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Comparable interface

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

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

• You do not write this interface. You use the one in the Java library
• Call the compareTo 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 object1 should come after object2.

Activity - Comparable interface in the String class

Record your participation in Piazza clicker question Lesson22 Q1

• String class implements Comparable. How could you find out for yourself if it implemented Comparable?
• Let's look at how Strings are compared
• 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 a library utility to sort an array or ArrayList containing objects of your class.
• Arrays.sort for arrays or Collections.sort for ArrayLists
• Download and open comparable_interface.zip
• Look at ComparableTester.
• We want to sort the array of BankAccounts.

Activity - Try to sort BankAccounts

Record your participation in Piazza clicker question Lesson22 Q2

• Open the comparable_interface project
• What happens when you compile ComparableTester?
• What happens when you execute the main method?
• Why?

Activity - Implementing Comparable interface in BankAccount

Record your participation in Piazza clicker question Lesson22 Q3

Let's fix the problem by implementing the Comparable interface.

• 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. Why? What message do you get?
• Add the required method from the interface, compareTo, as a stub. Compile
• Now implement the body of the compareTo method ordering by balance
• If the balances are equal, order by accountId

Activity - A trick for comparing doubles

Record your participation in Piazza clicker question Lesson22 Q4

• A simplier way 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 Lesson22 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 and sorts it.

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

Activity - Creating Comparable objects

Record your participation in Piazza clicker question Lesson22 Q6

What would happen if you tried 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)