Easy-to-follow video tutorials help you learn software, creative, and business skills.Become a member

Casting subclass objects

From: Java Essential Training

Video: Casting subclass objects

Earlier in this video course when I was describing the use of primitive variables, I described the process of conversion as either upward or downward. I described the conversion of an integer say to a float or double as an upward conversion and said that this sort of conversion is implicit, automatic, and safe, but if you try to go in the other direction converting downward where you take a very specific value and try to squeeze it into a less specific value that's where the conversion can lose data, and so you have to be explicit in your syntax.

Casting subclass objects

Earlier in this video course when I was describing the use of primitive variables, I described the process of conversion as either upward or downward. I described the conversion of an integer say to a float or double as an upward conversion and said that this sort of conversion is implicit, automatic, and safe, but if you try to go in the other direction converting downward where you take a very specific value and try to squeeze it into a less specific value that's where the conversion can lose data, and so you have to be explicit in your syntax.

This same thing is true of complex objects that have an inheritance relationship, when you use an object that's a subclass and you treat it as its superclass, we don't call that converting we call it casting, but we still use the same directions, the specific version being used as the more general version is called upward casting and taking an object that's been seen as the superclass and casting it downward to the specific object is called down casting. And just as with primitive variables you have to be specific.

Let's take an example, I'm working in a version of my application that's called casting objects. In this version of the application I removed a little bit of code, but I still have a declaration of an olive variable and then I'm instantiating that variable three times, twice as a Kalamata and once as a Liguirian, I'm then adding those objects to an array list that explicitly is declared as only containing instances of the olive class. I'm allowed to pass in the subclass, because that's an upward conversion and it's implicit and it's safe.

But now I'll show you what happens when you try to recast downward. I'll go down to the bottom of the code and I'm going to get a reference to the first item in that array list. I know that it's an instance of Kalamata class, because that's how I declared it up here. So I'll start off below all the other code and put in a data type of Kalamata and I'll name the variable olive1. And then I'll get a reference to that object using the syntax olives.get and I'll pass in a value of 0 for the first item in the array list.

Now when I save my changes and build the project I get this, type mismatch cannot convert from Olive to Kalamata, it's a downward casting that I'm trying to do. Even though I know it's a Kalamata olive, I created it as a Kalamata olive, the compiler and the JVM at this point will think it's simply an olive. So I have to tell the compiler its okay, it's a Kalamata olive. Here is how you do it, place the cursor before the code that's returning the reference and then declare the native type of the object wrapped in parentheses.

I'm telling the compiler and getting the reference to this object, but I want you to treat it as the subclass, I'll save the changes, the error goes away and all I have left now is a warning saying the value of that variable isn't yet used. So let's go ahead and add a little bit specific functionality to Kalamata, that's not available in all other olives. I hold down the Ctrl key on Windows or Command on Mac and click the name of the class Kalamata and that takes me to the declaration of the class.

I'll add a new public method into the class after the override of the crush method. It'll be a public method that returns a String and it will be named getOrigin and I'll return a value simply of "Greece" because that where Kalamata's come from. I'll save my changes and go back to the main class and then I'll output the return value from that method using a System.out.println command and the output will look like this "Olive1 is from" and then I'll append to that the value of olive1.getOrigin.

I'll run the application and there is the result Olive 1 is from Greece. Now if I didn't have this casting syntax I wouldn't even be able to compile the application, but if I could I wouldn't be able to call that get origin method, the compiler would say, well you said that was an olive not a Kalamata and if it's not a Kalamata then it doesn't have to get origin method. So this is how you cast objects downward from the general type to their specific type.

As you continue using the inheritance model in Java you'll find that this casting becomes more and more important. The ability to cast upward and downward so that you can either use objects in their general form that is as their superclass or in their specific form their subclass.

Show transcript

This video is part of

Image for Java Essential Training
Java Essential Training

71 video lessons · 72528 viewers

David Gassner
Author

 
Expand all | Collapse all
  1. 10m 8s
    1. Welcome
      1m 3s
    2. Is this course for you?
      5m 35s
    3. Using the exercise files
      3m 30s
  2. 31m 24s
    1. The history of Java
      5m 19s
    2. Java compilation and syntax
      8m 54s
    3. Understanding the principles of Java
      8m 28s
    4. Choosing a development environment
      8m 43s
  3. 19m 5s
    1. Installing Java on Windows
      6m 42s
    2. Installing Eclipse on Windows
      3m 19s
    3. Exploring Java on Mac OS X Leopard and Snow Leopard
      2m 27s
    4. Installing Java on Mac OS X Lion
      3m 27s
    5. Installing Eclipse on Mac OS X
      3m 10s
  4. 46m 10s
    1. Creating a Hello World application
      11m 7s
    2. Exploring the Eclipse IDE
      8m 55s
    3. Compiling and running from the command line
      8m 2s
    4. Passing arguments to the application
      8m 17s
    5. Using the Java API documentation
      4m 5s
    6. Memory management and garbage collection
      5m 44s
  5. 58m 57s
    1. Everything is an object
      5m 59s
    2. Declaring and initializing variables
      9m 15s
    3. Working with numbers
      8m 32s
    4. Converting numeric values
      6m 40s
    5. Understanding operators
      7m 58s
    6. Working with character values
      5m 14s
    7. Working with boolean values
      5m 13s
    8. Outputting primitive values as strings
      5m 33s
    9. Creating a simple calculator application
      4m 33s
  6. 53m 40s
    1. Writing conditional code
      5m 35s
    2. Using the switch statement
      8m 50s
    3. Repeating code blocks with loops
      7m 35s
    4. Creating reusable code with methods
      6m 31s
    5. Declaring methods with arguments
      5m 41s
    6. Overloading method names with different signatures
      5m 53s
    7. Passing arguments by reference or by value
      5m 35s
    8. Creating a more complex calculator application
      8m 0s
  7. 20m 30s
    1. Using the String class
      5m 44s
    2. Building strings with StringBuilder
      3m 34s
    3. Parsing string values
      3m 19s
    4. Working with date values
      7m 53s
  8. 20m 44s
    1. Understanding compile-time vs. runtime errors
      4m 5s
    2. Handling exceptions with try/catch
      4m 55s
    3. Throwing exceptions in methods
      2m 50s
    4. Using the debugger
      8m 54s
  9. 32m 22s
    1. Using simple arrays
      4m 47s
    2. Using two-dimensional arrays
      6m 17s
    3. Managing resizable arrays with ArrayList
      7m 14s
    4. Managing unordered data with HashMap
      6m 5s
    5. Looping through collections with iterators
      7m 59s
  10. 52m 2s
    1. Understanding encapsulation
      5m 59s
    2. Creating and instantiating custom classes
      8m 8s
    3. Organizing classes with packages
      6m 47s
    4. Creating and using instance methods
      6m 52s
    5. Storing data in instance variables
      6m 56s
    6. Using constructor methods
      5m 40s
    7. Managing instance data with getter and setter methods
      8m 26s
    8. Using class variables and Enum classes
      3m 14s
  11. 41m 15s
    1. Understanding inheritance and polymorphism
      9m 12s
    2. Extending custom classes
      9m 1s
    3. Overriding superclass methods
      3m 8s
    4. Casting subclass objects
      5m 3s
    5. Understanding interfaces and implementing classes
      4m 2s
    6. Creating your own interfaces
      4m 14s
    7. Using abstract classes and methods
      6m 35s
  12. 32m 17s
    1. Managing files with the core class library
      7m 46s
    2. Managing files with Apache Commons FileUtils
      7m 32s
    3. Reading a text file from a networked resource
      7m 52s
    4. Parsing an XML file with DOM
      9m 7s
  13. 17m 39s
    1. Creating your own JAR files
      4m 54s
    2. Understanding the classpath
      5m 2s
    3. Documenting code with Javadoc
      7m 43s
  14. 47s
    1. Goodbye
      47s

Start learning today

Get unlimited access to all courses for just $25/month.

Become a member
Sometimes @lynda teaches me how to use a program and sometimes Lynda.com changes my life forever. @JosefShutter
@lynda lynda.com is an absolute life saver when it comes to learning todays software. Definitely recommend it! #higherlearning @Michael_Caraway
@lynda The best thing online! Your database of courses is great! To the mark and very helpful. Thanks! @ru22more
Got to create something yesterday I never thought I could do. #thanks @lynda @Ngventurella
I really do love @lynda as a learning platform. Never stop learning and developing, it’s probably our greatest gift as a species! @soundslikedavid
@lynda just subscribed to lynda.com all I can say its brilliant join now trust me @ButchSamurai
@lynda is an awesome resource. The membership is priceless if you take advantage of it. @diabetic_techie
One of the best decision I made this year. Buy a 1yr subscription to @lynda @cybercaptive
guys lynda.com (@lynda) is the best. So far I’ve learned Java, principles of OO programming, and now learning about MS project @lucasmitchell
Signed back up to @lynda dot com. I’ve missed it!! Proper geeking out right now! #timetolearn #geek @JayGodbold
Share a link to this course

What are exercise files?

Exercise files are the same files the author uses in the course. Save time by downloading the author's files instead of setting up your own files, and learn by following along with the instructor.

Can I take this course without the exercise files?

Yes! If you decide you would like the exercise files later, you can upgrade to a premium account any time.

Become a member Download sample files See plans and pricing

Please wait... please wait ...
Upgrade to get access to exercise files.

Exercise files video

How to use exercise files.

Learn by watching, listening, and doing, Exercise files are the same files the author uses in the course, so you can download them and follow along Premium memberships include access to all exercise files in the library.


Exercise files

Exercise files video

How to use exercise files.

For additional information on downloading and using exercise files, watch our instructional video or read the instructions in the FAQ.

This course includes free exercise files, so you can practice while you watch the course. To access all the exercise files in our library, become a Premium Member.

Join now "Already a member? Log in

Are you sure you want to mark all the videos in this course as unwatched?

This will not affect your course history, your reports, or your certificates of completion for this course.


Mark all as unwatched Cancel

Congratulations

You have completed Java Essential Training.

Return to your organization's learning portal to continue training, or close this page.


OK
Become a member to add this course to a playlist

Join today and get unlimited access to the entire library of video courses—and create as many playlists as you like.

Get started

Already a member?

Become a member to like this course.

Join today and get unlimited access to the entire library of video courses.

Get started

Already a member?

Exercise files

Learn by watching, listening, and doing! Exercise files are the same files the author uses in the course, so you can download them and follow along. Exercise files are available with all Premium memberships. Learn more

Get started

Already a Premium member?

Exercise files video

How to use exercise files.

Ask a question

Thanks for contacting us.
You’ll hear from our Customer Service team within 24 hours.

Please enter the text shown below:

The classic layout automatically defaults to the latest Flash Player.

To choose a different player, hold the cursor over your name at the top right of any lynda.com page and choose Site preferencesfrom the dropdown menu.

Continue to classic layout Stay on new layout
Exercise files

Access exercise files from a button right under the course name.

Mark videos as unwatched

Remove icons showing you already watched videos if you want to start over.

Control your viewing experience

Make the video wide, narrow, full-screen, or pop the player out of the page into its own window.

Interactive transcripts

Click on text in the transcript to jump to that spot in the video. As the video plays, the relevant spot in the transcript will be highlighted.

Are you sure you want to delete this note?

No

Your file was successfully uploaded.

Thanks for signing up.

We’ll send you a confirmation email shortly.


Sign up and receive emails about lynda.com and our online training library:

Here’s our privacy policy with more details about how we handle your information.

Keep up with news, tips, and latest courses with emails from lynda.com.

Sign up and receive emails about lynda.com and our online training library:

Here’s our privacy policy with more details about how we handle your information.

   
submit Lightbox submit clicked
Terms and conditions of use

We've updated our terms and conditions (now called terms of service).Go
Review and accept our updated terms of service.