About class and object relationship [duplicate] - java

This question already has answers here:
The difference between Classes, Objects, and Instances
(16 answers)
Closed 4 years ago.
Can I say that class is a manual for building up an object?
I agreed that object is one of the instance of class.But if some properties that defined by classes may not make sense for particular object, for example,I have an object called "Duck" which use the class "Bird" for build, but Duck cannot fly,so if I use class "Bird" for build up an object "Duck".In the other words, the manual cannot provided the guideline for build up the object.
So,can i say that class is a manual for building up an object?As well as the manual should provided all the functions that the users need,in case there shouldn 't have any defects inside.

You are mixing two things at the same time:
On one hand you have to know that a class is just some kind of a template for an object instance, as it will show you what are the attributes and methods that any instance of that object can do. For example, you can think about the class "Car". Each car will have a different colour and a different plate number. That's a good sign for us to think that those two can be attributes of the class.
On the other hand, you have to know that sometimes classes are related between them and you have some tools to resolve your problems. That's when inheritance and interfaces come along in your example. As you can imagine, any duck is a bird, but not any birds are ducks. Therefore, you can establish a class inheritance between Bird (as a "father" class) and Duck (as a "child" class), because a lot of the behaviour between birds can be defined for all of them. In your specific example, you will define the "flying" methods in Bird subclasses.
Moreover, you can say that this example should have Bird as an abstract class, because Bird is a "concept" but it cannot exist by itself (you can also think in "polygon" and "triangle", polygon doesn't make sense by itself).

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Interfaces and methods in Java [duplicate]

This question already has answers here:
Is there more to an interface than having the correct methods
(17 answers)
Closed 7 years ago.
For Java!!
We can implement different methods in different classes. In interface we create an abstract method and provide its implementation in the class that implements the particular interface. What is the purpose of creating an interface if we can create and implement methods in classes separately?
Plz help me I'm new in Java?
Consider an example :
Suppose you are creating some application which is concerned with the animal kingdom.
Now you are asked to create a dog, cat, lion etc objects.
So first thing will come to your mind, since these all belong to animal kingdom I can create a base class named ' Animal ', and everything will inherits it. Now yo created something like this
class Animal {
legs;
family;
eat();
roam();
sleep();
makeNoise();
}
So all the animals inheriting the animal class will have these features. You can call this as a "IS-A" relationship. Like Dog IS-A Animal.
Now suppose you are asked to use your animal simulation program for some science-fair. You can use this design in that too.
Now suppose someone asked you to use your simulator in a pet-shop.
Since you don't have any pet behavior. What you did is add the pet features in the base class and thought this will work.
So now you program can create a lion which has the pet behavior. STRANGE!!
Now what you need to put all the pet behavior in one place and make sure that all the pet animals should posses it.
A way to do is create another superclass with all pet features and extend it. This is multiple inheritance, which JAVA don't allow (just Google deadly diamond of death). So comes the interface.
Interface is more like a set of behaviors which your object implements.
And since every object can have its own set of implementations, all these methods should be abstract.
It gives you polymorphic benefits without deadly diamond of death problem. It is a more like a contract which defines that your object must implements following features.
So now what you can do
interface PetBehavior{
befriend();
play();
}
and classes from different inheritance tree can implement this interface.
We do that in order to organize data. This the ability to perform operations lots of times and to structure your data. There is another thing called Vector. If objects implement the same method, they can be iterated and sorted via this Vector
Because with a single interface you can have multiple implementation.
For example if you have a list interface, implementation could be ArrayList or LinkedList.
They have different performances and are used based on the context. By having the same interface, if you want to change something you have to simply change from this
List<String> arrayList = new ArrayList<String>();
to this
List<String> arrayList = new LinkedList<String>();
bacause they have the same methods but implemented in different ways
I used to have this question too!
There are 3 main reasons why we need interfaces:
it makes more sense. Interfaces create a "can be used as" or "have the ability to" relationship between the implementing class and the interface. For example, you can have an interface called Flyable and all the things that can fly implements this interface. E.g. Bird, airplane and balloons. These implementing classes "have the ability to fly" because they all have a fly method or whatever is defined in the interface. Also, if a method requires a Flyable object, the programmer can just pass in a bird, an airplane, etc. with his/her common sense. But still, this isn't very practical, just like whether you should write the { on a new line doesn't matter.
It makes it easier for you, and other programmers using your API or library or whatever you are creating, to create custom behaviors. Java Swing is a very good example. If you don't know what swing is, it is an API used to create programs that has a "window" or GUI. How are you going to tell the computer what to do when the user clicks on a button? Via an interface! There is an interface called ActionListener. It let you create your own things to do when the users clicks on a button. You just pass an ActionListener object to a method, with your own implementation, and it will be run when the button is clicked! If my words don't make sense, let me use one sentence to summarize all this.
interfaces provide a way to pass around different methods (with custom implementations) as parameters in methods.
EDIT
Oh I missed one point!
interfaces aid polymorphism. Say you have a dog, a cat, and a fish class and all of them don't implement any interfaces but have similar methods (move, sleep, eat etc). If you want to create an array of all your animals, you can only create an array of Object because all Java classes inherits Object. This is unsafe because then you can add whatever you want into the array, and you need to do casting in order to use those move sleep eat methods. So sad! :( If you create an interface called Animal which contain the three common methods and make all three classes implement it, you can just create an array of Animal. And you don't even need to cast it to the right type before you can access move sleep eat! How cool is that!
So remember to create interfaces when a lot of your classes have similar methods but different implementations to just UNITE THEM ALL!

An empty super class?

I want to have an observable list that can list TableObjects (Self made class). I will send this observable list to a TableView object. My question: is there any point in having an empty super class?
I'm going to create three different tables, which shows three different types of objects. My plan is to create these types from the TableObjects, meaning that I will create subclasses that extends TableObject. So TableObjectA, B and C will have String values like for example name or type.
I could actually just drop the super TableObject super class, but isn't it in my interest for design specific reasons? I've actually learned little about software design, and how to design a software that will be future development friendly from the courses I've been taking.
You just describe the Marker interface pattern and, in your case, it should not be an empty superclass but an interface.
If your super-class represents a common behavior you can have it in place. If you want all your classes to incorporate any specific behavior you can have an interface in place. But, if you want to have a super-class only for the sake of clubbing few child classes together or for any reason alike, I would recommend to adhere to appropriate class naming convention to do so and NOT to have super-class.

multiple inheritance casting , constructor chaining [duplicate]

This question already has answers here:
Java Multiple Inheritance
(17 answers)
Closed 8 years ago.
I have gone through various sites to understand java does not support multiple inheritance. One of the reasons was to avoid issues like casting and constructor chaining. How does multiple inheritance would cause the issue of casting and constructor chaining in Java? Can anybody explain me with example.
There are multiple reasons why multiple inheritance could be an issue, and to prevent these, Java simply does not allow it at all. The main one is diamond inheritance aka the diamond problem. Lets say you have superclass animal, and subclasses land and water. Now, lets say you create another class, frogs. Because frogs can live in water, or on land, you decide that the frog class will inherent from both land and water. So it will be a subclass of land and water.
Lets extend our scenario to say that the animal class has a move method. This method has the animal traveling a distance without specifying how they do so. The land subclass overrides this move method to specify that the animal is walking. Meanwhile, the water subclass overrides to specify that the animal is swimming. If a frog were to try to move, it would not know whether it was swimming, or walking. This can cause some serious issues, and is the basic problem to which EJP was referring.

What is the use of a sub class object being referred by a super class variable in java? [duplicate]

This question already has answers here:
Using superclass to initialise a subclass object java [duplicate]
(5 answers)
Closed 9 years ago.
Suppose B extends A and we have a declaration A a=new B(); What is the use of referencing sub class object by super class variable?And what are the fields and methods accessible by the object a((only child class methods and variables) Or (from both child and parent class))?
Thank u
The reason is abstraction. The idea is that you don't need to know every little tiny detail about the object. For example, say you're driving a car. For the most part, the pedal on the right makes you go faster, the pedal on the left slows you down, and the big round thing in front of you steers the car. How that happens isn't important for the driver (a.k.a. user) to know, but it is very important for those details to work properly in order for the car to actually move.
The use of using super class reference type is that you dont have to worry about implementation class (or specific class), like List a = new ArrayList();
only methods exposed by super class will be accessible in this case.
So this is mainly to achieve: Polymorphism as well as Abstraction.
The main principal is that client coding against a higher label interface and does not worry about the low level implementation.
This gives a flexibility to change the implementation without changing the clients. And also client has a flexibility to try multiple implementations.
In your example, the members that will be accessible from a are the ones defined by the A class.
Hi In simple words we can say that All B's are A's and not all A's are B's. That is how the whole inheritence works. It givces the required abstarction.

Conceptual Difference: Fully Abstract Class v. Interface

I am wondering if I have the correct understanding of the theoretical difference between a fully abstract class and an interface. I understand the technical differences.
My understanding is that classes are used for concrete objects and interfaces for features those objects can exhibit. So If I was making a car class, and I wanted that car to have a navigation feature, I would make a navigation interface rather than an abstract class correct? (Replace with any feature, i.e. automatic parking, etc)
You are correct.
A boat or a plane could also have an automatic parking feature or a navigation system.
But in most models, something is either a boat or a car (let's leave flying cars out...).
Practically, in Java an abstract class is a good way to force an extension in a particular (unique) category. If you have AbstractPlane and AbstractBoat, you are sure an object is either one or the other.
The theoretical difference between the two is off-topic here, you might want to ask at http://programmers.stackexchange.com.
The closest on-topic question for so is what you can do with them, which you probably already know -- a class can implement two interfaces, it can extend only one class.
Fully abstract class is very similar to interface, practically identical. When it comes to particular languages, differences arises. For example, in Java class can extend only one other class but can implement many interfaces.
Regarding your example is correct. Class is collection of 2 main things: data and methods to work with that data.
Yes, you are on the right track. Think of it this way. You have a fruit. But you don't know which one it might be. So you make an interface fruit which is upto the user now how may they use this interface. It might be apple, oranges or peach. The interface fruit will have a definite size, color and environment it grows. Though this information changes according to different types of fruits.
In other words, interface is like a skeleton of something very very specific that you are trying to accomplish in the long run.

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