inheritance, what is going on after every step

Question

i have an exercise to solve, i have the correct answer, but I can't get it... Could someone help me what is happening after every "step". here's the exercise. We have 2 classes: enter code here

public class CA
{ 
    protected String descricao;
    private float valor;
    public CA(String descricao, float valor)
    {
        this
        .descricao = descricao;
        this.valor = valor;
    }

    public String getDescricao()
    {
        return descricao;
    }

    public float getValor()
    {
        return valor;
    }

    public float teste(float a)
    {
        return soma(a);
    }

    public float soma(float a)
    {
        return valor + a;
    }
}

and the second :

public class CB extends CA
{
    private final int maxStock = 15;
    private int stock;
    public CB(String descricao, float valor)
    {
        super(descricao,valor);
        stock = 0;
    }

    public int getStock()
    {
        return stock;
    }

    public void setStock(int actual)
    {
        stock = actual;
    }

    public int emFaltaStock()
    {
        return (maxStock-stock);
    }

    public float soma(float a)
    {
        return getValor() + a * 2;
    }

    public boolean noLimite(int minStock)
    {
        return ((minStock-stock) <= 0);
    }
}

The question is what will be the result of these statements:

CB cb1 = new CB("cb1",10);
CA ca1 = cb1;
float v1= ca1.soma(2);

I know it will be 14, but why? Can anyone tell me?

You mean you are getting 14 as answer ? And thats what you have expected ? And you want the reason why the answer is 14 ? — Shirishkumar Bari, Jan 09 '15 at 06:35
possible duplicate of [About casting sub-class to super-class in JAVA](http://stackoverflow.com/questions/14151013/about-casting-sub-class-to-super-class-in-java) — ajb, Jan 09 '15 at 06:38
Basically, you have created a `CB`. Calling it a `CA` doesn't change it to a `CA`. It's still a `CB`, and when you use `soma`, it uses the one defined in `CB`. — ajb, Jan 09 '15 at 06:40

score 1 · Answer 1 · answered Jan 09 '15 at 06:50

It is an exhibition of what we call (polymorphism). While you are setting CB values (Subclass values) in first statement. and calling the subclass instance with superclass ca1 refernce in 2nd statement. In 3rd statement when you call method with superClass ca1 reference, the subclass CB method is called, due to polymorphism in java. hence the result is 14.

Shirishkumar Bari · Answer 2 · 2015-01-09T06:46:27.220

0

Please try to understand the flow

CB cb1 = new CB("cb1",10); // Object of CB has been created 
CA ca1 = cb1;             // Referencing  CA instance to CB instance
float v1= ca1.soma(2);   // Calling *soma* Method

as soma is an overriden method ... method of CB will be called even with ca1.soma(2); as ca1 is actually referring to CB instance. Method Calling depends on Instance and not on Reference .

edited Jan 09 '15 at 06:46

answered Jan 09 '15 at 06:40

Shirishkumar Bari

2,692
1
28
36

score 0 · Answer 3 · edited May 23 '17 at 10:33

0

I know it will be 14, but why? Can anyone tell me?

Why because, that is the beauty of Polymorphism.

Step 1: CB cb1 = new CB("cb1",10);

Here the value of valor = 10

Step 2: CA ca1 = cb1;

here reference of CA is reference of CB.

Step 3: float v1= ca1.soma(2);

So when you call soma, the method of CB will be called and return the value as 14.0.

Go through: What is polymorphism?

edited May 23 '17 at 10:33

Community

1
1

answered Jan 09 '15 at 07:03

TSKSwamy

225
2
21

score 0 · Answer 4 · answered Jan 09 '15 at 08:16

This is Because you created an object of a child class and assign it to a super class like so :

Father obj = new Child();

if you have a declaration like this which yours is exactly the same :

CB cb1 = new CB("cb1",10);
CA ca1 = cb1;
float v1= ca1.soma(2);
OR
CA ca1 = new CB("cb1",10);
float v1= ca1.soma(2);

and there is a method with the similar signature sharing in father and child, always the child method will be proceeded ~

this technique is so useful in Object Oriented Programming because you may have many child class which are extending a Father class and each of those child classes might have an overwriting method with different algorithms

This approach calls Polymorphism in Object Oriented Programming.

inheritance, what is going on after every step

4 Answers4