What is the difference between a declaration using the Generic Class type and Generic Collections

Question

I will show you two generic generic styles. In part 1, I use the general declaration of the upper bound in the list as follows:

List<? extends Animal> totList = new ArrayList<Animal>();

But this will result in an error, as shown below, if you try to add an Animal object to the list:

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
    The method add(capture#1-of ? extends Animal) in the type List<capture#1-of ? extends Animal> is not applicable for the arguments (Animal)

    at GenericsType.main(GenericsType.java:39)

But, as in Part2, if I declare a list inside a general class in the following format, there will be no errors when adding (Animal objects) or (subclass of Animal objects) to the list.

class GenericAnimal<T extends Animal> 
{    
   List<T> genList = new ArrayList<T>();
}

Why in part2 he did not give an error and what is the difference between the two ad styles.

Code example:

1.Animal.java

public class Animal {
    private String name;
    private int height;

    public void animalJump()
    {
        if(height>100)
        {
            System.out.println(name+" with height-"+height+" can JUMP");
        }
        else
            System.out.println(name+" with height-"+height+" cannot jump");         
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setHeight(int height) {
        this.height = height;
    }

    public Animal(String name, int height) {
        setName(name);
        setHeight(height);
    }

}

2.GenericsType.java

import java.util.*;
import Animal;
import Animal.Cannine;
import Animal.Feline;
import Animal.Feline.Cat;
import Animal.Cannine.Dog;

public class GenericsType {

    public static List<? extends Animal> totList = new ArrayList<Animal>();

    public static void processAllfunc1()
    {
        for(Animal a : totList)
        {
            a.animalJump();
        }

    } 

    public static void main(String args[])
    {    
       // Part 1
        totList.add(new Animal("Animal1",21));  // Error: Unresolved compilation problem:           
        processAllfunc1();


       // Part 2
        GenericAnimal<Animal> genericanimal = new GenericAnimal<Animal>();
        genericanimal.genList.add(new Animal("Animal2",22));  // No Error, why?
        genericanimal.genList.add(new Cat("Cat4",204));      // No Error for Cat also, why?

        genericanimal.processAllfunc2();
    }
}

3.GenericAnimal.java

public class GenericAnimal<T extends Animal> {  

    public List<T> genList = new ArrayList<T>();

    public void processAllfunc2()  {
        for (T a : genList) {
            a.animalJump();
        }
    }
}

Answer 1

2 genericanimal.genList List<T>= List<Animal>. 1 List<? extends Animal>.

, List<? extends Animal> " Animal, ". , List<? extends Animal> list = new ArrayList<Cat>(). - . List<? extends Animal>, , , , , , , - .

Answer 2

part1, Animal ?

List<? extends Animal> totList = new ArrayList<Animal>();
totList.add(new Animal("Animal1",21));  // Error: Unresolved compilation problem:

, (? extends Animal). , Animal, Cat Dog.

, , , .

:

• , ( ) .
• Animal Dog Cat.
• Animal .

part2 Animal, Cat?

public List<T> genList = new ArrayList<T>();

genericanimal.genList.add(new Animal("Animal2",22));  // No Error, why?
genericanimal.genList.add(new Cat("Cat4",204));

genList Animal,

List<Animal> genList = new ArrayList<Animal>();

, , .

, Animal Cat, Animal (Cat - Animal)

<T extends Animal> :

• .
• , .
• Cat Dog .
• Animal .