Understanding Multilevel Inheritance in Java

package ch9.l4;

class A {
	int a;
	void setA(int a) {this.a=a;}
}

class B extends A {
	int b;
	void setB(int b) {this.b=b;}
}

class C extends B {
	int c;
	void setC(int c) {this.c=c;}
}

class D extends C{
	int d;
	void setD(int d) {this.d=d;}
	
	int findSum() {
		return (a+b+c+d);
	}
}

public class MultiLevel {
	public static void main(String[] args) {
		D objD=new D();
		objD.setA(10);
		objD.setB(20);
		objD.setC(30);
		objD.setD(40);
		System.out.println("A+B+C+D: "+objD.findSum());
	}
}

A+B+C+D: 100

package ch9.l4;

class Animal {
    void breathe() {
        System.out.println("Animal breathes");
    }
}

class Mammal extends Animal {
    void feed() {
        System.out.println("Mammal feeds");
    }
}

class Dog extends Mammal {
    void bark() {
        System.out.println("Dog barks");
    }
}

public class MultilevelTestInheritance {
	public static void main(String[] args) {
		Dog d = new Dog();
        d.breathe(); // From Animal class
        d.feed();    // From Mammal class
        d.bark();    // From Dog class
	}
}

Animal breathes
Mammal feeds
Dog barks