Regarding super() and this()

[Ashwin Rao]

I'm preparing for the OCA 7 exam. I learnt from the OCA 7 book by K&B that you are only allowed to access static variables and methods as a part of the call to super() or this(). I think I would find it easier to understand if they had given a complete example but they just gave (super(Animal.NAME); // OK because NAME is static.) but I am not finding the complete code in the book. So if anyone could give a complete example I would be really grateful. Also another query..Is this actually used in real-world programs?

[Rehan Zahoor]

class Animal{
     static String defaultAnimalName = "";
     String name;
     public Animal(){
          this(defaultAnimalName);
     } 
     public Animal(String name){
          this.name = name;
     }
}

This is a simple example of use of static variable in constructor of class Animal. Note that no argument constructor calls the other constructor using static variable.
This technique is used in java classes. There can be many occasions when use of static variable in the super() or this() is the most appropriate choice.

[Campbell Ritchie]

Rehan Zahoor wrote: . . . There can be many occasions when use of static variable in the super() or this() is the most appropriate choice.

Really? Please explain why that is an appropriate use of a static field.

I think OP has misunderstood the passage in the book. Please tell us exactly what the book says about static fields being only permitted in this(...) and super(...).

[Ashwin Rao]

There is no passage. It written as just one sentence or point and it goes like this:

Only static variables and methods can be accessed as part of the call to super() or this().(Example:super(Animal.NAME) is OK, because NAME is declared as a static variable)

[Junilu Lacar]

That may be as opposed to accessing instance variables and methods as part of a call to super() or this().

[Dave Tolls]

Ashwin Rao wrote:There is no passage. It written as just one sentence or point and it goes like this:

Only static variables and methods can be accessed as part of the call to super() or this().(Example:super(Animal.NAME) is OK, because NAME is declared as a static variable)

Because the rules of Java state that you can't access a Child's attributes before the supertype's constructor has been called (they don't exist before then).

[Henry Wong]

Yeah, quite frankly, I never understood the restriction -- as it can be easily circumvented via a getter method. Obviously, the instance variables exists at the call, and they have default values.

Henry

[Dave Tolls]

Henry Wong wrote:
Yeah, quite frankly, I never understood the restriction -- as it can be easily circumvented via a getter method. Obviously, the instance variables exists at the call, and they have default values.

Henry

Not in the super() or this() call, eg super(getId()). That won't compile.

Yes, if you have an abstract method which the supertype calls in its constructor. But then that's considered a no no anyway.

Frankly I'm not concerned as passing a subtype's attributes into a supertype strikes me as just plain wrong.