Java OOP – Understanding Abstract Classes and Abstract Methods

abstract classjavaobject-orientedobject-oriented-design

I got several explanations but so far I'm not able to understand that what are the abstract classes and methods in Java.
Some said it has to do something with the security of the program, other said it isn't anything like that.
Even from the Dietel & Dietel's book I don't get it's purpose.
When, where and why do we use it?

Best Answer

First of all, the examples will be in C#. But I think you'll have no problem understanding it. So...

You CANNOT instantiate objects of an abstract class. You must have a subclass that derives from the class, if you want to instantiate it. But this probably won't help you since you may have come across that before.

So let's try an example.

Suppose you want to lose weight and your nutritionist asks you to track your food intake. When you go to her again, you can't tell her that you ate a "food". Although it's not wrong, since all food has calories, it is too "abstract" for her. So you need to tell her WHICH food.

So, if she were to code that... She'd have an abstract class Food. 

abstract class Food
{
    public int CaloriesPerPound { get; set; }
    //...
}

Why? Because that wouldn't allow you to say "I ate a food". 

{
    Food fries = new Food(); //This won't even compile
}

So you need concrete classes:

class Fruit : Food
{
 //Other stuff related only to fruits
}

class Bread : Food
{ 
 //Other stuff related only to Bread
}

So you'll have to tell exactly which food it is. 

{
   Apple desert = new Apple(); //Now we are talking
}

But she still needs the food class, it is still interesting to her: 

class Meal
{
    public List<Food> Foods { get; set; }
    //Other meal related stuff
    public int TotalCalories()
    {
        int calories = 0;
        foreach (Food food in Foods)
        {
            calories = calories + food.CaloriesPerPound * FoodQuantity;
        }
        return calories;
    }
}

That's an example of when you need an abstract class. When you want to have a base class, but you don't want anyone creating an object of your class.

An abstract CAN have abstract methods. It doesn't need to. Is totally ok to have an abstract class without abstract methods. Speaking of which...

Abstract method is something that it's too broad. There's no similarities on how things work, so the classes that derive from your class that has an abstract method won't be able to call the super class implementation. And you're also forcing all the sub classes to implement their version of that method.

Let's try another example? An Animal Band Simulator. You'd have animals, and each one of them would make a sound. But those are very different "implementations", they have nothing in common A dog differs from a cat, a cat from an elephant, an elephant from a cricket. So you'd have an abstract method MakeSound.

class Animal
{
    public abstract void MakeSound();
}

But, if you have an abstract method in your class, that makes the whole class abstract! 

abstract class Animal
{
    public abstract void MakeSound();
}

So the rest of the code might be:

class Dog : Animal
{
    public void MakeSound()
    {
        //bark at the moon
    }
}

class Cat : Animal
{
    public void MakeSound()
    {
        //meowing for food
    }
}

And the BAND!

class AnimalBand
{
    public List<Animal> Animals { get; set; }
    public void RockOn()
    {
        foreach (Animal animal in Animals)
        {
            animal.MakeSound();
        }
    }
}

So this is an example of when you use an abstract method.

I guess this is the essence. It may be a little forced/convoluted, but I think it gets the point across. Go back to the examples that didn't help you before and see if they make sense now.

I'm go out on a limb and say you're a beginner. This stuff may seem strange to you, and you may not see any benefit in using abstract classes.

It's ok. It's strange for almost all of us when we start. But in time, you'll learn to appreciate it.

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