Can someone explained, as detailed as possible, the differences between the following types?
List
List<Object>
List<?>
Let me make this more specific. When would I want to use
// 1
public void CanYouGiveMeAnAnswer(List l) { }
// 2
public void CanYouGiveMeAnAnswer(List<Object> l) { }
// 3
public void CanYouGiveMeAnAnswer(List<?> l) { }
As the other posts have noted, you are asking about a Java feature called generics. In C++, this is called templates. This feature in Java is usually easier to work with than the that found in C++.
Let me answer your questions functionally (if that's not a naughty word for OO discussions).
Before generics, there were concrete classes like Vector.
Vector V = new Vector();
Vectors hold any object you give them.
V.add("This is an element");
V.add(new Integer(2));
v.add(new Hashtable());
They do this by casting all values given to it into an Object (the root of all Java classes). When you attempt to retrieve the values stored in your Vector, you need to cast the value back into the original class (if you want to do anything meaningful with it).
String s = (String) v.get(0);
Integer i = (Integer) v.get(1);
Hashtable h = (Hashtable) v.get(2);
Casting gets old fast. More than that, the compiler complains to you about unchecked casts. The most urgent problem with casting like this is that consumers of your Vector have to know the classes of its values at compile time in order to cast correctly. In cases where the producer of the Vector and the consumer of the same are completely isolated from each other (think RPC messages), this can be a fatal issue.
Enter generics. Generics attempt to create strongly typed classes to do generic operations.
ArrayList<String> aList = new ArrayList<String>();
aList.add("One");
String element = aList.get(0); // no cast needed
System.out.println("Got one: " + element);
The Design Patterns book encourages the reader to think in terms of contracts, not concrete types. There is wisdom (and code re-use) in divorcing variables from their implementing class.
With this in mind, you might think that all implementations List objects should do the same set of things: add(), get(), size(), etc. With a little reflection, you can imagine many implementations of List operations that obey the List contract in various ways (e.g. ArrayList). However, the type of data these objects deal with is orthogonal to the actions performed on them.
Put it all together and you'll see the following kinds of code frequently:
List<String> L = new ArrayList<String>();
You should read that as "L is a kind of List that deals with String objects". When you start dealing with Factory classes, it is critical to deal with contracts rather than specific implementations. Factories produce objects of various types at runtime.
Using generics is pretty easy (most of the time).
One day you may decide you want to implement your own generic class. Perhaps you want to write a new database abstraction interface that elides the differencesbetween various data stores. When you define that generic class, you will use <t> as a placeholder for the kind of object that will be manipulated by the methods.
If you are still confused, use the generic classes for List until you are comfortable. Later, you can dive into the implementation with a bit more confidence. Or you can look at the source code for the various List classes that ship with the JRE. Open source is great that way.
Have a look at the Oracle/Sun docs about generics. Cheers.
In my own simple terms:
List
Would declare an ordinary collection, can hold any type, and will always return Object.
List<Object>
Will create a list that can hold any type of object, but can only get assigned a another List<Object>
For instance this doesn't work;
List<Object> l = new ArrayList<String>();
Of course you can add anything but only can pull Object.
List<Object> l = new ArrayList<Object>();
l.add( new Employee() );
l.add( new String() );
Object o = l.get( 0 );
Object o2 = l.get( 1 );
Finally
List<?>
Will let you assign any type, including
List <?> l = new ArrayList();
List <?> l2 = new ArrayList<String>();
This would be called collection of unknown and since the common denominator of unknown is Object you will be able to fetch Objects ( a coincidence )
The importance of unknown comes when its used with subclassing:
List<? extends Collection> l = new ArrayList<TreeSet>(); // compiles
List<? extends Collection> l = new ArrayList<String>(); // doesn't,
// because String is not part of *Collection* inheritance tree.
I hope using Collection as the type doesn't create confusion, that was the only tree that came to my mind.
The difference here, is that l is a collection of unknow that belongs to the Collection hierarchy.
I refer you to the excellent Java Generics tutorial, and the "advanced" Generics tutorial, both available from Sun Microsystems. Another great resource is the Java Generics and Collections book.
To add to the already good answers here:
Method arguments:
List<? extends Foo>
good choice if you don't intend to alter the list, and only care that everything in the list is assignable to type 'Foo'. This way, the caller can pass in a List<FooSubclass> and your method works. Usually the best choice.
List<Foo>
good choice if you intend to add Foo objects to the list in your method. The caller may not pass in a List<FooSubclass>, as you intend to add a Foo to the List.
List<? super Foo>
good choice if you intend to add Foo objects to the list, and it's not important what else is in the list (ie, you are ok getting a List<Object> that contains a 'Dog' that has nothing to do with Foo).
Method return values
just like method arguments, but with the benefits reversed.
List<? extends Foo>
Guarantees that everything in the returned List has type 'Foo'. It might be List<FooSubclass> though. Caller cannot add to the List. This is your go-to choice and the most common case by far.
List<Foo>
Just like List<? extends Foo> but also allows the caller to add to the List. Less common.
List<? super Foo>
allows the caller to add Foo objects to the List, but does not guarantee what will be returned from list.get(0)... it could be anything from Foo to Object. The only guarantee is that this won't be a list of 'Dog' or some other choice that would prevent list.add(foo) from being legal. Very rare use case.
I hope that helps. Good luck!
ps. To sum up... two questions...
do you need to add to the List? Do you care what is in the list?
yes yes - use List<Foo>.
yes no - use List<? super Foo>.
no yes - use <? extends Foo> --- most common.
no no - use <?>.
Simplest explanation which is not "RTFM":
List
Will generate lots of compiler warnings, but is mostly equivalent to:
List<Object>
While:
List<?>
basically means its something generic, but you don't know what the generic type is. Its great for getting rid of compiler warnings when you cant modify the return types of other things that just returned List. Its much more useful in the form:
List<? extends SomeOtherThing>
The shortest possible explanation is: The second item is a list that can hold any type, and you can add objects to it:
List<Object>
The first item you list is treated as essentially equivalent to this, except you will get compiler warnings because it is a "raw type".
List
The third is a list that can hold any type, but you cannot add anything to it:
List<?>
Basically, you use the second form (List<Object>) when you truly have a list that can contain any object and you want to be able to add elements to the list. You use the third form (List<?>)when you receive the list as a method return value and you will iterate over the list but never add anything to it Never use the first form (List) in new code compiling under Java 5 or later.
I'd put it this way: While List and List<Object> can contain any type of objects, List<?> contains elements of an unknown type, but once that type is captured, it can only contain elements of that type. Which is why it is the only type safe variant of those three, and therefore generally preferable.
To complement the tutorials mentioned by Rob, here's a wikibook explaining the topic:
http://en.wikibooks.org/wiki/Java_Programming/Generics
Edit:
No restrictions on type of items in list
Items in list must extend Object
Wildcard used by itself, so it matches anything
Would it be naive of me to conclude at this point that there's hardly any/no difference at all?
When would I want to use
public void CanYouGiveMeAnAnswer( List l ){}
When you cant to do all the casting your self.
When would I want to use
public void CanYouGiveMeAnAnswer( List l<Object> ){}
When you want to restrict the type of the List. For instance, this would be an invalid argument.
new ArrayList<String>();
When would I want to use
public void CanYouGiveMeAnAnswer( List l<?> ){}
Mostly never.
In terms of List functionality differences, the other responses already answered this question. In terms of applicable rules of Java generics, it's a complicated topic. I wrote an in-depth article about Java generics rules, this is the link: https://medium.com/@royalilin/java-generics-rules-1d05de86e9cb