Get Generic Type from a Run-Time type Parameter in Java

So I've read a lot of other questions out there, but I cannot find one with similar case as mine. Assume I have the following code:

public class TestClass
{
    public Class clazz;

    public TestClass(Object input)
    {
        this.clazz = ........ ?? // how to get String.class from "input" parameter?
    }

    public static void main(String[] args)
    {
        List<String> all = new ArrayList<>();
        new TestClass(all);
    }
}

The constructor of TestClass has 1 parameter which is an Object type. But at runtime, it receives a variable of type List, but the actual instance is an ArrayList. Inside the TestClass constructor, I want to extract the generic type of that ArrayList from input parameter. The expected result should be String.class object, which can be stored in clazz variable.