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javacollectionsrandom-access

RandomAccess Interface, Why no methods?

I was reading Collections.shuffle(List) javadoc and then took a look at the RandomAccess javadoc:

Marker interface used by List implementations to indicate that they support fast (generally constant time) random access. [...]

I am wondering why this interface (like Serializable) does not have methods? What is the design reason for this?

Even if only Lists "implement" this interface, why not setting E get() as a method? I know that not every list is random access but how can I use this interface if there is no methods?

Something like this:

if(object instanceof RandomAccess){
    // should I cast it if no operations can be done? why?
}

Also, only Lists can be RandomAccess? What about Files?

Solution

  • A "Marker" interface is a technique that predates annotations; it is meant to mark a Class as conforming to some standard, where the standard isn't about methods.

    In this case, Shuffle may act differently on a list that doesn't support quick random access. Consider how you would shuffle a linked list; it's hard, right? You can't just say "get me a random element" without walking through the list, following pointers to the next element. Now contrast this with an ArrayList. Getting a random element is much easier, because of the way the list is stored.

    There isn't a way to qualify "how the list is stored" or "how fast or slow different access patterns might be" in a method name. So instead, Java uses marker interfaces to provide this information.

    In this case, ArrayList would be a RandomAccess, and LinkedList would not.

    EDIT

    Those interested in the differences between marker interfaces and marker annotations would enjoy Item 37:"Use marker interfaces to define types" in Effective Java 2nd Edition by Joshua Bloch.