How does class object relation work?

Question

What happens when we create an instance of a class? I mean, will every field and method of that class be inside that object (with allocated memory) or will it not have anything inside and have a reference to its class, instead. (First option looks like a waste of memory.)

Answer 1

Whenever a new object is created, new memory is allocated in the heap space (dynamic memory). This space is reserved for everything that's specific to this single instance of a class. That means every field (instance field, not the static one) would have its own separate location in memory.

For methods, things are different since they are common for all instances of a class, which means you would have one method in memory which would be referred to by each instance of a class.
If you wonder where are local variables of a method stored: they are stored on the stack, meaning they are not shared between invocations of that method.

Also, methods are stored in the 'code memory', separate from instance fields.

Answer 2

The full concept of OOP (Object Oriented Programming) is being able to abstract the reality by "describing" the objects.

To accomplish this purpose we're able to declare an object attributes, for example. You can have an object Person, this person has different attributes, such as name, age, address. Also you can describe the different actions of this person, such as eating, taking a bath, etc. The abstraction of this would look like:

class Person(){
     int age;
     String name;
     String address;

     void eating(){
         //describe the process of eating
     }

     void takeBath(){
          //describe the process of taking a bath
     }

}

The whole purpose of this is that afterwards you can instantiate an Object Person and it will have all of it's attributes. You can call the object in another class and instantiate it, this call will look like:

Person person1 = new Person(); //here you are saying that you have a new variable of type person
person1.name = "Eduardo"; //You're saying that his name is Eduardo
person1.age= 28; //he is 28 years old
person1.eating(); //he's eating, I like to eat tho.

Person person2 = new Person(); //You are saying that there's another person
person2.name = "Maria"; //her name is Maria
person2.age = 31; //She's 31 years old
person2.takeBath(); She's taking a bath

As you can see I didn't have to declare again the methods or attributes of the class, simply created a new object and started setting the attributes (please be aware of the difference between declaring and setting, to declare is to say there is an attribute of X type; setting is giving a value to that attribute).

There's another very useful property in OOP called heritage. Lets say we want to describe a Programmer; a programmer is a Person and he can do whatever a person does but also he codes and has a preferred language this would look like:

class Programmer extends Person { // I'm declaring the class but also I'm saying it's a person, so it will have all the attributes and methods of the Person class.

       String preferredLanguage;  //I will only declare the new attribute that is preferred Language, name, age and address are set because it's a person



       void code(){
              //the process of making very awesome things
        }
}

And then I can instantiate another object Programmer

Programmer person3 = new Programmer();   //I called it person3 so you can understand it's only a variable, you can call it as you wish.
person3.name = "Berkay";  //Set the name without the need of declaring it in the method programmer, it's been inherit by the class Person
person3.code();//it can code!
person3.takeBath(); //A method of Person
person3.code(); //method of Programmer

At this stage I'd like to point that Programmer having the attributes of Person doesn't mean the same way around. For example, trying to make this would be a mistake:

person1.code(); //person1 is a Person and it doesn't have the code method.

PS: There are a lot of things to improve in these code, it's far from using best practices. All I intend to do is to give a clear example of the way OOP works. I didn't test the code, it's only illustrative, it may contain typos