How do I get a class instance of generic type T?

Question

I have a generics class, Foo. In a method of Foo, I want to get the class instance of type T, but I just can\'t call T.

Answer 1

Imagine you have an abstract superclass that is generic:

public abstract class Foo {}

And then you have a second class that extends Foo with a generic Bar that extends T:

public class Second extends Foo {}

You can get the class Bar.class in the Foo class by selecting the Type (from bert bruynooghe answer) and infering it using Class instance:

Type mySuperclass = myFoo.getClass().getGenericSuperclass();
Type tType = ((ParameterizedType)mySuperclass).getActualTypeArguments()[0];
//Parse it as String
String className = tType.toString().split(" ")[1];
Class clazz = Class.forName(className);

You have to note this operation is not ideal, so it is a good idea to cache the computed value to avoid multiple calculations on this. One of the typical uses is in generic DAO implementation.

The final implementation:

public abstract class Foo {

    private Class inferedClass;

    public Class getGenericClass(){
        if(inferedClass == null){
            Type mySuperclass = getClass().getGenericSuperclass();
            Type tType = ((ParameterizedType)mySuperclass).getActualTypeArguments()[0];
            String className = tType.toString().split(" ")[1];
            inferedClass = Class.forName(className);
        }
        return inferedClass;
    }
}

The value returned is Bar.class when invoked from Foo class in other function or from Bar class.