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Abstract class in Java
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?
Kindly explain it like you're teaching a beginner, I would really appreciate your help.
- Abstract class is one which can't be instantiated, i.e. its object cannot be created.
- Abstract method are method's declaration without its definition.
- A Non-abstract class can only have Non-abstract methods.
- An Abstract class can have both the Non-abstract as well as Abstract methods.
- If the Class has an Abstract method then the class must also be Abstract.
- An Abstract method must be implemented by the very first Non-Abstract sub-class.
- Abstract class in Design patterns are used to encapsulate the behaviors that keeps changing.
An abstract class is a class that can't be instantiated. It's only purpose is for other classes to extend.
Abstract methods are methods in the abstract class (have to be declared abstract) which means the extending concrete class must override them as they have no body.
The main purpose of an abstract class is if you have common code to use in sub classes but the abstract class should not have instances of its own.
You can read more about it here: Abstract Methods and Classes
An abstract method is a method signature declaration with no body. For instance:
public abstract class Shape {
. . .
public abstract double getArea();
public abstract double getPerimeter();
}
The methods getArea() and getPerimeter() are abstract. Because the Shape class has an abstract method, it must be declared abstract as well. A class may also be declared abstract without any abstract methods. When a class is abstract, an instance of it cannot be created; one can only create instances of (concrete) subclasses. A concrete class is a class that is not declared abstract (and therefore has no abstract methods and implements all inherited abstract methods). For instance:
public class Circle extends Shape {
public double radius;
. . .
public double getArea() {
return Math.PI * radius * radius;
}
public double getPerimeter() {
return 2.0 * Math.PI * radius;
}
}
There are many reasons to do this. One would be to write a method that would be the same for all shapes but that depends on shape-specific behavior that is unknown at the Shape level. For instance, one could write the method:
public abstract class Shape {
. . .
public void printArea(PrintStream out) {
out.println("The area is " + getArea());
}
}
Admittedly, this is a contrived example, but it shows the basic idea: define concrete behavior in terms of unspecified behavior.
Another reason for having an abstract class is so you can partially implement an interface. All methods declared in an interface are inherited as abstract methods by any class that implements the interface. Sometimes you want to provide a partial implementation of an interface in a class and leave the details to subclasses; the partial implementation must be declared abstract.
abstract method do not have body.A well defined method can't be declared abstract.
A class which has abstract method must be declared as abstract.
Abstract class can't be instantiated.
An abstract class is a class that you can't create an object from, so it is mostly used for inheriting from.(I am not sure if you can have static methods in it)
An abstract method is a method that the child class must override, it does not have a body, is marked abstract and only abstract classes can have those methods.
With abstract classes you can have some kind of skeleton for other classes to extend.
You can't instantiate them but you can put some common implementation which you can use in the classes that extend them.
Once you get what abstract means in Java, you would ask: why they put this in ? Java may work without abstract stuff, BUT it makes part of a certain OO style or vocabulary. There exists really situations where an abstract class or method is an elegant way to express the program authors intention. Most when you are programming a framework or a library that will be used by others.
ABSTRACT CLASSES AND ABSTARCT METHODS FULL DESCRIPTION GO THROUGH IT
abstract method do not have body.A well defined method can't be declared abstract.
A class which has abstract method must be declared as abstract.
Abstract class can't be instantiated.
Related
Is this a valid abstract class?
I know that abstract classes cannot be instantiated, so I'm suspicious of the instance variable language and constructor Programmer. It also implements a writeCode method that is not declared as default. If I recall correctly, the only methods that can be implemented in an abstract class are those with default implementations.
public abstract class Programmer {
private String language;
public Programmer (String language) {
this.language = language;
}
public void writeCode() {
System.out.println("Written in " + language);
}
}
If it is a valid abstract class, can someone explain why it contains a constructor?
Also, more broadly speaking, can abstract classes have instance variables? If so, why? Doesn't that seem counter to the idea that abstract classes cannot be instantiated?
Finally, I would love it if someone addresses the writeCode method. Why is it implemented, without a default modifier?
Thanks!
Yes, this is a valid abstract class.
Abstract classes can have constructors, instance variables and concrete methods.
The main difference with regular classes is that they can also declare abstract methods, delegating implementation to the non-abstract child classes (this is not the case here, you have no abstract methods).
Another difference is that they cannot be initialized directly, even if they do provide an accessible constructor.
The constructor(s) of an abstract class are typically used to initialize values internally, and to invoke from child classes or anonymously.
See documentation here.
Example
Given...
public abstract class Programmer {
private String language;
public Programmer(String language) {
this.language = language;
}
public void writeCode() {
System.out.println("Written in " + language);
}
}
... and...
public class JavaProgrammer extends Programmer {
public JavaProgrammer() {
super("Java");
}
}
Concrete child class
new JavaProgrammer().writeCode(); // prints "Java"
Anonymous class (note the empty class body {})
new Programmer("JavaScript"){}.writeCode(); // prints "JavaScript"
As you say, abstract classes cannot be instantiated. However, when a subclass of any abstract class is created, the first sentence in its constructor is a call to super(), which is nothing but a representation of the constructor of its parent class, the abstract class.
An abstract class can have instance variables and methods. It is even possible to have an abstract class without any abstract method. However, an abstract method can only be declared in an abstract class.
You are mixing abstract classes and interfaces concepts. An interface cannot have instance variables, and any implemented method must be prefixed with the static or default modifier.
This is correct example of abstract class. Answering your questions:
default keyword is used (as of java 8) in interfaces, where you can implement default method implementation, abstract class can have a method implementation just as any normal java class
having constructor in abstract class imposes having a constructor in extending class so that the underlying abstract class can be properly constructed (e.g. fields instantiated etc)
abstract class cannot be instantiated but as any other class can have private fields and internally make use of them, should they be protected then extending classes will also be able to directly access them.
It seems to me you're confusing abstract class with interface.
The abstract class contains a constructor because when the instantiated class based on the abstract class is created it will call super() to execute the code from the abstract class.
The instance variable is a similar thing. The class that is developed from the abstract class would then have access to the language and be able to work with it.
In terms of the default, that would be best practice but there is no absolute requirement for it to be there.
Abstract classes are partial implementations. Sometimes, as in the case above, the only thing that prevents the class from being instantiated is the abstract modifier itself!
If an abstract class has constructors then it means that subclasses must invoke one of the constructors (by calling super(...) in their constructor.
More broadly it seems that you're confusing Interfaces and Abstract Classes. Interfaces are a contract, they specify how a class should behave but provide no implementation whatsoever. Abstract Classes are a specific partial implementation of some code.
Interfaces are used (broadly) when you require someone else to provide behaviour but you don't care how it works. Abstract Classes are used when you want to assist people in providing behaviour, but you still require them to provide some details.
Since these definitions overlap, it's not unusual to see both Abstract Classes and Interfaces provided, e.g. the various Adapters in java.swing.*.
I am told that: a class can extend a concrete superclass and override an implemented method to make it abstract.
If the method becomes abstract, then the corresponding superclass should become abstract too.
Is this correct? If so, is it meaningful to make the superclass or the method in superclass to become abstract?
To contain abstract methods the class itself must also be declared abstract.
Abstract methods can be used where behaviour is specific to each subclassing object. Although this can also be achieved through interfaces, you may prefer to use an abstract class as they can also contain already implemented methods, or you may wish to declare fields which are not static and final. Beware that when a class becomes abstract it can no longer be instantiated.
Read more about abstract classes here.
Whether or not you make your class abstract if it contains abstract methods is not a choice, it is a must. You cannot have a non-abstract class that has abstract methods.
Whether or not you should declare a method abstract or implement it in your super class comes down to your concrete use case. Is there a default behavior that makes sense for all possible concrete implementations of the super class? Then you might want to implement it. If that's not the case, declare it abstract and let the sub classes take over.
I am told that: a class can extend a concrete superclass and override an implemented method to make it abstract.
If the method becomes abstract, then the corresponding superclass should become abstract too.
Is this correct?
I think it's incorrect.
Take the following code for example.
public class Father {
public void print() {
System.out.println("Father");
}
}
public abstract class Son extends Father {
#Override
public abstract void print(); /* no need to abstract the superclass */
// and other abstract or concrete methods
}
public class Grandson extends Son {
#Override
public void print() {
System.out.println("Grandson");
}
}
I have two questions really. I'm trying to get a handle on how inheritance works.
If I have an abstract class to inherit from, and it has a method that is not labelled abstract does this method still need to be implemented in the subclass?
If I have a subclass that is inheriting from another subclass, which is then inheriting from an abstract class, does the lowest subclass need to implement the methods in the abstract class? Or because the methods have been implemented in the middle subclass, they don't need to be implemented again?
Thank you!
An abstract class is a class that is declared abstract. It may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.
An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this:
abstract void moveTo(double deltaX, double deltaY);
If a class includes abstract methods, the class itself must be declared abstract, as in:
public abstract class GraphicObject {
// declare fields
// declare non-abstract methods
abstract void draw();
}
When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, the subclass must also be declared abstract
If the method is not abstract it has been implemented already, when you subclass the abstract class you inherit the method implementation, re-implementing it would be overriding it. If the method was declared abstract you must implement or get compile-time error if the subclass is also not declared abstract.
If you are inheriting from a class A extends AbstractClass that is not abstract then A must have implemented any abstract methods or again compileerror. If it hasn't implemented any abstract classes then A must also be abstract and responsibility of implementing the abstract methods fall on subclassers of A. Any sublcassers that do not implement the method must also be declared abstract untill finally a subclass implements it.
What is difference between abstract class and non abstract class when extending derived classes? Both class I didn't use the method overriding and abstract methods (i.e. abstract class). Just I inherited the properties. What and why did prefer the class?
Ex:
Code 1:
abstract class a {
protected int empnno;
protected String empname;
}
class b extends a {
...
}
Code 2:
class a {
protected int empnno;
protected String empname;
}
class b extends a {
...
}
what is difference to extend abstract class and non abstract class?
Abstract classes may have abstract methods. Abstract methods are methods without implementations and these must be implemented by your subclass (unless you make your subclass abstract too).
Since your a class have no abstract methods, there is no difference what so ever from a subclass-perspective. (The only difference is that if a is abstract it may no longer be instantiated as is. It may only be instantiated in terms of subclasses.)
Suppose there is a class B, and class A, where A extends be. The following are the possible scenarios:
1. B is abstract
1.1. B doesn't have abstract methods
1.1.1. A is abstract
1.1.1.1. You don't want to instantiate A. Everything is fine.
1.1.1.2. You want to instantiate A. That's not possible, you can't create abstract objects
1.1.2. A is not abstract. Everything is fine
1.2. B has at least an abstract method
1.2.1. A is abstract
1.2.1.1. You don't want to instantiate A. Everything is fine.
1.2.1.2. You want to instantiate A. That's not possible, you can't create abstract objects
1.2.2. A is not abstract
1.2.2.1. A doesn't implement all the abstract methods. You can't run your project until you change this
1.2.2.2. A implements all the abstract methods. Everything is fine.
2. B is not abstract
2.1. A is abstract
2.1.1. You want to instantiate A. Error.
2.1.2. You don't want to instantiate A. No problem
2.2. A is not abstract. No problem.
To have a better understanding, we can compare Abstract Classes with Interfaces, the main differences are:
While abstract classes may contain fields/properties and concrete
methods, interfaces may contain only abstract methods (method
signatures).
One class can implement several interfaces, whereas it can extend just one class, abstract or not.
Therefore, when you create a subclass extended an abstract class, you need to implement the abstract method that was in the abstract class(if any), otherwise, the subclass would be still an abstract class -- which cannot be instantiated!!
Also, you can use interfaces instead of abstract classes if you only want to declare some methods signatures.
Actually they are the same, but you cannot instanciate Abstract classes. So if you want nobody tries to instanciate your class, you would like to make it Abstract.
An abstract class will contain abstract methods that do not have an implementation.
When you extend this class, sometimes you may decide only to provide implementations for some of those abstract methods. In this case you've extended an abstract class and yet the subclass is still abstract.
If you implement all the abstract methods, your subclass is typically not abstract (although there's nothing stopping you from declaring it as such, AFAIK).
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