Class as a generic type

April 27th, 2007 by Sam

I just learnt something great about generics and thought I’d share it.

We have several systems that take care of persistence and caching, which usually involve getting hold of a store through a factory method something like this

public static <K extends Serializable, V extends Serializable> Store<K, V>
    getInstance(String name, Class keyClass, Class valueClass)

Up until now, I was very frustrated that there was no way to check that the Class parameters are actually related to the generic types K and V, and I had to explicitly check they extended Serializable.

But it turns out that the Class object itself can have a generic type K forced upon it. For example Class<List> will only accept the class of an object that implements List. The method then becomes

public static <K extends Serializable, V extends Serializable> Store<K, V>
    getInstance(String name, Class<K> keyClass, Class<V> valueClass)

and suddenly things become a lot cleaner… specifically there is no need to check that the Classes are subclasses of Serializable or that generic objects can be cast to one of the Class parameters in every set method. That said, it would be much better to be able to lose the Class parameters altogether and allow runtime access of generic types, but unfortunately Java’s legacy fails us there.

Oh, and while I have you in generic mode, isn’t the “Type mismatch” error here just the most annoying thing ever?

List<Object> stooges = Arrays.asList("Larry", "Moe", "Curly");

We all know it is perfectly fine to go and do this

List<Object> stooges = new ArrayList();
stooges.add("Larry");
stooges.add("Moe");
stooges.add("Curly");

A correction is to do

List<? extends Object> stooges = Arrays.asList("Larry", "Moe", "Curly");

but having to explicitly say the type extends is just annoying. Does anyone know of a valid case where it would be bad to allow a sub-type to be assigned in place of a type?

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This entry was posted by by Sam on Friday, April 27th, 2007 at 6:22 pm, and is filed under Factories, Generics, Java. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

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List<Object> stooges = Arrays.<Object>asList("Larry", "Moe", "Curly");

List<Number> numbers=new ArrayList<Double>(); //not allowed, but imagine
List<Double> doubles=numbers;
numbers.add(5);
double d=doubles.get(0); //ClassCastException, we don't want this.

List<String> strList = new ArrayList<String>() // Valid
List<Object> objList = strList; // NOT valid 
objList.add("Bob"); // This would be safe
objList.add(Boolean.TRUE); // This is OK for a string list,
                           // but not for an object list