Generic methods question

[megha joshi]

Hi all,

Can anyone please explain why D. is not valid.

  
 import java.util.*;
 4. class Car { }
 5. class Honda extends Car { }
 6. public class Test {
 7.   public static void main (String[] args) {
 8.     List<Car> cars = new ArrayList<Car>();
 9.     List<Honda> cars2 = new ArrayList<Honda>();
 10.     List<Object> cars3 = new ArrayList<Object>();
 11.     takeCars(cars);
 12.     takeCars(cars2);
 13.     takeCars(cars3);
 14.   }
 15.   //  insert code here
 16. }
 
 
 
  

Which two, inserted independently at line 15, allow the file to compile?

A. public static void takeCars(List<?> list) { }
B. public static void takeCars(List<Object> list) { }
C. public static void takeCars(List<? extends Car> list) { }
D. public static void takeCars(List<T extends Object> list) { }
E. public static void takeCars(List<? extends Object> list) { }

Source: Sun Practice exams.
Ans : A,E

[ April 26, 2007: Message edited by: megha joshi ]
[ April 26, 2007: Message edited by: megha joshi ]

[Keith Lynn]

You don't list the extends clause like that.

If you modify d to be this, it will compile.

public static <T extends Object> void takeCars(List<T> list) { }

[Garrett Rowe]

Its because T has not been declared a type variable for the method. If the method signature was:

public static <T> void takeCars(List<T extends Object> list) { } 

there would be no compilation error.

[Garrett Rowe]

Originally posted by Garrett Rowe:
Its because T has not been declared a type variable for the method. If the method signature was:

public static <T> void takeCars(List<T extends Object> list) { } 

there would be no compilation error.

Sorry, that won't work either. Thats what I get for trying to post before I tested. Keith's answer is the correct one.

This will also work

    public static <T> void takeCars(List<T> list) { }

[ April 26, 2007: Message edited by: Garrett Rowe ]

[megha joshi]

Thanks for explaining everyone...

So to sum up..

1) If <?> or <? extends something> or <? super something> is used in method's args its not declaring the type and hence its okay.
2) If anything other than ? or like <T> or <T extends something> is used then in method args then , it is a type declaration and must be done before the return type of the method or in the class type declaration.Just <T> should be written in the method args in this case...for eg.

public <T extends Integer> void methodA(<T >
or
public <T> void methodA(<T >

Let me know if I am mistaken..
Thanks again!!
[ April 26, 2007: Message edited by: megha joshi ]

[Chandra Bhatt]

Exactly,

Let me add...

1- class UrType<? extends Something> {}
2- class MyType<T super Something>{}
 
3- void <? super Something> doo(List<?> list) {}
4- void <T super Something> doo(List<T> list) {}

Always wrong syntax!!!


Regards,
cmbhatt

[Burkhard Hassel]

Hi ranchers,

Keith wrote:

If you modify d to be this, it will compile.

public static <T extends Object> void takeCars(List<T> list) { }

isn't that the same as

public static        <T>         void takeCars(List<T> list) { }

?
Will compile, by the way.

Bu.

[Keith Lynn]

Sure, it's the same.

I was just modifying the way they had it in the original statement.