why we use abstract classes ?

Question

abstract class Animal {
int legs = 0;
abstract void makeSound();
}
public class Cat extends Animal{
public void makesound(){
System.out.println("meow");
}
}
here in above code we can also define seprate method makesound for cat,dog and other animals etc................then why define abstract method and implement all details later

Answer 1

animals have some common properties that abstract classes can capture succinctly. each animal is different so there is no meaning in making animal class concrete. but for a user, all animals can be same. in case new discovery happens, animal can still capture it.