We Keep Coding

Abstract Class Vs. [Interface+Inheritance] in Java

In context of Java, could a class replace the need of extending an abstract class by extending another non-abstract class and implementing an interface together, both of which combined have all the methods(abstract and implemented), of an abstract class?

In context of Java, could a class replace the need of extending an
abstract class by extending another non-abstract class and
implementing an interface together, both of which combined have all
the methods(abstract and implemented), of an abstract class?
Can it? Yes
Should it? No
An abstract class can be replaced by a concrete one, but you will be altering your system.
Do remember: an abstract class can not be instantiated, nor should it be, since it's not 'concrete enough' to make sense for your business. (If it does, it shouldn't have been an abstract class to begin with)
If you make it concrete, you risk that developers will use instances of the (what-should-be) abstract class.
If you change it the way you propose:
public void doSomething(MyAbstractClass instance){
// here we know there is an implementation provided by a subclass
}
would become
public void doSomething(MyShouldBeAbstractClass instance){
// here they can pass instances of the base class, which might have unsupported methods
}
For instance:
public String getConcreteInformation(){
throw new UnsupportedOperationException("Should be called on a child class");
}
and could lead to a lot of nasty bugs

If I extend an abstract class, that is implementing an interface, do I need to rewrite the methods?

I have an interface with several methods. I then have an abstract class that defines some of those methods, but also leaves some of the methods abstract. Now I have a class that extends the abstract class. Do I need to write all the methods from the original interface and define them, or does java know that some of them have been implemented by the abstract class?

At the point that you write a concrete (non abstract) class that implements an interface and/or is derived from an abstract base class, that concrete class must provide an implementation for every method it is specified as having. Specified means either by being declared abstract in a base class, or being specified by an interface that the class or one of its base classes declares it to implement.
If you write an abstract class that implements an interface and/or is derived from an abstract base class, your new class need not provide an implementation for any of the methods it is specified as having; those remaining methods are either explicitly or implicitly considered as being declared abstract by your new class.

Interfaces methods must be implemented. If the abstract class already implements some of them, you're free to override them in the subclasses or not.

In Interfaces, you declare some methods(), no actual implementation should be there.
And when you implement the interface into a abstract class, the declaration of those methods is present in that abstract class. It is up to you to decide whether to define the methods here or not.
when you extend these methods in concrete class(one which will be used to create objects), every methods needed to be defined and since if you already defined some methods in abstract class you don't need to do it again,but you can do it.

Normal Class extend to another abstract class

public abstract class A {
abstract void polo();
}
class B extends A {
}
My doubt is if I give abstract keyword in class B means it wont shows anything
abstract class B extends A {
}
Why its not showing the message here to implement methods in class A - this is my doubt.
If I didn't give abstract keyword means class B indicates like this
The type B must implement the inherited abstract method A.polo()
What's wrong with that one. Can abstract class extend another abstract class or not.

If a non-abstract class extends an abstract class, it must implement all abstract methods, in your case class B must implement polo()
If you make class B abstract, then you don't have to. But in this case you can't create an instance of B and the classes that extends B should implement polo

You can extend an abstract class, but you must decide what the new one will do about the abstract methods. You can choose to implement all those methods, so the abstract keyword won't be required. You can also implement some or none of the methods, but you must mark the class as abstract.
Depending of the business logic, you may put the abstract methods into an Interface that required classes will implement.

if B is abstract
not mandatory to implement the method
but you can implement the method or left implementation for child class
but if B is not abstractit is mandatory you implement the abstract method

One handy trick I use in this situation is to make class B non abstract to start with. I then use the IDE to generate the over-rides for all the abstract methods and then afterwards I make class B abstract and then go through the overridden methods and decide for each one whether to implement them or leave them abstract at this level too.

abstract classes ,constructor and interface

I'm really confused, and I've read a TON of questions on this topic and I have not been able to pinpoint anything specifically that an interface can do that an abstract class cannot do.

A class can implement multiple interfaces, but it cannot implement multiple abstract classes.

Interface itself cannot do anything. It just defines kind of contract between the class(es) that provide some functionality and the caller.
Abstract class is the class that defined as abstract. If class has at least one abstract method (i.e. method without implementation) it must be defined as abstract. But abstract class can contain implementations as well.
Interface cannot contain implementation. Only abstract methods and constants (static final fields).
Class can implement several interfaces and extend only one class (including abstract class).
I hope this helps.

Abstract class can also contain function implementation rather than just defining the functions that have to be implemented by inheriting classes

Abstract classes are partially implemented classes, that will be extended by concrete classes(non-abstract), to be implemented.
Example:
This example does not mean that the sub classes must implement those methods(as it happens when implementing an interface). You can declare a subclass abstract, and the implementation will be done later by annother sub-sub-class. (For example: Boat can have subclasses "SpeedBoat" and FisshingBoat, and the may implement honk() in different ways)
The interface are the eyes of class to the outside world. What classes can do is declared in the interface, but implemented in the class.
A class can implement many interfaces, but can extend only one class.
See this little example of interfaces:
As you can see when we use interfaces we can have a lot of flexibility. Some Enemies can do things that some Heroes can do too(DarkKnight can throw arrows).
I hope you now feel the difference between the abstract classes and interfaces.
Remember this about interfaces and Abstract classes:
Interfaces dont have variables, just non implemented methods(abstract methods implicitly)
Classes that implement interfaces must have all the methods of the interface in its body
One class can extend only one class but implement more than one interface
If a class has an abstract method, it must bee declared as abstract.
Abstract classes can implement interfaces
Interfaces can extend other interfaces(more than one)
I dont know if i forget something, i hope this information helps.

An abstract class can everything that a Interface can do. However inverse of this is not correct.

Abstract class can contain abstract methods, abstract property as well as other members (just like normal class).
Interface can only contain abstract methods, properties but we don’t need to put abstract and public keyword. All the methods and properties defined in Interface are by default public and abstract.
We can see abstract class contains private members also we can put some methods with implementation also. But in case of interface only methods and properties allowed

Interface and abstract class almost both are same the major difference is using interface we are not able to define the body of the method but using abstract class we can define the body of the method inside the abstract class or while implementing it.
e.g
Interface abc()
{
string xyz();
}
abstract abc()
{
string xyz()
{
// define body
}
}
or
abstract abc()
{
string xyz();
}

An abstract class is a class - it defines all or part of an implementation of some behaviour for a class of objects, but with some extension points for concrete subclasses to provide.
An interface is a type - it defines the set of operations which are provided by any class implementing the interface.
You're almost asking whether there is anything that a candidate can do that the job description can't. Creating an abstract class says 'here is a template for some implementation'. Creating an interface says 'I expect an object to provide these capabilities'. You can use virtual methods in an abstract class to implement some aspects of a type, but the intention is different.

Defining an abstract class without any abstract methods

Can I define an abstract class without adding an abstract method?

Of course.
Declaring a class abstract only means that you don't allow it to be instantiated on its own.
Declaring a method abstract means that subclasses have to provide an implementation for that method.
The two are separate concepts, though obviously you can't have an abstract method in a non-abstract class. You can even have abstract classes with final methods but never the other way around.

Yes you can. The abstract class used in java signifies that you can't create an object of the class. And an abstract method the subclasses have to provide an implementation for that method.
So you can easily define an abstract class without any abstract method.
As for Example :
public abstract class AbstractClass{
public String nonAbstractMethodOne(String param1,String param2){
String param = param1 + param2;
return param;
}
public static void nonAbstractMethodTwo(String param){
System.out.println("Value of param is "+param);
}
}
This is fine.

Yes you can do it. Why don't you just try doing that?

YES You can create abstract class with out any abstract method the best example of abstract class without abstract method is HttpServlet
Abstract Method is a method which have no body, If you declared at least one method into the class, the class must be declared as an abstract its mandatory BUT if you declared the abstract class its not mandatory to declared the abstract method inside the class.
You cannot create objects of abstract class, which means that it cannot be instantiated.

Yes we can have an abstract class without Abstract Methods as both are independent concepts. Declaring a class abstract means that it can not be instantiated on its own and can only be sub classed. Declaring a method abstract means that Method will be defined in the subclass.

Yes, you can declare a class you cannot instantiate by itself with only methods that already have implementations. This would be useful if you wanted to add abstract methods in the future, or if you did not want the class to be directly instantiated even though it has no abstract properties.

yes, we can declare an abstract class without any abstract method. the purpose of declaring a class as abstract is not to instantiate the class.
so two cases
1) abstract class with abstract methods.
these type of classes, we must inherit a class from this abstract class and must override the abstract methods in our class,
ex: GenricServlet class
2) abstract class without abstract methods.
these type of classes, we must inherit a class from this abstract class,
ex: HttpServlet class
purpose of doing is although you if you don't implement your logic in child class you can get the parent logic
please check the HttpServlet source code

You can, the question in my mind is more should you. Right from the beginning, I'll say that there is no hard and fast answer. Do the right thing for your current situation.
To me inheritance implies an 'is-a' relationship. Imagine a dog class, which can be extended by more specialized sub types (Alsatian, Poodle, etc). In this case making the dog class abstract may be the right thing to do since sub-types are dogs. Now let's imagine that dogs need a collar. In this case inheritance doesn't make sense: it's nonsense to have a 'is-a' relationship between dogs and collars. This is definitely a 'has-a' relationship, collar is a collaborating object. Making collar abstract just so that dogs can have one doesn't make sense.
I often find that abstract classes with no abstract methods are really expressing a 'has-a' relationship. In these cases I usually find that the code can be better factored without using inheritance. I also find that abstract classes with no abstract method are often a code smell and at the very least should lead to questions being raised in a code review.
Again, this is entirely subjective. There may well be situations when an abstract class with no abstract methods makes sense, it's entirely up to interpretation and justification. Make the best decision for whatever you're working on.

yes you can do that.
declaring class abstract means that class will not be instantiated by any other class.
and there should be at least one abstract method inside that and meaning of that you can declare abstract method in that class if you are not declaring method than its ok.
example:
public abstract class abs {
protected int cx = 0, cy = 0;
public void p() {
System.out.print("hello");
}
}
this will work for sure.

Yes you can. Sometimes you may get asked this question that what is the purpose doing this?
The answer is: sometimes we have to restrict the class from instantiating by its own. In that case, we want user to extend our Abstract class and instantiate child class

Actually there is no mean if an abstract class doesnt have any abstract method . An abstract class is like a father. This father have some properties and behaviors,when you as a child want to be a child of the father, father says the child(you)that must be this way, its our MOTO, and if you don`t want to do, you are not my child.

Yes, you can define an abstract class without an abstract method. However, if there is no method inside you might better go with an interface