Java Generics-我可以根据变量类型动态创建列表

Question

Is there any way by which we can create specific instance of generic type at runtime?

For example.

Cacheable instance = getCacheable(someInput);

getCacheble method will return me the instance of Cacheable. So it can be any class implementing Cacheable.for eg Customer,Product etc.Now I want to create a list of specific type returned by getCacheable as below.Is that possible? If yes how to do that?

List<? extends Cacheable> cacheList = new ArrayList<>();

I want to create ArrayList<Product> or ArrayList<Customer> based on the instance returned by getCacheable method.

Answer 1

You can do this:

Cacheable instance = getCacheable(someInput);
List<? extends Cacheable> l = new ArrayList<>();
l = Collections.checkedList(l, instance.getClass());

Because of type erasure, all information accessible at compile time is lost at runtime. The checkedList method will ensure that your list will receive only instances of the class of instance variable.

UPDATE: You can do this also:

public static <T extends Cacheable> MyOwnCustomGeneric<T> createMyOwnCustomGeneric(Class<T> type) {
    return new MyOwnCustomGeneric<T>();
}

// ...
IMyOwnCustomGeneric foo = createMyOwnCustomGeneric(instance.getClass());

Answer 2

Use generic helper function:

public static <T extends Cacheable>ArrayList<T> createList(Class<T> claz)
{
       return new ArrayList<>();
}

ArrayList<? extends Cacheable>alist = createList(instance.getClass());

Answer 3

if (instance instanceof Product) { 
   // create ArrayList<Product>; 
} else { 
   // create ArrayList<Customer>;
}

This is not really general and dynamic though.

Answer 4

The only way to do it is by switching on the type of instance but there isn't much you can do with it:

List<? extends Cacheable> cacheList = null;
if (instance instanceof Product)
  cacheList = new ArrayList<Product>();
// etc

But what can you do with this list? You cannot add anything to it. Maybe you add items to it while creating it, but that is probably not the use case since it is a cache list.

If you need to add items to it later, see this answer is the right approach.

If you are not going add items later, it does not matter if the type is List<Cacheable> or List<? extends Cacheable> List<? extends Cacheable>