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

If you are extending or implementing any class/interface that are using generics , you may get the Generic Type of parent class/interface, without modifying any existing class/interface at all.

There could be three possibilities,

Case 1 When your class is extending a class that is using Generics

public class TestGenerics {
    public static void main(String[] args) {
        Type type = TestMySuperGenericType.class.getGenericSuperclass();
        Type[] gTypes = ((ParameterizedType)type).getActualTypeArguments();
        for(Type gType : gTypes){
            System.out.println("Generic type:"+gType.toString());
        }
    }
}

class GenericClass {
    public void print(T obj){};
}

class TestMySuperGenericType extends GenericClass {
}

Case 2 When your class is implementing an interface that is using Generics

public class TestGenerics {
    public static void main(String[] args) {
        Type[] interfaces = TestMySuperGenericType.class.getGenericInterfaces();
        for(Type type : interfaces){
            Type[] gTypes = ((ParameterizedType)type).getActualTypeArguments();
            for(Type gType : gTypes){
                System.out.println("Generic type:"+gType.toString());
            }
        }
    }
}

interface GenericClass {
    public void print(T obj);
}

class TestMySuperGenericType implements GenericClass {
    public void print(Integer obj){}
}

Case 3 When your interface is extending an interface that is using Generics

public class TestGenerics {
    public static void main(String[] args) {
        Type[] interfaces = TestMySuperGenericType.class.getGenericInterfaces();
        for(Type type : interfaces){
            Type[] gTypes = ((ParameterizedType)type).getActualTypeArguments();
            for(Type gType : gTypes){
                System.out.println("Generic type:"+gType.toString());
            }
        }
    }
}

interface GenericClass {
    public void print(T obj);
}

interface TestMySuperGenericType extends GenericClass {
}