Generics Question

[Chandra Bhatt]

Basket b = new Basket(); // 1

Basket<Apple> bA = b; // 2

Basket<Orange> bO = b; // 3

bA.setElement(new Apple()); // 4

Orange orange = bO.getElement(); // 5

a)The lines 2 and 3 will cause a compile error.

b)The line 4 will cause a compile error.

c)The line 5 will cause a compile error because a cast is missing.

d)The source code will be compiled with warning(s). During the runtime a ClassCastException will be thrown in the line 5.

e)The soure code will be compiled with warning(s). No exception will be thrown during the runtime.

Source: Java Beat

Please guide me to understand this question!


Regards,
cmbhatt

[Chandra Bhatt]

My two questions (generics) are left orphaned, nobody cares to answer!

Isn't it like Sun has removed the generics from exam ***


Regards,

cmbhatt

***Don't panic please, it is nothing like that. I was eagerly waiting for the answer.

[Sam Sunamin]

    List list = new ArrayList(); //1
    List<String> listStr = list; //2
    List<Integer> listInt = list; //3
    listStr.add("ABC");           //4
    Integer in=listInt.get(0);    //5

You code is just like the code above. The code compile with warning and will get runtime exception java.lang.ClassCastException on line 5.
[ April 11, 2007: Message edited by: Sam Sunamin ]

[Keith Lynn]

I would say option 5.

Consider these test classes.

import java.util.*;
 
public class Basket<T> {
   private List<T> list;
 
   public Basket() {
      list = new ArrayList<T>();
   }
 
   public <E extends T> void setElement(E e) {
      list.add(e);
   }
 
   public T getElement() {
      return(list.get(list.size()-1));
   }
}

public class Fruit {
}

public class Apple extends Fruit {
}

public class Orange extends Fruit {
}

public class Test27 {
 
   public static void main(String[] args) {
      Basket b = new Basket(); // 1
 
      Basket<Apple> bA = b; // 2
 
      Basket<Orange> bO = b; // 3
 
      bA.setElement(new Apple()); // 4
 
      Orange orange = bO.getElement(); // 5
   }
 
}

In line 1, you create a new instance of a Basket but all type information is erased because you created a raw Basket.

In line 2, you cause a compile-time warning to be generated because you are assigning a type safe reference equal to a raw reference.

You also do the same thing in line 3.

Line 4 is fine because bA has reference type Basket<Apple> so you can add an instance of an Apple to it.

But in line 5, the getElement method called on the reference type Basket<Orange> tries to cast the element in the Basket, which is of type Apple, to an Orange.

So you get a ClassCastException.

[Chandra Bhatt]

Thanks Keith,
And for you! specially for making me clear on the Line 5 issue!
I got why runtime "ClassCastException" there!



Regards,
cmbhatt