这个 Java 语法是什么意思？ (`类<? extends E>克拉兹`）

Question

I need a custom method to check for a list containing an instance of a class and call this method but I do not understand this syntax "Class clazz" and I do not understand What's the second parameter of this method

public static <E> boolean containsInstanceOfOidInrArraylist(List<E> Arraylist, Class<? extends E> clazz) {
        for (E e : Arraylist) {
            if (clazz.isInstance(e)) {
                return true;
            }
        }
        return false;
    }

Answer 1

Ok, so the first parameter of the function is a List<E> named Arraylist (You shouldn't capitalize variables in Java, name it as such arrayList ).

The second parameter is a Class<? extends E> Class<? extends E> named clazz .

Check manub's explaination of Class<?> :

Class is a parameterizable class, hence you can use the syntax Class<T> where T is a type. By writing Class<?> , you're declaring a Class object which can be of any type ( ? is a wildcard). The Class type is a type that contains meta-information about a class.

So now you know what Class<?> means, but what about Class<? extends E> Class<? extends E> ?

<? extends E> <? extends E> basically means any class which extends E (or E itself).

So Class<? extends E> clazz Class<? extends E> clazz means you have a varaible named clazz which is E class or a sub class of E .

Answer 2

See this tutorial: https://docs.oracle.com/javase/tutorial/java/generics/upperBounded.html

"Upper Bounded Wildcards
You can use an upper bounded wildcard to relax the restrictions on a variable. For example, say you want to write a method that works on List<Integer> , List<Double> , and List<Number> ; you can achieve this by using an upper bounded wildcard.

To declare an upper-bounded wildcard, use the wildcard character ('?'), followed by the extends keyword, followed by its upper bound. Note that, in this context, extends is used in a general sense to mean either "extends" (as in classes) or "implements" (as in interfaces)."

The second parameter is a class that is the same as the class of the objects in the list, or extends it. clazz is used as a variable name because class is a reserved keyword and cannot be used.