Is abstraction possible without inheritance? This is my code
abstract class whatever
{
abstract void disp1();
abstract void disp2();
abstract void disp3();
}
class what {
void disp1()
{
System.out.println("This is disp1");
}
}
public class threeClasses {
public static void main (String args[])
{
what obj =new what();
obj.disp1();
}
}
Please note above, how i:
did not extend the class "what" from abstract class "whatever" and yet the code runs perfectly with no errors
Did not declare class "what" as abstract (since it's not declaring the other two methods disp2() and disp3())
I am very confused. Please help.
You aren't using whatever (and Java naming conventions should be respected). The idea behind an abstract class (and inheritance) is that there is an interface contract. Let's examine it with a more practical example,
abstract class Whatever {
abstract void disp1();
void disp2() {
System.out.println("disp2");
}
void disp3() {
System.out.println("disp3");
}
}
Then make What extend it. Override two methods for demonstration (the annotation is a useful compile time safety check)
class What extends Whatever {
#Override
void disp1() {
System.out.println("This is disp1");
}
#Override
void disp2() {
System.out.println("This is disp2");
}
}
Finally, invoke methods on a What instance through the Whatever contract
public static void main(String args[]) {
Whatever obj = new What();
obj.disp1();
obj.disp2();
obj.disp3();
}
Which outputs
This is disp1
This is disp2
disp3
Note that What is providing the implementation for disp1 and disp2 while Whatever provides disp3.
There is no relationship between your abstract class and your concrete class. Whatever your definition of "abstraction", it actually represents a relationship between types. The abstract keyword does not establish that relationship between classes, it represents that relationship, and not by itself. The relationship needs to be extended from both sides.
abstract is a declaration from one side about a promise that must be kept, for an inheriting type either to implement abstract methods or to ask for that promise from its inheriting types.
The other side makes the promise by being a class that inherits from the abstract type. Without inheritance, the concrete type loses the is-a connection.
You will get the compiler error you're complaining about missing if you correct one major mistake you made. You failed to use the #Override annotation. Always use the #Override annotation when you intend to override a method, or you will forever enjoy just the sort of bug you show here.
I think what he meant was if we can implement abstract class's method without inheriting abstract class.
You might be thinking if we can do it with composition/association/aggregation relation.
To that, I will answer: NO because you can't create an object of abstract class as in these relations you have to make object or reference of the object.
So, the only way to implement abstract methods is through inheritance.
I am programming a simple platformer game, and I have several types of platforms. I created a class for the most simple type and made the rest subclasses, with the only difference between each class being the value of their variables (so they all share the same variable and method names).
In my collision detection, I loop through a HashMap. Within that HashMap are ArrayLists of the instances of each class. But when I use a nested loop to loop through try to call their methods implicitly, I have found that I cannot access these methods without explicitly declaring which class I want to call the method from.
I have done research, although the only way I can see of doing this is to loop through the instances of each class separately, meaning one loop per class; I would rather not do this, since it would be a lot more code than I feel is necessary.
In order to be able to call a common method on classes of different types you need to give your objects a common supertype declaring the common method - i.e. they should have a common superclass, or implement a common interface.
Interfaces provide an easier way of declaring common functionality, because a class can implement multiple interfaces, but it can extend only one class.
Provide an interface with the common method, then declare the map to use objects of that interface, i.e.
interface CommonInterface {
void commonMethod(int arg);
}
class One implements CommonInterface {
public void commonMethod(int arg) {
...
}
}
class Two implements CommonInterface {
public void commonMethod(int arg) {
...
}
}
Here is what you can do now:
Map<String,CommonInterface> myMap = new HashMap<>();
myMap.put("one", new One());
myMap.put("two", new Two());
for (Map.Entry<String,CommonInterface> e : myMap.entrySet()) {
System.out.println(e.getKey());
CommonInterface c = e.getValue();
c.commonMethod(123);
}
Simple, make each platform class implement an IPlatform interface or extand a base class. Look up java polymorphism and interfaces.
Are your subclasses overriding the common methods from the super class?
In other words, are your subclass' common methods declared in your simpler class?
If it is the case, you can simply call the method as if it is a simple class:
public abstract class Fruit {
public abstract void method();
}
public class Apple extends Fruit {
#Override
public void method() {
System.out.println("I'm an apple");
}
}
public class Orange extends Fruit {
#Override
public void method()
System.out.println("I'm an orange");
}
}
Using this you can simply call your method from any fruit, since it has your method declared. No need to know which fruit it is. The following code:
Fruit fruit = new Orange();
fruit.method();
will output: "I'm an orange".
Any body can tell me
when to use the abstract class and when to use interface?
So many websites having only differences. I am not able to get these terms
"when to use the abstract class and when to use interface"
Thanks in Advance
Interfaces
An interface is kinda* like a template. Say for example that you want to make a 'Shape' class. Not all shapes use the same formulas for calculating area, so you just establish that there has to be a "getArea" method, but you don't define it.
A simple example:
public interface Shape
{
public int getArea();
}
Then you can have a class that implements the Shape interface:
public class Rectangle implements Shape
{
//this works for rectangles but not for circles or triangles
public int getArea()
{
return this.getLength() * this.getHeight();
}
}
Abstract Classes
Abstract methods can be extended by subclasses.* They differ from interfaces in that they can also contain defined methods.
You can still leave undefined methods, but you must label them abstract.
An example:
public abstract class Vegetable
{
public String vegName;
public boolean edible = true;
public Vegetable(final String vegName, final boolean edible)
{
this.vegName = vegName;
this.edible = edible;
}
public void printName()
{
System.out.println(this.vegName);
}
//to be determined later when implemented
public abstract void drawOnScreen();
}
Then we can extend this abstract class.
public class Carrot extends Vegetable
{
//we must define the abstract methods
public void drawOnScreen()
{
//we can still use our other methods
this.printName();
//do some other thing that is specific to this class
}
}
Interfaces cannot contain implementation (at least prior to Java 8) so if you need "common" method implementation, you have to have it in a super class (be it abstract or concrete)
However, any class can have only one super class (but many interfaces). so an interface is the solution for polymorphism when you have a class that already got a super class
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
What does it mean to “program to an interface”?
Interface vs Abstract Class (general OO)
I'm new to learn JAVA and now I'm confused about interface. I have searched and read many materials but still not clear.
When I try to find some information about interface, I see many people talked about the relationship between interface and abstract class. But I even don't know why they contrast these two. Because I think abstract class is used to tell other people you can not create an object of this abstract class and if you want, you must modify the abstract class. This is something about inheritance, right?
But I don't know the meaning of interface. There is a interface a, and if a class B is going to implement the interface a, it must use the reserved word class B implements a, and then complete all the methods that the interface requires. But my question is, if class B have to complete all the methods by itself, what's the meaning of interface? I think we don't need it.
I don't understand it very much. I read many sentences like: "interface can reflect the core thought of object-oriented language", "interface can help make the program easier" and so on. But I can not really understand the meaning.
So, does anyone can show me some examples to let understand interface? Or you can tell me some useful links or the books that describe the interface clearly. I really hope to figure it out. THANK YOU!
Suppose you have a Car class and Vegetable class which is unrelated in real life and there is a common behaviour called wash(). Because we can wash a car and wash a vegetable too. But washing a car and washing a vegetable is totally different process/behaviour.
For ex: Car should be washed with a power pump, Vegetables under your kitchen sink. So the way of washing is different. So you make the washing process as a method wash() in the interface say Washable and you implement them in both Car and Vegetable class.
interface Washable {
public void wash();
}
public class Car implements Washable {
public void wash() {
// wash the car with a power pump
}
}
public class Vegetable implements Washable {
public void wash() {
// wash the vegetable under a kitchen sink
}
}
As a person, you would want to wash a car as well as vegetable.
public class Person {
Washable washableObject = new Car();
washableObject.wash();
washableObject = new Vegetable();
washableObject.wash();
}
So interface is a way to connect unrelated classes which has a common behavior.But the behavior will be differently implemented or can be changed in future.
One day you decide to change the way you wash a Car.Suppose you have purchased a "car washing machine". So the implementation changes inside the method wash() in the Car class.
public class Car implements Washable {
public void wash() {
// wash the car with my new car washing machine !!
}
}
But as a Person , you still call the wash() method. The way the wash() method is being implemented changed ( washing the car with your new car washing machine ), this implementation change did not affect your Person class.
Hope you are clear why we use interfaces now.
Basically, interface is a way to accomplish multiple inheritance without actually having to do it. (Which is not to say that the Java folks "wimped out" -- multiple inheritance is exceedingly messy and inefficient to implement.)
What this means is that you can have two totally separate classes A and B, with no common ancestor other than Object, and, if they implement the same interface, you can substitute one for the other (so long as you reference only methods in the interface).
A simple code to understand the interface and class.
import java.util.List;
import java.util.ArrayList;
public class Q01 {
List<Shape> shapes= new ArrayList();
public void add() {
shapes.add(new Square(3.0));
shapes.add(new Circle(2.0));
}
public void print() {
for (int i = 0; i < shapes.size(); i++) {
Shape shape = shapes.get(i);
System.out.println(shape.getClass().getSimpleName() + " --->" + shape.getArea());
}
}
public static void main(String[] args) {
Q01 q01= new Q01();
q01.add();
q01.print();
}
public interface Shape {
double getArea();
}
public class Square implements Shape{
private double edge;
public Square(double edge) {
this.edge = edge;
}
public double getArea() {
return edge*edge;
}
}
public class Circle implements Shape{
private double radius;
public Circle(double radius) {
this.radius = radius;
}
public double getArea() {
return 3.14 * radius * radius;
}
}
}
Think in interfaces like contracts between a class and the user. When a class implements an interface is telling that it promise to offer all the methods defined by the interface.
Assuming you understand class inheritance, I think of an Interface like a skeleton class, where the structure of a class is described but not actually written/implemented.
Another class can then work with any class that implements a particular Interface even if it hasn't been implemented yet.
For example, someone may create an Interface called Animal. Its methods maybe: Talk(), Walk() and Eat(). You could write a Dog class that implements Animal that prints "woof" when the Talk() method is called. Therefore another class will know how to work with all classes that implements the Animal interface.
UPD
A good real world example is the JDBC Database Statement Interface. This sets out a number of required properties that a database manufacturer will have to implement, such as execute(String sql). Oracle will implement this differently from Postgresql. This allow the database to be swapped for another one but the user code remains the same.
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