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Generic method question

 
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In a recent example I read I was trying to figure out what the different parts of this line were:

public static <T> List<T> backwards(List<T> input);

Does this line say that it can take a type <T> as an argument and must return at List<T>, or am I misinterpreting this method?

If this is not enough information to answer my question I am glad to list the entire example question.

Thanks,
brian
 
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You got it correctly. You must return List<T>, not List<String> or
List<Object>.


Thanks,
 
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Or, put a different way, the generic type of the formal parameter determines the generic type of the return value. So if you passed a reference to a List<String> to the backwards method it will return a List<String>, if you passed a reference to a List<Rectangle> to the backwards method it will return a List<Rectangle>.
 
Brian Deer
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So why the <T> before the List<T> return type?

public static <T> List<T> backwards(List<T> input);

[ July 01, 2007: Message edited by: Brian Deer ]
Is it a type parameter that must be used if you don't put a type parameter in the class declaration?
[ July 01, 2007: Message edited by: Brian Deer ]
 
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Originally posted by Brian Deer:
Is it a type parameter that must be used if you don't put a type parameter in the class declaration?



Yes, it it is a type parameter declaration. It is used in the case you didn't use that parameter in the class declaration. What I mean is that it is used in one of these circumstances:
- The class is not generic, i.e. it does not have a type parameter.
- The class is generic but it does not declare that type parameter, i.e. it declares another type parameter. For instance:

- The method is static, i.e. in that case the type parameter does not apply to the method. Class' type parameters refer to a particular instantiation of a class (while static methods belong to all instantations of the class).
[ July 01, 2007: Message edited by: Sergio Tridente ]
 
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The <T> before the return type is the declaration, that this method is generic.
It also declares that T can be used as a parameter variable.
Generic methods can occur also in non-generic classes.


Usually a T is used (for "type") in non-collections and an E (for "element") in collections.
But this is only a name convention, you may use any legal identifier.

For clarification: instead of
public static <T> List<T> backwards(List<T> input);

you may equally type
public static <Daisy> List<Daisy> backwards(List<Daisy> input);

but in that case be sure that you don't use a class named Daisy or you get problems.


Yours,
Bu.
 
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