Java generic method declaration

Question

I'm learning Java generics and I ask myself this question.

What is the difference between these two method declarations?

public static void someMethod(List<? extends Number> numberList);

and

public static <E extends Number> void someMethod(List<E> numberList);

using the second one allows you to reference E in your `someMethod`s code. — Oren, Feb 20 '13 at 12:01

score 23 · Accepted Answer · answered Feb 20 '13 at 12:02

23

In the latter you have a reference to the type within the scope of someMethod, namely E. In the former you do not.

answered Feb 20 '13 at 12:02

Rich O'Kelly

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score 14 · Answer 2 · answered Feb 20 '13 at 12:19

14

The main difference is that the latter is a generic method the former is not.

So for example in the latter method you can do something like this:

public static <E extends MyObject> void someMethod(List<E> someList) {
    E myObject = someList.iterator().next(); // this can actually lead to errors
    myObject.doSomething();                  // so treat it as an example
}

This means that you can substitute an arbitrary type E which conforms to the rule in the generic method declaration and be able to use that type in your method.

Be advised though that you should call the generic method with type arguments like this:

someClass.<MyArbitraryType>someMethod(someList);

You can find a nice overview of generic methods here.

answered Feb 20 '13 at 12:19

Adam Arold

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1

In java Se 7 and above you can call this `someClass.somemethod(someList);` i belive, don't have se7 compiler atm, please confirm? – ZeDonDino Feb 20 '13 at 12:22
1

I don't have se7 compiler atm either. – Adam Arold Feb 20 '13 at 12:23
Thanks for the explanation. But why would I want E? With the former version I can do `for(Number numb: numberList) { numb.anyMethodOfNumber(); }` Isn't it the same? – Boss Man Jul 01 '15 at 00:29

score 5 · Answer 3 · answered Feb 20 '13 at 12:09

5

With the second version you can do something like:

public static <E extends Number> void someMethod(List<E> numberList) {
  E number = numberList.get(0); 
  numberList.add(number);
}

This isn't possible with the first version.

answered Feb 20 '13 at 12:09

Landei

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score 2 · Answer 4 · edited Jan 13 '19 at 11:18

2

Using the first method declaration will not allow you to add anything new to the list. For example this will not compile.

public static void someMethod(List<? extends Number> numberList, Number number) {
    numberList.add(number);
}

while the second allows you to do this:

public static <T extends Number> void someMethod(List<T> numberList, T number) {
    numberList.add(number);
}

edited Jan 13 '19 at 11:18

GabrielBB

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answered Feb 20 '13 at 12:06

Denis Rosca

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Using the second wont either. – ZeDonDino Feb 20 '13 at 12:08
You can add elements to the list, as long as the element to be added is of type E or a subclass. – Denis Rosca Feb 20 '13 at 12:09
I'm using java SE 6 and its not possible. – ZeDonDino Feb 20 '13 at 12:11
are you sure we're talking about the same thing? ` void method(List list, T number) {list.add(number);}` should compile just fine. – Denis Rosca Feb 20 '13 at 12:14
mine doesn't compile, I have to cast number. numbers.add((E)number1); is fine – ZeDonDino Feb 20 '13 at 12:19
Btw number isnt type T, its type Number, as stated in the answer. Please look above – ZeDonDino Feb 20 '13 at 12:20
@ZeDonDino ofcourse you cannot add a number to a `List` because of up-casting. i never said this is possible. – Denis Rosca Feb 20 '13 at 12:25
By my analogy logic I thought, that you thought this. : ) Sorry mate, didn't want to insult you – ZeDonDino Feb 20 '13 at 12:36

Java generic method declaration

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