How Can I Determine the Class of a Generic Type at Runtime?

Determining Class of a Generic Type

In generic programming, it's often desirable to access the class of the generic type being used. However, generic type information is erased at runtime, making this task inherently challenging. This article explores an effective workaround to overcome this limitation.

The Challenge

Consider the following code snippet:

public class MyGenericClass<T> {
  public void doSomething() {
    T bean = (T)someObject.create(T.class);
  }
}

This code attempts to create an instance of type T using a third-party library's create method, which requires the actual class of T as an argument. However, accessing T.class directly results in an Illegal class literal for type parameter T error.

The Workaround

Since generic type information is erased at runtime, an alternative approach is necessary. The following workaround involves creating a static factory method that accepts the class of the generic type as an argument:

public class MyGenericClass<T> {

    private final Class<T> clazz;

    public static <U> MyGenericClass<U> createMyGeneric(Class<U> clazz) {
        return new MyGenericClass<U>(clazz);
    }

    protected MyGenericClass(Class<T> clazz) {
        this.clazz = clazz;
    }

    public void doSomething() {
        T instance = clazz.newInstance();
    }
}

Usage

To use this workaround, create an instance of MyGenericClass using the createMyGeneric factory method:

MyGenericClass<MyClass> genericInstance = MyGenericClass.createMyGeneric(MyClass.class);

This provides access to the actual class of the generic type via the clazz field within the MyGenericClass instance.

Drawbacks

While this workaround is functional, it adds some boilerplate code to the solution. Additionally, it limits the use of generic methods or constructors within the doSomething method, as they cannot access the generic type information directly.

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