Welcome to the Treehouse Community

Want to collaborate on code errors? Have bugs you need feedback on? Looking for an extra set of eyes on your latest project? Get support with fellow developers, designers, and programmers of all backgrounds and skill levels here with the Treehouse Community! While you're at it, check out some resources Treehouse students have shared here.

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and join thousands of Treehouse students and alumni in the community today.

Start your free trial

Posted December 2, 2015 10:41am by

Can someone tell me what's wrong with my code (Inheritance)

Im messing around trying to familiarize myself with inheritance and I keep getting an error in Workspace telling me "Cannot find symbol". Iv looked over my code many times and cannot figure out what's wrong.

Example.java

import java.util.*;
import java.io.*;

public class Example {

    public static void main(String[] args) {
       Cars superBike = new Motorcycle("Black", 100);
       superBike.zoomZoom();
    }
}

Cars.java

import java.util.*;
import java.io.*;


public class Cars{
public String mColor;
public int mSpeed;

public void takeOff(){
  System.out.println("The car takes off the speed of " + this.mSpeed);
}  

public Cars(String color, int speed){
  mColor = color;
  mSpeed = speed;
}
}

public class Motorcycle extends Cars{
  public void zoomZoom(){
    System.out.println("The " + this.mColor + " motorcycle takes off at the speed of " + this.mSpeed);
  }
}

They are both in the same location as far as package goes. So I believe I do not need to import my Cars.java

December 2, 2015 3:21pm

Hi Lucas,

what happens if you do it like this:

blic class Example {

    public static void main(String[] args) {
       Motorcycle superBike = new Motorcycle("Black", 100);
       superBike.zoomZoom();
    }
}

Tell us if it does the trick ..

Grigorij

December 3, 2015 6:44am

Hey Grigorij

It worked but it does not make sense. Everything that iv learned about casting in the Data Structure course says you can instantiate an instance like that because Cars is a Superclass of Motorcycle which is the subclass

So I had

Cars superBike = new Motorcycle("Black", 100);

Then you changed Cars to the subclass of Motorcycle like so:

Motorcycle superBike = new Motorcycle("Black", 100);

But what I had should have worked according to what I learned in the Java courses?

December 3, 2015 6:07pm

Hey Lucas,

your Motorcycle class is inheriting ("extend") from the superclass Cars right. So if you instatializing a new Motorcycle object, this object is inheriting from the superclass too.

The Motorcycle object can use methods from the Motorcycle class or the super class Cars.

And look here:

http://stackoverflow.com/questions/2701182/call-a-method-of-subclass-in-java

This explains the topic very good

Grigorij

4 Answers

December 2, 2015 10:51am

I think you have to declare a Motorcycle constructor that take those two arguments and call super(color, speed) in the first line. Cause, by default, you just have a basic constructor with no arguments.

December 2, 2015 10:54am

tried that already and I end up getting a lot more errors,

public class Motorcycle extends Cars{
  public void zoomZoom(){
    System.out.println("The " + this.mColor + " motorcycle takes off at the speed of " + this.mSpeed);
  }
    public MotorCycle(mColor, mSpeed){
    super(mColor,mSpeed);
  }
}

December 2, 2015 10:56am

What kind of errors? Can we see them?

December 2, 2015 10:59am

To see it better just open Workspae and create a new project and simply paste in my code.

Here are the errors after I try adding the super() on the Motorcycle constructor. Went from 2 errors to 8 after that.

./Motorcycle.java:5: error: invalid method declaration; return type required                     
    public MotorCycle(mColor, mSpeed){                                                           
           ^                                                                                     
./Motorcycle.java:5: error: <identifier> expected                                                
    public MotorCycle(mColor, mSpeed){                                                           
                            ^                                                                    
./Motorcycle.java:5: error: <identifier> expected                                                
    public MotorCycle(mColor, mSpeed){                                                           
                                    ^                                                            
./Motorcycle.java:5: error: cannot find symbol                                                   
    public MotorCycle(mColor, mSpeed){                                                           
                      ^                                                                          
  symbol:   class mColor                                                                         
  location: class Motorcycle                                                                     
./Motorcycle.java:5: error: cannot find symbol                                                   
    public MotorCycle(mColor, mSpeed){                                                           
                              ^                                                                  
  symbol:   class mSpeed                                                                         
  location: class Motorcycle                                                                     
Example.java:8: error: cannot find symbol                                                        
       superBike.zoomZoom();                                                                     
                ^                                                                                
  symbol:   method zoomZoom()                                                                    
  location: variable superBike of type Cars                                                      
./Motorcycle.java:6: error: cannot reference mColor before supertype constructor has been called 
    super(mColor,mSpeed);                                                                        
          ^                                                                                      
./Motorcycle.java:6: error: cannot reference mSpeed before supertype constructor has been called 
    super(mColor,mSpeed);                                                                        
                 ^                                                                               
8 errors

December 2, 2015 11:15am

Oh yes, you missed types

public MotorCycle(String mColor, int mSpeed){

December 2, 2015 11:32am

That fixed most of my errors but now there is only one error left and this is my code as of now.

import java.util.*;
import java.io.*;


public class Cars{
public String mColor;
public int mSpeed;

public void takeOff(){
  System.out.println("The car takes off the speed of " + this.mSpeed);
}  

public Cars(String color, int speed){
  mColor = color;
  mSpeed = speed;
}
}


public class Motorcycle extends Cars{
  public void zoomZoom(){
    System.out.println("The " + this.mColor + " motorcycle takes off at the speed of " + this.mSpeed);
  }
    public Motorcycle(String color, int speed){
    super(color, speed);
  }
}

Then in my Example.java

import java.util.*;
import java.io.*;

public class Example {

    public static void main(String[] args) {
       Cars superBike = new Motorcycle("Black", 100);
       superBike.zoomZoom();
    }
}

Now I am only left with one error and that is.

Example.java:8: error: cannot find symbol                                                        
       superBike.zoomZoom();                                                                     
                ^                                                                                
  symbol:   method zoomZoom()                                                                    
  location: variable superBike of type Cars                                                      
1 error

December 3, 2015 6:40pm

Especially this explanation is good:

When dealing with inheritance/polymorphism in Java there are basically two types of casts that you see:

Upcasting:

Superclass x = new Subclass();

This is implicit and does not need a hard cast because Java knows that everything the Superclass can do, the Subclass can do as well.

Downcasting

Superclass x = new Subclass();
Subclass y = (Subclass) x;

In this case you need to do a hard cast because Java isn't quite sure if this will work or not. You have to comfort it by telling it that you know what you're doing. The reason for this is because the subclass could have some weird methods that the superclass doesn't have.

In general, if you want to instantiate a class to call something in its subclass, you should probably just instantiate the subclass to begin with -- or determine if the method should be in the superclass as well.

Thanks to artgon from stack overflow

Hope it helps a little bit ...

Grigorij

Posting to the forum is only allowed for members with active accounts.
Please sign in or sign up to post.

Welcome to the Treehouse Community

Looking to learn something new?

Lucas Santos

Lucas Santos

Can someone tell me what's wrong with my code (Inheritance)

Grigorij Schleifer

Grigorij Schleifer

Lucas Santos

Lucas Santos

Grigorij Schleifer

Grigorij Schleifer

4 Answers

ginopino

ginopino

Lucas Santos

Lucas Santos

ginopino

ginopino

Lucas Santos

Lucas Santos

ginopino

ginopino

Lucas Santos

Lucas Santos

Grigorij Schleifer

Grigorij Schleifer