Chapter 8 - Question 5 - Page 695

[Glen Iris]

Hey Guys,

public class HorseTest{
	public void main (String [] args){
		class Horse{
			public String name;
			public Horse(String s){
				name == s;
			}
		}
	Object obj = new Horse("Zippo");
	System.out.println(obj.name);
	}
}

Compilation fails because of an error on line 10.

I do not understand the explaination by the book. Could someone please elaborate on it.

"If you use a reference variable of type Object, you can access only those members defined in class Object."

Is this saying that the code fails to compile as the Object class does not have a public String variable named 'horse'?

Thanks.

[John Jai]

The concept is that when you use your base class reference to refer child class instance polymorphically, you can call only those methods defined in the base class using the base class reference.
Though the base class reference is holding a child class instance it's not possible and allowed to call child class specific methods.

See below program, a football player is a person and so is a tennis player. you cannot ask a football player to play tennis and vice versa.

class Person {
	private String name;
	Person (String name){
		this.name = name;
	}
	void sleep(){
		System.out.println("Sleeping...");
	}

	void eat(){
		System.out.println("Eating...");
	}
	
	String getName(){
		return name;
	}
}

class TennisPlayer extends Person{
	TennisPlayer(String name){
		super(name);
	}
	void playTennis(){
		System.out.println("Playing Tennis...");
	}
}

class FootballPlayer extends Person{
	FootballPlayer(String name){
		super(name);
	}
	void playFootball(){
		System.out.println("Playing Football...");
	}
}


public class PolySample {
	public static void main(String...args){
		Person john = new Person("John");
		System.out.println("person name -> " + john.getName());
		john.eat();
		john.sleep();
		
		Person nadal = new TennisPlayer("Nadal");
		System.out.println("person name -> " + nadal.getName());
		nadal.eat();
		nadal.sleep();
		
		Person messi = new FootballPlayer("Messi");
		messi.eat();
		messi.sleep();
					
		// Suppose for argument's sake if I allow reference of Person to able to call other methods of child
		messi.playTennis();
		john.playFootball();
		
		// Right one is to...
		TennisPlayer federer = new TennisPlayer("Federer");
		federer.playTennis();
		System.out.println("Person name -> " + federer.getName());		
	}
}

[Hiral Jhaveri]

Yes. You understood it right. Since the reference is of classtype Object.

So, using this reference we can only access the fields (or methods) that are defined in class Object.

Though the actual object is of classtype Horse, the compiler doesn't know this, so it gives error!

In order to access name, our reference must be first type-casted to Horse, and then name can be accessed.

System.out.println((Horse)obj.name);

Again, here type-casting works (without compilation error or Exception at runtime) because the actual object is of classtype Horse.

[Joseph Arnold]

Object obj = new Horse("Zippo");

Here is the reference type is Object and hence you can only access members of object and not horse.
If you want to use a access a method/property of Horse, you should simple use the horse reference type itself.
Look at it this way, the only reason Java allows you have to have a base class refer to a sub class type is to make use of polymorphism. Conceptually, a base class reference is used to access a property that is common to all subclasses and not a property pertaining to a particular sub class(here name belongs only to horse).
Common properties of several classes are identified and are put in a common class called the Base class. That is the idea behind inheritance.

[Joanne Neal]

Hiral Jhaveri wrote:

System.out.println((Horse)obj.name);

Check your brackets. That won't compile either.