Assume an abstract class has only abstract methods declared in it and the interface has abstract methods.
How does this affect the abstract method m?
abstract class A {
abstract void m();
}
interface A {
abstract void m();
}
Even if the abstract class is a pure abstract, i.e. contains only abstract methods and does not contain state, your subclass extends this class and therefore you have single inheritance only.
However you can implement several interfaces.
Therefore if you want to force your classes to be a subclass of your A and not to be subclass of something else, use abstract class. Otherwise define interface.
Interfaces are also more useful when you don't exactly know if all your entities will need to override a specific method. Consider for instance birds, some can fly and some can't. You don't want to force overriding a fly() method in all Bird subclasses.
public abstract class Bird(){
...
public abstract void eat(); //all birds eat
public abstract void sleep(); //all birds sleep
public abstract void fly(); //is wrong here since not all birds can fly
}
A more correct way would be to define interfaces for specific cases, so in this case
public interface Flyable{
abstract void fly();
}
Then you can define your subclasses as birds that can fly or not based on the fact they implement the Flyable interface or not.
public class Eagle extends Bird implements Flyable{
#override
public void fly(); //needed
-----
}
public class Ostrich extends Bird{
//does't fly
}
I have a concrete class which extends an abstract class, which in turn extends an abstract class. This top class implements an interface. Can I implement the interface method in the top abstract class, so that the concrete sub-class does not have to?
Also, if the interface method takes an Object type as it's parameter, can I have the implementation in the top abstract class take another type (which seems right to me), or would I have to use exactly the same method signature?
Can I implement the interface method in the top abstract class, so that the concrete sub-class does not have to?
Yes
Also, if the interface method takes an Object type as it's parameter, can I have the implementation in the top abstract class take another type (which seems right to me), or would I have to use exactly the same method signature?
It has to be the same signature. However, you can trick with generics, if you want to and if you explain your plan in more detail.
for your fist quetion
Can I implement the interface method in the top abstract class, so that the concrete sub-class does not have to?
you can and you should if its a common feature for all the subclasses.
below is the example for the same
interface X{
void show();
}
abstract class A implements X{
public void show(){
System.out.println("show");
}
}
abstract class B extends A{
}
class C extends B{
}
for your second question
No , signature must be same always.
so below program illustares it
interface X{
void print(Object obj);
}
abstract class A implements X{
#Override
public void print(Object obj) {
System.out.println("print");
}
}
below program wont compile if you change type in subclass
interface X{
void print(Object obj);
}
abstract class A implements X{
#Override
public void print(A obj) {
System.out.println("print");
}
}
Below is the code snippet:
public abstract class MyAbstractClass {
public abstract void a();
public abstract void b();
}
public class Foo extends MyAbstractClass {
public void a() {
System.out.println("hello");
}
public void b(){
System.out.println("bye");
}
}
public class Bar extends MyAbstractClass {
public void a() {
System.out.println("hello");
}
public void delta() {
System.out.println("gamma");
}
}
There are couple of questions that I have:
Q-1 :- Should I implement ALL the methods in abstract class?
Q-2 :- Can the implementing class have its own methods?
When you extend an Interface or an Abstract class you are creating a contract of sorts with that superclass. In the contract you are saying:
"I will implement all unimplemented methods in my superclass"
If you do not, implement all the unimplemented methods, then you are breaking your contract. A way to not break your contract is make your subclass Abstract as well as a way of saying
"I have not implemented all the classes in my contract, I am going to
have my subclasses implement them".
For your class bar right now, you must implement b() or make bar an Abstract class or you are not fulfilling your contract with MyAbstractClass
The basic idea is:
Interface: None of my methods are implemented. A subclass must implement all my methods in order to implement me. (Note: I believe default interfaces have been added to Java 8 which may change this a bit)
Example:
public interface myInterface
{
//My subclasses must implement this to fulfill their contract with me
public void methodA();
//My subclasses must implement this to fulfill their contract with me
public void methodB();
}
Abstract: I may implement some of my methods, but I will also leave methods as abstract so that my subclasses must implement because they can implement those classes to suit their needs better than I can.
Example:
public abstract class myAbstractClass
{
//My subclasses must implement this to fulfill their contract with me
public abstract void methodC();
public void helloWorld()
{
System.out.println("Hello World");
}
}
Abstract classes can also extend interfaces so they can implement some of their methods. But they can also leave some of the methods unimplemented so the subclass can implement them. If you leave an interface method unimplemented, there is not need to declare it abstract, it is already in the contract.
Example:
public abstract class myAbstractClass2 implement myInterface
{
#Override
public void methodA()
{
// this fulfills part of the contract with myInterface.
// my subclasses will not need to implement this unless they want to override
// my implementation.
}
//My subclasses must implement this to fulfill their contract with me
public abstract void methodD();
}
So in essence, an abstract class doesn't have as strict a contract with it's superclass because it can delegate its methods to its subclasses.
Regular Class: (I use regular to mean non-interface, and non-abstract). I must implement all unimplemented methods from all of my superclasses. These classes have a binding contract.
Example:
public class mySubClass extends myAbstractClass2
{
#Override
public void methodB()
{
//must be implemented to fulfill contract with myInterface
}
#Override
public void methodD()
{
//must be implemented to fulfill contract with myAbstractClass2
}
public void myMethod()
{
//This is a method specifically for mySubClass.
}
}
Q-1:- Should I implement all methods in abstract class?
Yes, you must implement all abstract methods.
Q-2 :- Can the implementing class have its own methods?
Yes, you can declare own (more specfic) methods.
You not only should, but have to implement all abstract methods (if the subclass is non-abstract). Otherwise an object of that subclass wouldn't know what to do if that method was called!
The only way to prevent this is if the subclass is also declared abstract, so that it cannot be instantiated in the first place.
You don't have to implement all methods of an abstract class. But you must implement all abstract methods of it.
In fact extending an abstract class has no difference then extending a normal class. It's not like implementing interfaces. Since you're extending you are creating a subclass thus you can add as many methods and attributes as you need.
Ya definately implementing class can define its own method as well and if are not implementing all the methods of your abstract class in the derived class then mark this derived class also as Abstract
but at the end of chain you have to make one concrete class which implements all the method that was not implement in abstract sub-parent
public interface I{
public void m();
}
public abstract class A1 implements I{
//No need to implement m() here - since this is abstract
}
public class B1 extends A1{
public void m(){
//This is required, since A1 did not implement m().
}
}
public abstract class A11 extends A1{
//Again No need to implement m() here - since this is abstract
public abstract void newA11Method()
}
public class B11 extends A11{
//This class needs to implement m() and newA11Method()
}
Yes, the implementing class need only implement the methods labeled as abstract in the abstract class.
Yes you must implement all the methods present in an abstract class. As the purpose of abstract class is purely to create a template for the functions whose implementation is decided by the class implementing them. So if you don't implement them, then you are breaking the concept of abstract class.
To answer your second question, yes you can create any number of your own methods irrespective of the abstract class you are extending.
Yes, When you are implementing an Interface you will have to implement all its methods. That is the purpose of defining an Interface. And yes, you can have your own methods in the class where you implement an Interface.
Yes, when you extends abstract you should give implementation for all abstract methods which are presents in abstract class. Othrewise you should make it implementation class as abtract class.
You must implement all the abstract methods you have in the abstract class. But you can directly use the concrete methods that are implemented.
For more information please refer to the :
https://www.javatpoint.com/abstract-class-in-java
Let's look at the following simple code snippet in Java.
interface Sum
{
abstract public void showSum();
}
interface Mul
{
abstract public void showMul();
}
abstract class Super implements Sum
{
protected int x;
protected int y;
public Super(int x, int y)
{
this.x=x;
this.y=y;
}
//No error, though the method showSum() of the implementing iterface Sum is commented. Why?
/*public void showSum()
{
System.out.println("Sum = "+(x+y));
}*/
}
final class Calculation extends Super implements Mul
{
public Calculation(int x, int y)
{
super(x,y);
}
public void showSum()
{
System.out.println("Summation = "+(x+y));
}
//If showMul() is commented , it would issue a compile-time error. Why?
public void showMul()
{
System.out.println("Multiplication = "+(x*y));
}
}
final public class Main
{
public static void main(String[] args) throws IOException
{
Scanner s=new Scanner(System.in);
System.out.print("\nEnter a number : ");
int x=s.nextInt();
System.out.print("\nEnter another number : ");
int y=s.nextInt();
Calculation c=new Calculation(x,y);
c.showSum();
c.showMul();
}
}
Since the interface Sum containing only one method showSum(); is being implemented by the abstract class Super , there should be necessary for the abstract class Super to implement that method showSum();. The compiler however doesn't complain at all and the program is working well with no problem at all. Why?
Similarly, the non-abstract final class Calculation is implementing the Mul interface and contains the actual implementation of the method showMul(); presented in it's implementing interface. In case, if this method showMul() in the class Calculation is commented, it issues a compile-time error. Why is the same thing not applicable to that abstract class Super?
The abstract class is not real implementation class. It may contain abstract methods and doesnot need to implement the methods from the interface. It is concern of the REAL implementing class to define the abstract/interface methods.
See this difference between abstract class and interface
Interface:
public interface InterfaceClass {
void interfaceMethod();
//no method definition
}
//Abstract Class implementing InterfaceClass
abstract class AbsClass implements InterfaceClass{
abstract void abstractMethod();
public void doSomethingCommon() {
System.out.println("Abstract class may contain method definition");
}
//no need to implement methods of InterfaceClass because AbsClass is abstract
}
And here is real class that extends AbsClass: Its duty of RealClass to define all abstract methods and interface methods. Additionally, it may override the defined methods in abstract class as well.
public class RealClass extends AbsClass{
#Override
public void interfaceMethod() {
//implement interface method here
}
#Override
void abstractMethod() {
}
// you may override the doSomethingCommon() of AbsClass too
#Override
public void doSomethingCommon() {
// TODO Auto-generated method stub
super.doSomethingCommon();
}
}
Why there is no compile time error on AbsClass:
We cannot create instances of abstract class. That's why there is no meaning of displaying error at compile time.
Abstract classes behavior resembles interfaces in this case.
From the Java Tutorial: ...abstract classes are similar to interfaces, except that they provide a partial implementation, leaving it to subclasses to complete the implementation. If an abstract class contains only abstract method declarations, it should be declared as an interface instead
You don't implement methods in an interface that extends another interface. And you don't HAVE to implement methods in an abstract class that implements an interface.
Because it's abstract. An abstract class is a class which is allowed to declare some methods without providing any implementation of those methods, forcing concrete subclasses doing it. You could add the method
#Override
public abstract void showSum();
to the abstract class, but it would just be redundant with the method already declared in the interface.
This ought to be comment to JB Nizet's answer, but I cant post comments yet :(
Note that you could provide the implementation for the showSum() method in your abstract class, and still have that class remain abstract without any problems. An abstract class could have all methods implemented and still be abstract!
From Oracle's Java tutorial:
a class that implements an interface must implement all of the
interface's methods. It is possible, however, to define a class that
does not implement all of the interface's methods, provided that the
class is declared to be abstract.
Reference document: https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html , section When an Abstract Class Implements an Interface.
What is an "abstract class" in Java?
An abstract class is a class which cannot be instantiated. An abstract class is used by creating an inheriting subclass that can be instantiated. An abstract class does a few things for the inheriting subclass:
Define methods which can be used by the inheriting subclass.
Define abstract methods which the inheriting subclass must implement.
Provide a common interface which allows the subclass to be interchanged with all other subclasses.
Here's an example:
abstract public class AbstractClass
{
abstract public void abstractMethod();
public void implementedMethod() { System.out.print("implementedMethod()"); }
final public void finalMethod() { System.out.print("finalMethod()"); }
}
Notice that "abstractMethod()" doesn't have any method body. Because of this, you can't do the following:
public class ImplementingClass extends AbstractClass
{
// ERROR!
}
There's no method that implements abstractMethod()! So there's no way for the JVM to know what it's supposed to do when it gets something like new ImplementingClass().abstractMethod().
Here's a correct ImplementingClass.
public class ImplementingClass extends AbstractClass
{
public void abstractMethod() { System.out.print("abstractMethod()"); }
}
Notice that you don't have to define implementedMethod() or finalMethod(). They were already defined by AbstractClass.
Here's another correct ImplementingClass.
public class ImplementingClass extends AbstractClass
{
public void abstractMethod() { System.out.print("abstractMethod()"); }
public void implementedMethod() { System.out.print("Overridden!"); }
}
In this case, you have overridden implementedMethod().
However, because of the final keyword, the following is not possible.
public class ImplementingClass extends AbstractClass
{
public void abstractMethod() { System.out.print("abstractMethod()"); }
public void implementedMethod() { System.out.print("Overridden!"); }
public void finalMethod() { System.out.print("ERROR!"); }
}
You can't do this because the implementation of finalMethod() in AbstractClass is marked as the final implementation of finalMethod(): no other implementations will be allowed, ever.
Now you can also implement an abstract class twice:
public class ImplementingClass extends AbstractClass
{
public void abstractMethod() { System.out.print("abstractMethod()"); }
public void implementedMethod() { System.out.print("Overridden!"); }
}
// In a separate file.
public class SecondImplementingClass extends AbstractClass
{
public void abstractMethod() { System.out.print("second abstractMethod()"); }
}
Now somewhere you could write another method.
public tryItOut()
{
ImplementingClass a = new ImplementingClass();
AbstractClass b = new ImplementingClass();
a.abstractMethod(); // prints "abstractMethod()"
a.implementedMethod(); // prints "Overridden!" <-- same
a.finalMethod(); // prints "finalMethod()"
b.abstractMethod(); // prints "abstractMethod()"
b.implementedMethod(); // prints "Overridden!" <-- same
b.finalMethod(); // prints "finalMethod()"
SecondImplementingClass c = new SecondImplementingClass();
AbstractClass d = new SecondImplementingClass();
c.abstractMethod(); // prints "second abstractMethod()"
c.implementedMethod(); // prints "implementedMethod()"
c.finalMethod(); // prints "finalMethod()"
d.abstractMethod(); // prints "second abstractMethod()"
d.implementedMethod(); // prints "implementedMethod()"
d.finalMethod(); // prints "finalMethod()"
}
Notice that even though we declared b an AbstractClass type, it displays "Overriden!". This is because the object we instantiated was actually an ImplementingClass, whose implementedMethod() is of course overridden. (You may have seen this referred to as polymorphism.)
If we wish to access a member specific to a particular subclass, we must cast down to that subclass first:
// Say ImplementingClass also contains uniqueMethod()
// To access it, we use a cast to tell the runtime which type the object is
AbstractClass b = new ImplementingClass();
((ImplementingClass)b).uniqueMethod();
Lastly, you cannot do the following:
public class ImplementingClass extends AbstractClass, SomeOtherAbstractClass
{
... // implementation
}
Only one class can be extended at a time. If you need to extend multiple classes, they have to be interfaces. You can do this:
public class ImplementingClass extends AbstractClass implements InterfaceA, InterfaceB
{
... // implementation
}
Here's an example interface:
interface InterfaceA
{
void interfaceMethod();
}
This is basically the same as:
abstract public class InterfaceA
{
abstract public void interfaceMethod();
}
The only difference is that the second way doesn't let the compiler know that it's actually an interface. This can be useful if you want people to only implement your interface and no others. However, as a general beginner rule of thumb, if your abstract class only has abstract methods, you should probably make it an interface.
The following is illegal:
interface InterfaceB
{
void interfaceMethod() { System.out.print("ERROR!"); }
}
You cannot implement methods in an interface. This means that if you implement two different interfaces, the different methods in those interfaces can't collide. Since all the methods in an interface are abstract, you have to implement the method, and since your method is the only implementation in the inheritance tree, the compiler knows that it has to use your method.
A Java class becomes abstract under the following conditions:
1. At least one of the methods is marked as abstract:
public abstract void myMethod()
In that case the compiler forces you to mark the whole class as abstract.
2. The class is marked as abstract:
abstract class MyClass
As already said: If you have an abstract method the compiler forces you to mark the whole class as abstract. But even if you don't have any abstract method you can still mark the class as abstract.
Common use:
A common use of abstract classes is to provide an outline of a class similar like an interface does. But unlike an interface it can already provide functionality, i.e. some parts of the class are implemented and some parts are just outlined with a method declaration. ("abstract")
An abstract class cannot be instantiated, but you can create a concrete class based on an abstract class, which then can be instantiated. To do so you have to inherit from the abstract class and override the abstract methods, i.e. implement them.
A class that is declared using the abstract keyword is known as abstract class.
Abstraction is a process of hiding the data implementation details, and showing only functionality to the user. Abstraction lets you focus on what the object does instead of how it does it.
Main things of abstract class
An abstract class may or may not contain abstract methods.There can be non abstract methods.
An abstract method is a method that is declared without an
implementation (without braces, and followed by a semicolon), like this:
ex : abstract void moveTo(double deltaX, double deltaY);
If a class has at least one abstract method then that class must be abstract
Abstract classes may not be instantiated (You are not allowed to create object of Abstract class)
To use an abstract class, you have to inherit it from another class. Provide implementations to all the abstract methods in it.
If you inherit an abstract class, you have to provide implementations to all the abstract methods in it.
Declare abstract class
Specifying abstract keyword before the class during declaration makes it abstract. Have a look at the code below:
abstract class AbstractDemo{ }
Declare abstract method
Specifying abstract keyword before the method during declaration makes it abstract. Have a look at the code below,
abstract void moveTo();//no body
Why we need to abstract classes
In an object-oriented drawing application, you can draw circles, rectangles, lines, Bezier curves, and many other graphic objects. These objects all have certain states (for ex -: position, orientation, line color, fill color) and behaviors (for ex -: moveTo, rotate, resize, draw) in common. Some of these states and behaviors are the same for all graphic objects (for ex : fill color, position, and moveTo). Others require different implementation(for ex: resize or draw). All graphic objects must be able to draw or resize themselves, they just differ in how they do it.
This is a perfect situation for an abstract superclass. You can take advantage of the similarities, and declare all the graphic objects to inherit from the same abstract parent object (for ex : GraphicObject) as shown in the following figure.
First, you declare an abstract class, GraphicObject, to provide member variables and methods that are wholly shared by all subclasses, such as the current position and the moveTo method. GraphicObject also declared abstract methods, such as draw or resize, that need to be a implemented by all subclasses but must be implemented in different ways. The GraphicObject class can look something like this:
abstract class GraphicObject {
void moveTo(int x, int y) {
// Inside this method we have to change the position of the graphic
// object according to x,y
// This is the same in every GraphicObject. Then we can implement here.
}
abstract void draw(); // But every GraphicObject drawing case is
// unique, not common. Then we have to create that
// case inside each class. Then create these
// methods as abstract
abstract void resize();
}
Usage of abstract method in sub classes
Each non abstract subclasses of GraphicObject, such as Circle and Rectangle, must provide implementations for the draw and resize methods.
class Circle extends GraphicObject {
void draw() {
//Add to some implementation here
}
void resize() {
//Add to some implementation here
}
}
class Rectangle extends GraphicObject {
void draw() {
//Add to some implementation here
}
void resize() {
//Add to some implementation here
}
}
Inside the main method you can call all methods like this:
public static void main(String args[]){
GraphicObject c = new Circle();
c.draw();
c.resize();
c.moveTo(4,5);
}
Ways to achieve abstraction in Java
There are two ways to achieve abstraction in java
Abstract class (0 to 100%)
Interface (100%)
Abstract class with constructors, data members, methods, etc
abstract class GraphicObject {
GraphicObject (){
System.out.println("GraphicObject is created");
}
void moveTo(int y, int x) {
System.out.println("Change position according to "+ x+ " and " + y);
}
abstract void draw();
}
class Circle extends GraphicObject {
void draw() {
System.out.println("Draw the Circle");
}
}
class TestAbstract {
public static void main(String args[]){
GraphicObject grObj = new Circle ();
grObj.draw();
grObj.moveTo(4,6);
}
}
Output:
GraphicObject is created
Draw the Circle
Change position according to 6 and 4
Remember two rules:
If the class has few abstract methods and few concrete methods,
declare it as an abstract class.
If the class has only abstract methods, declare it as an interface.
References:
TutorialsPoint - Java Abstraction
BeginnersBook - Java Abstract Class Method
Java Docs - Abstract Methods and Classes
JavaPoint - Abstract Class in Java
It's a class that cannot be instantiated, and forces implementing classes to, possibly, implement abstract methods that it outlines.
Simply speaking, you can think of an abstract class as like an Interface with a bit more capabilities.
You cannot instantiate an Interface, which also holds for an abstract class.
On your interface you can just define the method headers and ALL of the implementers are forced to implement all of them. On an abstract class you can also define your method headers but here - to the difference of the interface - you can also define the body (usually a default implementation) of the method. Moreover when other classes extend (note, not implement and therefore you can also have just one abstract class per child class) your abstract class, they are not forced to implement all of your methods of your abstract class, unless you specified an abstract method (in such case it works like for interfaces, you cannot define the method body).
public abstract class MyAbstractClass{
public abstract void DoSomething();
}
Otherwise for normal methods of an abstract class, the "inheriters" can either just use the default behavior or override it, as usual.
Example:
public abstract class MyAbstractClass{
public int CalculateCost(int amount){
//do some default calculations
//this can be overriden by subclasses if needed
}
//this MUST be implemented by subclasses
public abstract void DoSomething();
}
From oracle documentation
Abstract Methods and Classes:
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, then the class itself must be declared abstract, as in:
public abstract class GraphicObject {
// declare fields
// declare nonabstract 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, then the subclass must also be declared abstract.
Since abstract classes and interfaces are related, have a look at below SE questions:
What is the difference between an interface and abstract class?
How should I have explained the difference between an Interface and an Abstract class?
Get your answers here:
Abstract class vs Interface in Java
Can an abstract class have a final method?
BTW - those are question you asked recently. Think about a new question to build up reputation...
Edit:
Just realized, that the posters of this and the referenced questions have the same or at least similiar name but the user-id is always different. So either, there's a technical problem, that keyur has problems logging in again and finding the answers to his questions or this is a sort of game to entertain the SO community ;)
Little addition to all these posts.
Sometimes you may want to declare a
class and yet not know how to define
all of the methods that belong to that
class. For example, you may want to
declare a class called Writer and
include in it a member method called
write(). However, you don't know how to code write() because it is
different for each type of Writer
devices. Of course, you plan to handle
this by deriving subclass of Writer,
such as Printer, Disk, Network and
Console.
An abstract class can not be directly instantiated, but must be derived from to be usable. A class MUST be abstract if it contains abstract methods: either directly
abstract class Foo {
abstract void someMethod();
}
or indirectly
interface IFoo {
void someMethod();
}
abstract class Foo2 implements IFoo {
}
However, a class can be abstract without containing abstract methods. Its a way to prevent direct instantation, e.g.
abstract class Foo3 {
}
class Bar extends Foo3 {
}
Foo3 myVar = new Foo3(); // illegal! class is abstract
Foo3 myVar = new Bar(); // allowed!
The latter style of abstract classes may be used to create "interface-like" classes. Unlike interfaces an abstract class is allowed to contain non-abstract methods and instance variables. You can use this to provide some base functionality to extending classes.
Another frequent pattern is to implement the main functionality in the abstract class and define part of the algorithm in an abstract method to be implemented by an extending class. Stupid example:
abstract class Processor {
protected abstract int[] filterInput(int[] unfiltered);
public int process(int[] values) {
int[] filtered = filterInput(values);
// do something with filtered input
}
}
class EvenValues extends Processor {
protected int[] filterInput(int[] unfiltered) {
// remove odd numbers
}
}
class OddValues extends Processor {
protected int[] filterInput(int[] unfiltered) {
// remove even numbers
}
}
Solution - base class (abstract)
public abstract class Place {
String Name;
String Postcode;
String County;
String Area;
Place () {
}
public static Place make(String Incoming) {
if (Incoming.length() < 61) return (null);
String Name = (Incoming.substring(4,26)).trim();
String County = (Incoming.substring(27,48)).trim();
String Postcode = (Incoming.substring(48,61)).trim();
String Area = (Incoming.substring(61)).trim();
Place created;
if (Name.equalsIgnoreCase(Area)) {
created = new Area(Area,County,Postcode);
} else {
created = new District(Name,County,Postcode,Area);
}
return (created);
}
public String getName() {
return (Name);
}
public String getPostcode() {
return (Postcode);
}
public String getCounty() {
return (County);
}
public abstract String getArea();
}
What is Abstract class?
Ok! lets take an example you known little bit about chemistry we have an element carbon(symbol C).Carbon has some basic atomic structure which you can't change but using carbon you can make so many compounds like (CO2),Methane(CH4),Butane(C4H10).
So Here carbon is abstract class and you do not want to change its basic structure however you want their childrens(CO2,CH4 etc) to use it.But in their own way
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.
In other words, a class that is declared with abstract keyword, is known as abstract class in java. It can have abstract(method without body) and non-abstract methods (method with body).
Important Note:-
Abstract classes cannot be used to instantiate objects, they can be used to create object references, because Java's approach to run-time Polymorphism is implemented through the use of superclass references. Thus, it must be possible to create a reference to an abstract class so that it can be used to point to a subclass object. You will see this feature in the below example
abstract class Bike{
abstract void run();
}
class Honda4 extends Bike{
void run(){
System.out.println("running safely..");
}
public static void main(String args[]){
Bike obj = new Honda4();
obj.run();
}
}
An abstract class is one that isn't fully implemented but provides something of a blueprint for subclasses. It may be partially implemented in that it contains fully-defined concrete methods, but it can also hold abstract methods. These are methods with a signature but no method body. Any subclass must define a body for each abstract method, otherwise it too must be declared abstract.
Because abstract classes cannot be instantiated, they must be extended by at least one subclass in order to be utilized. Think of the abstract class as the generic class, and the subclasses are there to fill in the missing information.
Class which can have both concrete and non-concrete methods i.e. with and without body.
Methods without implementation must contain 'abstract' keyword.
Abstract class can't be instantiated.
It do nothing, just provide a common template that will be shared for it's subclass