Pass generic type with actual literal value in type parameter-CodePudding

I'm trying to pass a type parameter to a function

public void foo(Class<T> class){
// do something
}

it's easy if i have to pass Integer or any simple Class by just passing foo(ClassName.class) but how can i pass this with generic type, i.e those class that have generic types? e.g


Map<String,Object> m = foo(Map<String,Object>.class) // achieve something like this
Map<String,Object> m = (Map<String,Object>)foo(Map.class) // instead of something like this

CodePudding user response：

Depending on what the receiver is doing, you may be able to get the necessary information from a ParameterizedType instance. ParameterizedType is a subclass of Type, as is Class, and many libraries that do binding or other reflection-intensive work use Type instead of Class.

In order to obtain a ParameterizedType instance, however, you have to define a type with the correct generic parameters. This is called a "type token," and often an anonymous instance is created.

Re-write your foo() method to inspect the class for generic type information:

<T> void foo(Class<T> clz) {
    Type type = clz.getGenericSuperclass();
    if (type instanceof ParameterizedType) {
        ParameterizedType generic = (ParameterizedType) type;
        for (Type t : generic.getActualTypeArguments()) {
            System.out.println(t);
        }
    } else {
        System.out.println(clz);
    }
}

Now you can create a type token and pass it to your method:

/* This doesn't work; type parameters are erased. */
Class<?> fail = Map<String, Object>.class 
class TypeToken<T> {}
Class<?> okay = new TypeToken<Map<String, Object>>() {}.getClass();
foo(okay);

CodePudding user response：

You can use getClass method.

import java.util.*;

public class Main{
   public static <T> void foo(Class<T> c){
      System.out.println(c);
   }
    public static void main(String[] args) {
        Map<String,String> m = new HashMap<>();
        foo(m.getClass());
    }
}

Output

class java.util.HashMap