CODEWITHSUNDEEP
javagenericsgeneric-list
victorio
victorio

Reputation: 6646

Get Java Class name from generic List type

I have a method with 1 List, generic class inside:

public static String classTypeOfList(List<T> list) {
    return T.getName(); //in my mind...
}

The code is wrong, but you can see, what I want. If i call this method like this:

List<MyObject> list;
System.out.println("the type of the list is: "+classTypeOfList(list));

I would like to get this result:

the type of the list is: MyObject

How could I get the name of a generic type class? Or if I can't get it this way, than could you show me an another option? Thank you!

Upvotes: 2

Views: 15234

Answers (5)

Prateek
Prateek

Reputation: 12242

Because of Type Erasure, you will not be able to get the type(in case of Empty List).As JLS says on Type Erasure :

4.6. Type Erasure

Type erasure is a mapping from types (possibly including parameterized types and type variables) to types (that are never parameterized types or type variables). We write |T| for the erasure of type T. The erasure mapping is defined as follows:

The erasure of a parameterized type (§4.5) G<T1,...,Tn> is |G|.

The erasure of a nested type T.C is |T|.C.

The erasure of an array type T[] is |T|[].

The erasure of a type variable (§4.4) is the erasure of its leftmost bound.

The erasure of every other type is the type itself.

Type erasure also maps the signature (§8.4.2) of a constructor or method to a signature that has no parameterized types or type variables. The erasure of a constructor or method signature s is a signature consisting of the same name as s and the erasures of all the formal parameter types given in s.

The type parameters of a constructor or method (§8.4.4), and the return type (§8.4.5) of a method, also undergo erasure if the constructor or method's signature is erased.

The erasure of the signature of a generic method has no type parameters.

In case of Non Empty List:

......
public static void main(String[] args) throws ClassNotFoundException {

List<MyObject> list= new ArrayList<MyObject>();
        list.add(new MyObject());
        System.out.println("the type of the list is: "+classTypeOfList(list));
}

public static <T> String classTypeOfList(List<T> list) throws ClassNotFoundException {
        return list.get(0).getClass().getCanonicalName(); 
}

OUTPUT

the type of the list is: MyObject

Upvotes: 3

Gaurav Gupta
Gaurav Gupta

Reputation: 4691

This answer may be little ( or very ) different from question, since it is long enough for the comment, so i am posting it as answer.

I found this question interesting and hence gave it a try. My attempt is following, i got the Type of the class, so i thought it is worth sharing and getting experts opinion back on my approach

public class A {
    public static void main(String[] args) {

    List<B> listB = new ArrayList<>();
    B b1 = new B();
    listB.add(b1);

    List<C> listC = new ArrayList<>();
    C c1 = new C();
    listC.add(c1);

    A a = new A();
    a.method(listB);
    a.method(listC);

}

public <T> void method(List<T> list) {

    System.out.println(list.get(0).getClass().getName());
}
}

class B {

}

class C {

}

Output i got was B and C

Upvotes: 2

Parth Bhagat
Parth Bhagat

Reputation: 509

This is how you can get class:

Class<T> clazz = ((Class<T>) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[0]);

You can read more about it on this link.

Upvotes: 0

Daniel van der Merwe
Daniel van der Merwe

Reputation: 1960

You need to use getClass() on the object. After you get the class object you can use getName() to retrieve the class name.

Upvotes: 1

Andrew Logvinov
Andrew Logvinov

Reputation: 21831

I'm afraid, this cannot be done. Generics only exist at compile time. At runtime this information is erased and at runtime your List<MyObject> will simply turn into List.

Upvotes: 2

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