The Big Programming Thread - Page 689

On December 07 2015 02:10 travis wrote:
But in that case, any time I do Animal.talk(); won't it just say "Animal" regardless of if it is a cat or a dog?

Or does it not work that way? do the subclasses get priority?

Animal heffalump = new Animal();
Animal felix = new Cat();
Animal scooby = new Dog();

heffalump.talk();
felix.talk();
scooby.talk();

On December 07 2015 02:17 Acrofales wrote:
And connecting this back to our discussion on setters in the constructor, this is the only reason I can really think of why calling setters from the constructor can be a problem. If you override the setters in your subclasses, then you run a (very real and hard to debug) risk of breaking the superclass' constructor. So in general, my answer earlier went a bit overboard on the use of setters (and getters): the constructor is probably the only place where you shouldn't do that... although there is a caveat in that if the way you are overriding getters and setters in the subclass breaks superclass functionality, you are probably messing stuff up in any case, whether that is in the constructor or otherwise and you should rethink your design.

On December 07 2015 02:16 travis wrote:
hmm okay.

so is this how I am supposed to do it if I want to print the fields that are both in the subclass and in the superclass:


public String getInfo(){
return this.getArtist()+this.getSongs()+super.getTitle()+super.getCopies();
}

public class Media {

protected String title;

public String getTitle() {
     return "Generic media: " + title;
}
}

public class Movie extends Media {

public String getTitle() {
    return "Movie: " + title;
}
}

On December 07 2015 02:44 Acrofales wrote:


Try to avoid calling super where necessary. Imagine the following:

public class Media {

protected String title;

public String getTitle() {
     return "Generic media: " + title;
}
}

public class Movie extends Media {

public String getTitle() {
    return "Movie: " + title;
}
}

Now, lets say you instantiate a Movie called "Terminator". If you explicitly call super.getTitle(), you will get "Generic media: Terminator" instead of "Movie: Terminato" from just calling getTitle().

A subclass inherits all public and protected methods and fields from its parent, so unless you explicitly want to refer to something that is being done differently in the parent and the child, and you want the parent's way of doing it, don't use super.

On December 07 2015 02:41 spinesheath wrote:

Generally, calling virtual methods from inside a constructor is a potential risk and not recommended. Some languages can deal with it, some can't, at least not always (an overriden method might access a field of the subclass while the subclass hasn't been constructed/initialized yet). And if they can deal with it, it usually is still prone to errors or not working as you'd expect. And if I remember correctly, methods in Java are virtual by default, so you have to be careful with just about any method call in a constructor.

public class Number {
  protected int value;

  public Number(int value) {
     setValue(value);
  }

  public setValue(int value) {
     this.value = value;
  }

  public getValue() {
    return value;
  }
}

public class BigNumber extends Number {

   private final Number size;

   public BigNumber(int value, Number size) {
     super(value);
     this.size = size;
   }

   public setValue(int value) throws ValueException{
      if(value > size.getValue()) {
         this.value = value;
      } 
      else throw new ValueException("not a big number");
   }
}