Kotlin
Java

Polymorphism
Java

Created By: Geoffrey Challen
/ Updated: 2021-09-29

Next we'll continue to practice with inheritance. We'll also introduce a new big (literally) idea—polymorphism. Polymorphism may sound scary, but it's not, and we'll work it out together like we always do, using a lot of examples.

Another Puzzle

Let's look at another example of puzzling Java code together:

Explain what is confusing about the code above. printIt takes an Object parameter. So why can we pass a String, a Pet, and even a Course?

Polymorphism

Polymorphism is a big word, and sounds a bit scary. But it's actually quite straightforward. Let's work it out together starting with the Wikipedia definition:

In programming languages and type theory, polymorphism is the provision of a single interface to entities of different types

Go through the Wikipedia definition of polymorphism.

"Is A"

One way to think about polymorphism and Java inheritance is to consider "is a" relationships. For example, every instance of any Java class "is a" Object, because every class is a subclass of Object. Other "is a" relationships depend on inheritance relationships established with extends:

Method Overriding and Polymorphism

One frequent confusion regarding polymorphism has to do with overriding inherited methods. Let's look at how that works:

Up and Down Casting

When we create an instance of a class, we can save it into a variable of any type that it can morph into:

This is referred to as upcasting. Java will automatically upcast an instance to any of its supertypes. Because Dog extends Pet and Pet extends Object, a Dog can be stored in a Dog, Pet, or Object variable.

However! The type of the variable determines what we can do with that object. Let's look at how.

Show how upcasting affects the methods that are available.

Don't worry if this seems a bit fuzzy now. We'll return to this topic a few lessons from now when we discuss object references.

Down Casting and instanceof

Consider the type hierarchy established below: Given a Pet variable, it might refer to a Dog, a Cat, a Pet or some other kind of pet!

Is there a way that we can tell? Yup! To test if an object is an instance of a particular class, we use the instanceof operator. And, once we determine that cat is actually an instance of Cat, there is another step that we have to take before we can call meow. Let's examine how that works:

Describe how to use the instanceof operator to distinguish between different Pet subtypes. Show both the old syntax and explict downcasting and the new syntax.

Steady There

The last two lessons have been pretty loaded with new ideas and concepts! Exciting, but also enough to make your head spin.

Don't worry. Over the next two lessons we'll slow down and review what we've learned. And then, over the lessons that follow we'll have even more opportunities to integrate this knowledge, but with a small twist. So be patient. This won't all make sense immediately. But it will all make sense eventually.

Show how to complete the homework problem above. Feel free to cover multiple approaches!

Solution Walkthrough

Show how to complete the homework problem above. Feel free to cover multiple approaches!

Solution Walkthrough