How To Instantiate a java.util.ArrayList with Generic Class Using Reflection

Question

How To Instantiate a java.util.ArrayList with Generic Class Using Reflection? I am writing a method that sets java.util.List on target object. A target object and a generic

Answer 1

Generics only work at compile time, therefore if you're using reflection, they won't be available.

See more about this here.

Answer 2

An object doesn't know about its generic type at execution time. Just create a new instance of the raw type (java.util.ArrayList). The target object won't know the difference (because there isn't any difference).

Answer 3

There's no need to do any reflection for creating your List. Just pass in some additional type information (usually done by passing a class of the correct type).

public static <T> List<T> createListOfType(Class<T> type) {
    return new ArrayList<T>();
}

Now you have a list of the required type you can presumably/hopefully set it directly on your targetObject without any reflection.

Answer 4

An object doesn't know about its generic type at execution time. Just create a new instance of the raw type (java.util.ArrayList). The target object won't know the difference (because there isn't any difference).

Basically Java generics is a compile-time trick, with metadata in the compiled classes but just casting at execution time. See the Java generics FAQ for more information.

Answer 5

Generics are a compile-time only "trick".

Reflection is runtime-only.

Basically, you can't - you can only create a "raw" ArrayList. If you need to pass it into methods that take generic parameters, casting it directly after construction will be safe (regardless of the "unchecked" warning). In this example, there's no compile-time type safety anyway due to using general Objects, so no casting is needed.

Answer 6

Generics is largely a compile time feature. What you are trying to do is the same as

public static void initializeList(Object targetObject, PropertyDescriptor prop, String gtype) {
    prop.getWriteMethod().invoke(targetObject, new ArrayList());
}

Note: This could change with Types in Java 7.