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".
I am just confused about abstract class concept. Please clear my doubt. Definition of Abstract class says we can not create object of such class, then what we called like A a = new A() { }. Example is below:
public abstract class AbstractTest {
public abstract void onClick();
public void testClick() {
}
}
public class A {
AbstractTest test = new AbstractTest() {
#Override
public void onClick() {
}
};
}
Then test is a object or what?
test is an object of an anonymous concrete sub-class of AbstractTest (note that it implements all the abstract methods of AbstractTest), which is why this sub-class can be instantiated.
On the other hand,
AbstractTest test = new AbstractTest();
wouldn't pass compilation, since that would be an attempt to instantiate an abstract class.
You are mixing up object and reference.
AbstractTest test = new AbstractTest() {
#Override
public void onClick() {
}
};
test here is a reference to a anonymous class that extends AbstractTest, the above code is like saying:
class MyClass extends AbstractTest {
#Override
public void onClick() {
}
}
AbstractTest test = new MyClass(); // test is a reference to a MyClass object
Abstract Class in my opinion needs to be explained together with Interface.
Interface allows you to specify operations that are supported/allowed on objects with that interface. More specifically Objects are instances of a Class which implements that Interface. Defining an interface allows you to describe a group of different classes of objects so that other objects can interact with them in the same manner.
Abstract Class is one step between interface and a Class (loosely speaking). Abstract Class allows you to specify operations that are supported by classes that extend it, but it also allows you to implement (some of) those operations. This way you can implement common methods for a group of classes in that abstract class. Other methods in the abstract class that are not implemented (aka abstract methods) need to be implemented by the class that extends it. The fact that you didn't implement all methods on an Abstract class naturally means you can't instantiate it (create an object of such class). There are other useful implementations for Abstract classes (i.e. callbacks).
In your example what you see there is that you are not really trying to just create an object that Abstract class you are also providing implementation of abstract method onClick();
That is the only way you can "create an instance of the abstract class" - technically speaking you are creating an instance of an Anonymous class (that is extending your abstract class) for which you provide implementation of inherited abstract methods.
Is there any way to NOT implement all of the methods of an interface in an inheriting class?
The only way around this is to declare your class as abstract and leave it to a subclass to implement the missing methods. But ultimately, someone in the chain has to implement it to meet the interface contract. If you truly do not need a particular method, you can implement it and then either return or throw some variety of NotImplementedException, whichever is more appropriate in your case.
The Interface could also specify some methods as 'default' and provide the corresponding method implementation within the Interface definition (https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html). These 'default' methods need not be mentioned while implementing the Interface.
The point of an interface is to guarantee that an object will outwardly behave as the interface specifies that it will
If you don't implement all methods of your interface, than you destroy the entire purpose of an interface.
We can override all the interface methods in abstract parent class and in child class override those methods only which is required by that particular child class.
Interface
public interface MyInterface{
void method1();
void method2();
void method3();
}
Abstract Parent class
public abstract class Parent implements MyInterface{
#Override
public void method1(){
}
#Override
public void method2(){
}
#Override
public void method3(){
}
}
In your Child classes
public class Child1 extends Parent{
#Override
public void method1(){
}
}
public class Child2 extends Parent{
#Override
public void method2(){
}
}
I asked myself the same question, and then learned about Adapters. It solved my problem, maybe it can solve yours. This explains it very well : https://blogs.oracle.com/CoreJavaTechTips/entry/listeners_vs_adapters
You can do that in Java8. Java 8 introduces “Default Method” or (Defender methods) new feature, which allows a developer to add new methods to the Interfaces without breaking the existing implementation of these interfaces.
It provides flexibility to allow Interface define implementation which will use as default in the situation where a concrete Class fails to provide an implementation for that method.
interface OldInterface {
public void existingMethod();
default public void DefaultMethod() {
System.out.println("New default method" + " is added in interface");
}
}
//following class compiles successfully in JDK 8
public class ClassImpl implements OldInterface {
#Override
public void existingMethod() {
System.out.println("normal method");
}
public static void main(String[] args) {
ClassImpl obj = new ClassImpl ();
// print “New default method add in interface”
obj.DefaultMethod();
}
}
Define that class as an abstract class. However, you must implement those unimplemented methods when you want to create an instance of it (either by using a subclass or an anonymous class).
It is possible and it is easy. I coded an example.
All you have to do is inherit from a class that does implement the method. If you don't mind a class that is not instantiable, then you can also define an abstract class.
If you want an instantiable class, it is not possible. You may try to define an abstract class, though.
If you try to implement an interface and you find yourself in a situation where there is no need to implement all of them then, this is a code smell. It indicates a bad design and it violates Liskov substitution principle. Often this happens because of using fat interface.
Also sometimes this happens because you are trying to implement an interface from an external dependency. In this case, I always look inside the source code to see if there is any implementation of that interface which I can either use it directly or subclass it and override methods to my needs.
We can use Adapter classes ,which reduces complexcity by not making mandatory to implement all the methods present in the interface
Adapter class is a simple java class that implements an interface with only EMPTY implementation .
Instead of implementing interface if we extends Adapter class ,we provide implementation only for require method
ex--- instead of implementing Servlet(I) if we extends GenericServlet(AC) then we provide implementation for Service()method we are not require to provide implementation for remaining meyhod..
Generic class Acts as ADAPTER class for Servlet(I).
yes possible below shown is the way
interface Test {
void m() throws NullPointerException;
}
class Parent {
// Parent class doesn't implements Test interface
public void m() {
System.out.println("Inside Parent m()");
}
}
class Child extends Parent implements Test {
}
public class Program {
public static void main(String args[]) {
Child s = new Child();
s.m();
}
}
can anybody tell me that. how can I call abstract class method to my own class in java?
thanks in advance
First of all look at you abstract class, it shall contain abstract methods and real methods. In the following sample the Foo class has an abstract method (FooMethod) and a real method (Yeee).
public abstract class Foo {
public abstract int FooMethod(int i);
public int Yeeee() {
for (int i = 0; i < 3; i++) {
int res = FooMethod(i);
// Do whatever
}
}
}
Abstract class are not meant to be directly used, so we have to inherit from them with a concrete class. The following inherits from the abstract (implementing the abstract method)
public class Bar extends Foo {
public int FooMethod(int i) {
// do something with i
}
public static void main (string [] args) {
Bar obj = new Bar();
obj.Yeeee();
}
}
Note: when in the main you call obj.Yeee() the base class method gets invoked, but in place of the abstract FooMethod, your own new implementation is used.
This is just the tip of the iceberg with abstract classes, but roughly should point you to the right direction.
Please take a good read here is a good tutorial and should give you some initial wisdom about inheritance and abstract classes.
You need to first create a subclass of the abstract class. This will then contain the methods of that abstract class. You use the "extends" keyword.
For example:
public class MyClass extends AbstractClass
{
//class content here...
}
For methods in abstract classes you need not to create the instance of the abstract class
So after importing the package in which the abstract class is present you can just call the method as below
YourAbstractClassName.methodName(args if any);
since abstract classes cant be instanciated in Java, You cant have member functions in this class and if you want to have one than their is a logical problem. However if you want to call the static methods, you can simply call them using class name, i.e.
YourClassName.fuctionName(parameters if any);
Do you mean how to implement that method in your class ?
if that is what you want to understand
then you just have to extend your class with the abstract one
for example
abstract class GraphicObject {....}
class Circle extends GraphicObject { ... }
try http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html
You can call the method in abstract class by creating an
object of subclasss of the abstract class
or
if u want to call from the abstract class then you have to make your method static then you can call from main method like
Abstract_className.methodName()
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