CS61B

11.6 Exercises

Exercises

Which of the following are examples of subtype polymorphism?
- Having a sqrt(int) and sqrt(double) method in the same class.
- Having a Dog override the makeSound method that it inherits from Animal.
- Creating a class that implements the built-in Comparable interface.
How would you compare two strings alphabetically in Java?
Suppose we correctly define a Comparator class called SixComparator for integers that compares them based on the number of 6s they contain. What will the code (new SixComparator()).compare(12345678, 45666678) return?
What is the main difference between the Comparable and Comparator interfaces?

Solutions

Problem 1

Having a Dog override the makeSound method that it inherits from Animal.

Overriding makeSound allows us to have different implementations of the same method via subtypes.

Creating a class that implements the built-in Comparable interface.

Implementing the Comparable interface involves overriding Comparables’ methods, allowing for differing behavior of the same method across types.

Problem 2

s1.compareTo(s2)

Problem 3

It will return some negative number, since 12345678 has less 6’s than 45666678. Note that there is no guarantee of what the negative number’s value is, only that it is less than 0.

Problem 4

An object that implements Comparable can compare another object to itself, whereas a Comparator compares two objects other than itself.

A good way to remember this is that a Comparable has an inherent property of being able to be compared, while a Comparator is an external source of truth.