Apex Inheritance: illegal Assignment

Question

Why is the following code returning an illegal assignment error in anonymous window?

Line: 3, Column: 9 illegal assignment from truck to vehicle

Super Class

public virtual class vehicle {

    public void model(){
         system.debug ('Model method of vehicle class ');
    }
    public void speed(){
        system.debug('Speed of the vehicle depends of the model');
    }
}

Child Class

public class truck extends vehicle 
{
    public override void speed()
    {
        system.debug('Truck has speed of 20 mile/hr');
    }
}

Execute Anonymous Script

vehicle v=new vehicle();
v.speed();
vehicle v2=new truck();
v2.speed();
v2.model();

Answer 1

I think you may have to mark your methods as virtual too in your base class.

**

In addition, the extending class can override the existing virtual methods by using the override keyword in the method definition.

**

public virtual void model(){
     system.debug ('Model method of vehicle class ');
}
public virtual void speed(){
    system.debug('Speed of the vehicle depends of the model');
}

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_extending.htm

Answer 2

The code you have posted is valid. You should be able to assign a child class to a super variable. Likely causes for this code to fail:

• Naming conflict
• Unnoticed compile fail

You can run the following script in Execute Anonymous to demonstrate the basic concept:

class Vehicle { }
class Truck extends Vehicle { }
Vehicle instance = new Truck();

Answer 3

Did your truck class even compile?

I just copied your code and I had to make the speed() method virtual, in order to override it in the truck class. Once I did this it worked as you would expect.

//vehicle class
public virtual class vehicle {

    public void model(){
        system.debug ('Model method of vehicle class ');
    }
    public virtual void speed(){
        system.debug('Speed of the vehicle depends of the model');
    }
}

//truck class
public class truck extends vehicle { 
    public override void speed() { 
        system.debug('Truck has speed of 20 mile/hr'); 
    } 
}

//execute anonymous
vehicle v=new vehicle(); 
v.speed(); 

//truck v2 = new truck(); or vehicle v2 = new truck(); both work. 
// I find the truck v2 =... cleaner
truck v2=new truck(); 
v2.speed(); 
v2.model();
system.debug(v2);