java - Why can I add an object of a class to a LinkedList of another class? -

i have class one:

public class 1 {     private int idone;     private string nameone;     //getter , setter } 

and class extends one:

public class 2 extends 1 {     private string morethings;     private string example;     //getter , setter } 

then have linkedlist<one> mylist = new linkedlist<>();

i dont understand why can add two objects list if specified list going have one objects linkedlist<one>. example:

linkedlist<one> mylist = new linkedlist<>();  2 t1 = new two(); 2 t2 = new two();  mylist.add(t1); mylist.add(t2); 

that cool dont know why can that.

consider problem real world example,let have class animal, having methods running() ,eating(), drinking() etc.now class like

class animal {   public void running()   {     //some code   }    public void eating()   {     //some code   }    public void drinking()   {     //some code   } } 

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let have 2 other classes dog , cat, type of animal can extend animal class. valid

animal animalobject=new dog();  // dog type of animal or animal animalobject=new cat();  //cat type of animal or dog dogobject=new dog();   //nothing special or cat catobject=new cat();    //nothing special 

however ,following invalid

dog dogobject=new animal(); or cat catobject=new animal(); 

because (read quote again.) , give eye this.. :)