Three options: Return default (or default(T) for older versions of C#) which means you’ll return null if T is a reference type (or a nullable value type), 0 for int, ‘\0′ for char, etc. (Default values table (C# Reference)) If you’re happy to restrict T to be a reference type with the where T : … Read more
You are correct. You can’t do new E(). But you can change it to private static class SomeContainer<E> { E createContents(Class<E> clazz) { return clazz.newInstance(); } } It’s a pain. But it works. Wrapping it in the factory pattern makes it a little more tolerable.
You could define callFriend this way: public <T extends Animal> T callFriend(String name, Class<T> type) { return type.cast(friends.get(name)); } Then call it as such: jerry.callFriend(“spike”, Dog.class).bark(); jerry.callFriend(“quacker”, Duck.class).quack(); This code has the benefit of not generating any compiler warnings. Of course this is really just an updated version of casting from the pre-generic days and … Read more
The obvious answer, of course, is not to do the unchecked cast. If it’s absolutely necessary, then at least try to limit the scope of the @SuppressWarnings annotation. According to its Javadocs, it can go on local variables; this way, it doesn’t even affect the entire method. Example: @SuppressWarnings(“unchecked”) Map<String, String> myMap = (Map<String, String>) … Read more
If your elements are value types, then you can just do: List<YourType> newList = new List<YourType>(oldList); However, if they are reference types and you want a deep copy (assuming your elements properly implement ICloneable), you could do something like this: List<ICloneable> oldList = new List<ICloneable>(); List<ICloneable> newList = new List<ICloneable>(oldList.Count); oldList.ForEach((item) => { newList.Add((ICloneable)item.Clone()); }); … Read more
What is a raw type? The Java Language Specification defines a raw type as follows: JLS 4.8 Raw Types A raw type is defined to be one of: The reference type that is formed by taking the name of a generic type declaration without an accompanying type argument list. An array type whose element type … Read more
If I understand correctly, your list has the same type parameter as the container class itself. If this is the case, then: Type typeParameterType = typeof(T); If you are in the lucky situation of having object as a type parameter, see Marc’s answer.
The short answer is, that there is no way to find out the runtime type of generic type parameters in Java. I suggest reading the chapter about type erasure in the Java Tutorial for more details. A popular solution to this is to pass the Class of the type parameter into the constructor of the … Read more
tl;dr: “PECS” is from the collection’s point of view. If you are only pulling items from a generic collection, it is a producer and you should use extends; if you are only stuffing items in, it is a consumer and you should use super. If you do both with the same collection, you shouldn’t use … Read more
No, a List<Dog> is not a List<Animal>. Consider what you can do with a List<Animal> – you can add any animal to it… including a cat. Now, can you logically add a cat to a litter of puppies? Absolutely not. // Illegal code – because otherwise life would be Bad List<Dog> dogs = new ArrayList<Dog>(); … Read more