What is the use of returning List<?> in a method ?

Question

e.g.

public interface CacheClient
{
    List<?> getKeysWithExpiryCheck();
}

Or should I return

List<Object>

Answer 1

Here is a good intro to Java Generics. [A] and [B] explain the difference between ? and Object. Basically, ? indicates that the type is unknown which is a problem if you need to add items to the list. However, if you only read from the list it is OK to treat the result as an Object. Although, I suggest to use some thing like

public interface CacheClient {
    List<? extends Key> getKeysWithExpiryCheck();
}

[A] http://java.sun.com/docs/books/tutorial/extra/generics/subtype.html

[B] http://java.sun.com/docs/books/tutorial/extra/generics/wildcards.html

Answer 2

Could it be to ensure type safety? Returning a List of object means that the list could hold anything.

Answer 3

If you declare your method as

List<Object> getKeysWithExpiryCheck();

You can only return List<Object> instances from it, and nothing else. If you e.g. try to return List<String>, you get a compilation error. This is because although Object is a supertype of String, List<Object> is not a supertype of List<String>.

Answer 4

Basically it means that the collection can hold any type. However when using it's impossible to modify the collection.

public void doSomething(Collection<?> col) {
    for (Object o : col) {
      System.out.println(o);
    }

    col.add("string"); //Compile Error here !
}