Java Generics: List, List<Object>, List<?

问题:

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) { }

回答1:

As the other posts have noted, you\'re asking about a Java feature called generics. In C++, this is called templates. The Java beasties are far tamer to deal with.

Let me answer your questions functionally (if that\'s not a naughty word for OO discussions).

Before generics, you had good old concrete classes like Vector.

Vector V = new Vector();

Vectors hold any old object you give them.

V.add(\"This is an element\");
V.add(new Integer(2));
v.add(new Hashtable());

However, they do this by casting everything you give it into an Object (the root of all Java classes). That\'s OK until you attempt to retrieve the values stored in your Vector. When you do, 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 pretty old fast. More than that, the compiler whines at you about unchecked casts. For a vivid example of this, use the XML-RPC library from Apache (version 2 anyway). The most important problem with this is that consumers of your Vector have to know the exact class of its values at compile time in order to cast correctly. In cases where the producer of the Vector and the consumer are completely isolated from each other, 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); 

Now, if you take a look at the infamous gang of four\'s Design Patterns book, you\'ll notice that there is some wisdom in divorcing variables from their implementing class. It\'s better to think of contracts rather than implementation. So, you might say that all List objects do the same things: add(), get(), size(), etc. However, there are many implementations of List operations that may choose to obey the contract in various ways (e.g. ArrayList). However, the type of data these object deal with is left as a runtime consideration to you, the user of the generic class. Put it all together and you\'ll see the following line of code very 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). However, one awful day you may decide you want to implement a generic class. Perhaps you\'ve thought of a great new List implementation. When you define that class, you use <t> as a placeholder for the kind of object that will be manipulated by the methods. If you\'re confused, use the generic classes for List until you\'re comfortable. Then, 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.

回答2:

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.

回答3:

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.

回答4:

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 <?>.

回答5:

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>

回答6:

I will try to answer this in detail. Before generics we were having only List (a raw list) and it can hold almost anything we can think of.

List rawList = new ArrayList();
rawList.add(\"String Item\");
rawList.add(new Car(\"VW\"));
rawList.add(new Runnable() {
            @Override
            public void run() {
               // do some work.
            }
        });

The major problem with the raw list is when we want to get any element out of such list it can only guarantee that it would be Object and for that reason we need to use casting as:

   Object item = rawList.get(0); // we get object without casting.
   String sameItem = (String) rawList.get(0); // we can use casting which may fail at runtime.

So conclusion is a List can store Object (almost everything is Object in Java) and always returns an Object.

Generics

Now lets talk about generics. Consider the following example:

List<String> stringsList = new ArrayList<>();
stringsList.add(\"Apple\");
stringsList.add(\"Ball\");
stringsList.add(new Car(\"Fiat\")); //error
String stringItem = stringsList.get(0);

In the above case we cannot insert anything other than String in stringsList as Java compiler applies strong type checking to generic code and issues errors if the code violates type safety. And we get error when we try to insert a Car instance in it. Also it eliminates cast as you can check when we invoke get method. Check this link for understanding why we should use generics.

List<Object>

If you read about type erasure then you will understand that List<String>, List<Long>, List<Animal> etc. will be having different static types at compile time but will have same dynamic type List at run time.

If we have List<Object> then it can store only Object in it and almost everything is Object in Java. So we can have:

 List<Object> objectList = new ArrayList<Object>();
 objectList.add(\"String Item\");
 objectList.add(new Car(\"VW\"));
 objectList.add(new Runnable() {
        @Override
        public void run() {

        }
 });
 Object item = objectList.get(0); // we get object without casting as list contains Object
 String sameItem = (String) objectList.get(0); // we can use casting which may fail at runtime.

It seems List<Object> and List are same but actually they are not. Consider the following case:

List<String> tempStringList = new ArrayList<>();
rawList = tempStringList; // Ok as we can assign any list to raw list.
objectList = tempStringList; // error as List<String> is not subtype of List<Obejct> becuase generics are not convariant.

You can see we can assign any list to raw list and major reason for that is to allow backward compatibility. Also List<String> will be converted to List at run time due to type erasure and assignment will be fine anyways.

But List<Object> means it can only refer to a list of objects and can also store objects only. Even though String is subtype of Object we cannot assign List<String> to List<Object> as generics are not covariant like arrays. They are invariant. Also check this link for more. Also check the difference between List and List<Object> in this question.

List<?>

Now we are left with List<?> which basically means list of unknown type and can refer to any list.

List<?> crazyList = new ArrayList<String>();
 List<String> stringsList = new ArrayList<>();
 stringsList.add(\"Apple\");
 stringsList.add(\"Ball\");
 crazyList = stringsList; // fine

The character ? is known as wildcard and List<?> is a list of unbounded wildcard. There are certain points to observe now.

We cannot instantiate this list as the following code will not compile:

List<?> crazyList = new ArrayList<?>(); // any list.

We can say a wildcard parameterized type is more like an interface type as we can use it to refer to an object of compatible type but not for itself.

List<?> crazyList2 = new ArrayList<String>();

We cannot insert any item to it as we don\'t know what actually the type would be.

crazyList2.add(\"Apple\"); // error as you dont actually know what is that type.

Now question arises When would I want to use List<?>?

You can think of this as a read-only list where you don\'t care about the type of the items. You can use it to invoke methods like returning the length of the list, printing it etc.

 public static void print(List<?> list){
        System.out.println(list);
    }

You can also check the difference between List, List<?>, List<T>, List<E>, and List<Object> here.

回答7:

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.

回答8:

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.

回答9:

To complement the tutorials mentioned by Rob, here\'s a wikibook explaining the topic:
http://en.wikibooks.org/wiki/Java_Programming/Generics

---

Edit:

1. No restrictions on type of items in list

2. Items in list must extend Object

3. 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?

回答10:

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.

回答11:

List, List<?>, and List<? extends Object> are the same thing. The second is more explicit. For a list of this type, you cannot know what types are legal to put into it, and you don\'t know anything about the types you can get out of it, except that they will be objects.

List<Object> specifically means that the list contains any sort of object.

Let\'s say we make a list of Foo:

List<Foo> foos= new ArrayList<Foo>();

It is not legal to put a Bar into foos.

foos.add(new Bar()); // NOT OK!

It is always legal to put anything into a List<Object>.

List<Object> objs = new ArrayList<Object>();
objs.add(new Foo());
objs.add(new Bar());

But you mustn\'t be allowed to put a Bar into a List<Foo> - that\'s the whole point. So that means that this:

List<Object> objs = foos; // NOT OK!

is not legal.

But it\'s ok to say that foos is a list of something but we don\'t know specifically what it is:

List<?> dontKnows = foos;

But that then means that it must be prohibited to go

dontKnows.add(new Foo()); // NOT OK
dontKnows.add(new Bar()); // NOT OK

because the variable dontKnows does\'t know what types are legal.

回答12:

List < Object > is meant to pass input type parameter of an Object. While List < ? > represents Wildcard type. The wildcard < ? > is of Unknown parameter type. The wildcard cannot be used as a type argument for a generic method and cannot be used to create a generic instance of a class. Wildcard can be used to extend a subtype class, List < ? extends Number >. To relax the restriction of an Object type and in this case to relax \"Number\" Object type.