List <?>

http://java.sun.com/j2se/1.5.0/docs/guide/language/generics.html

? means "any type".

CeciNEstPasUnProgrammeura at 2007-7-29 18:17:07 >

java.util.List

An ordered collection (also known as a sequence). The user of this interface has precise control

over where in the list each element is inserted. The user can access elements by their integer index

(position in the list), and search for elements in the list.

Unlike sets, lists typically allow duplicate elements. More formally, lists typically allow pairs of

elements e1 and e2 such that e1.equals(e2), and they typically allow multiple null elements if they

allow null elements at all. It is not inconceivable that someone might wish to implement a list that

prohibits duplicates, by throwing runtime exceptions when the user attempts to insert them, but

we expect this usage to be rare.

The List interface places additional stipulations, beyond those specified in the Collection interface,

on the contracts of the iterator, add, remove, equals, and hashCode methods. Declarations for

other inherited methods are also included here for convenience.

The List interface provides four methods for positional (indexed) access to list elements. Lists (like

Java arrays) are zero based. Note that these operations may execute in time proportional to the

index value for some implementations (the LinkedList class, for example). Thus, iterating over the

elements in a list is typically preferable to indexing through it if the caller does not know the

implementation.

The List interface provides a special iterator, called a ListIterator, that allows element insertion and

replacement, and bidirectional access in addition to the normal operations that the Iterator

interface provides. A method is provided to obtain a list iterator that starts at a specified position in

the list.

The List interface provides two methods to search for a specified object. From a performance

standpoint, these methods should be used with caution. In many implementations they will perform

costly linear searches.

The List interface provides two methods to efficiently insert and remove multiple elements at an

arbitrary point in the list.

Note: While it is permissible for lists to contain themselves as elements, extreme caution is advised:

the equals and hashCode methods are no longer well defined on a such a list.

Some list implementations have restrictions on the elements that they may contain. For example,

some implementations prohibit null elements, and some have restrictions on the types of their

elements. Attempting to add an ineligible element throws an unchecked exception, typically

NullPointerException or ClassCastException. Attempting to query the presence of an ineligible

element may throw an exception, or it may simply return false; some implementations will exhibit

the former behavior and some will exhibit the latter. More generally, attempting an operation on an

ineligible element whose completion would not result in the insertion of an ineligible element into the

list may throw an exception or it may succeed, at the option of the implementation. Such

exceptions are marked as "optional" in the specification for this interface.

skp71a at 2007-7-29 18:17:07 >

Sorry about it. I thought it's about java.util.List and I took the note from IBM RAD6.0 RSDP.

skp71a at 2007-7-29 18:17:07 >

What it says is that this is a reference to a list of some class of object. It's not really much difference from the old List without the generics stuff (it just saves you a warning if you assign a generic list too it).

You're kind of saying "Yes, I understand there's generics arround here, but I'm not using them this time".

malcolmmca at 2007-7-29 18:17:07 >

Just as a quick aside.... the ? applies to when using generic wildcards too...

Such as List<? extends Shape>

so you can then have a List of any object which extends Shape i.e. your list could contain objects of Circles and Rectangles both at the same time provided they extend Shape

c0demonk3ya at 2007-7-29 18:17:07 >

> Isn't List<?> shorthand for List<? extends Object> ?

Hadn't thought about it but yeah I suppose it does... I only mentioned it because it was in the Generics tutorial pdf.

Can't say I have ever actually used it myself as I try to keep my collections to just single types of objects in the first place.

c0demonk3ya at 2007-7-29 18:17:07 >