What is the purpose of List<?> if one can only insert a null value?

public static void printList(List<?> list) {
    for (Object elem: list)
        System.out.print(elem + " ");
    System.out.println();
}

You use the unbounded wildcards when the list (or the collection) is of unknown types.

 

https://stackoverflow.com/questions/29342117/what-is-the-purpose-of-list-if-one-can-only-insert-a-null-value

What is the purpose of List if one can only insert a null value?

https://stackoverflow.com/questions/35272848/java-generics-unbound-wildcards-vs-object

Java generics, Unbound wildcards vs

https://stackoverflow.com/questions/18176594/when-to-use-generic-methods-and-when-to-use-wild-card

When to use generic methods and when to use wild-card?

---

https://www.tutorialspoint.com/java_generics/java_generics_wildcards_guidelines.htm

Java Generics - Guidelines for Wildcard Use

package com.tutorialspoint;

import java.util.ArrayList;
import java.util.List;

public class GenericsTester {

   //Upper bound wildcard
   //in category
   public static void deleteCat(List<? extends Cat> catList, Cat cat) {
      catList.remove(cat);
      System.out.println("Cat Removed");
   }

   //Lower bound wildcard
   //out category
   public static void addCat(List<? super RedCat> catList) {
      catList.add(new RedCat("Red Cat"));
      System.out.println("Cat Added");
   }

   //Unbounded wildcard
   //Using Object method toString()
   public static void printAll(List<?> list) {
      for (Object item : list)
         System.out.println(item + " ");
   }

   public static void main(String[] args) {

      List<Animal> animalList= new ArrayList<Animal>();
      List<RedCat> redCatList= new ArrayList<RedCat>();

      //add list of super class Animal of Cat class
      addCat(animalList);
      //add list of Cat class
      addCat(redCatList);  
      addCat(redCatList);  

      //print all animals
      printAll(animalList);
      printAll(redCatList);

      Cat cat = redCatList.get(0);
      //delete cat
      deleteCat(redCatList, cat);
      printAll(redCatList); 
   }
}

class Animal {
   String name;
   Animal(String name) { 
      this.name = name;
   }
   public String toString() { 
      return name;
   }
}

class Cat extends Animal { 
   Cat(String name) {
      super(name);
   }
}

class RedCat extends Cat {
   RedCat(String name) {
      super(name);
   }
}

class Dog extends Animal {
   Dog(String name) {
      super(name);
   }
}

Cat Added
Cat Added
Cat Added
Red Cat 
Red Cat 
Red Cat 
Cat Removed
Red Cat