the Comparable interface

Record your participation in Piazza clicker question Lesson18 Q8

Can you use the Arrays.sort to find the Rectangle with the largest area in an array of rectangles by sorting the array?

1. Yes
2. No
3. It depends