How Can I Get Generic Parameter Types in Java Using Reflection?

Getting Generic Parameter Types in Java Using Reflection

In Java, generic types are a powerful feature that allows for more flexible and reusable code. However, working with generic parameters can be tricky sometimes. One common challenge is determining the actual type of a generic parameter at runtime.

Is it Possible?

Yes, it is possible to get the type of a generic parameter using reflection. Reflection provides a way to access and manipulate details about the structure, fields, and methods of a Java class at runtime.

Example:

Consider the following code snippet:

public class Voodoo {
    public static void chill(List<?> aListWithTypeSpiderMan) {
        // Here I'd like to get the Class-Object 'SpiderMan'
        Class typeOfTheList = ???;
    }

    public static void main(String... args) {
        chill(new ArrayList<SpiderMan>());
    }
}

In the chill method, we have a list of unknown type. We want to determine the actual type parameter, which in this case should be SpiderMan.

Solution:

The solution involves using the ParameterizedType interface, which is a type of Type that describes parameterized types, such as those using generics. Here's how we can achieve it:

Class typeOfTheList = ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0];

This code gets the superclass of the current class (Voodoo in this case), which should be a parameterized type, and then extracts the actual type arguments. The first type argument corresponds to the type parameter we're interested in, which is SpiderMan.

Note:

This technique is not fully supported before Java 16. The use of reflection for generic types was considered hacky earlier, but starting from Java 16, getActualTypeArguments() has been added to the Type interface, providing a cleaner and more streamlined way to access generic type information.

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