How to Get the Generic Type of a Java `List` Using Reflection?

How to Obtain Generic Type of java.util.List

Consider the following code:

List<String> stringList = new ArrayList<String>();
List<Integer> integerList = new ArrayList<Integer>();

A frequent question arises: is there an effortless method to retrieve the generic type of the list?

Using Reflection

If these lists are fields of a particular class, reflection can be utilized to access their generic types:

import java.lang.reflect.Field;
import java.lang.reflect.ParameterizedType;
import java.util.ArrayList;
import java.util.List;

public class Test {

    List<String> stringList = new ArrayList<>();
    List<Integer> integerList = new ArrayList<>();

    public static void main(String... args) throws Exception {
        Class<Test> testClass = Test.class;

        Field stringListField = testClass.getDeclaredField("stringList");
        ParameterizedType stringListType = (ParameterizedType) stringListField.getGenericType();
        Class<?> stringListClass = (Class<?>) stringListType.getActualTypeArguments()[0];
        System.out.println(stringListClass); // class java.lang.String

        Field integerListField = testClass.getDeclaredField("integerList");
        ParameterizedType integerListType = (ParameterizedType) integerListField.getGenericType();
        Class<?> integerListClass = (Class<?>) integerListType.getActualTypeArguments()[0];
        System.out.println(integerListClass); // class java.lang.Integer
    }
}

Alternative Approaches

• For parameter types and return types of methods, reflection can also be employed.
• However, if the lists are within the same scope of the class/method that requires their generic types, knowing them is redundant, as they have already been explicitly declared.

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