EDIT: This question really should be around Lombok and Guice instead of vanilla java inheritance.
I'm trying to implement a Java inheritance chain with Lombok and Guice injections, it works something like this:
Class 1
public abstract class Animal {
#NonNull protected String attr1;
protected abstract void method1();
void method0() {
// Some code that uses attr1
}
}
Class 2
public abstract class Mammal extends Animal {
#NonNull protected String attr2;
protected abstract void method2();
#Override
void method1() {
// some logic that uses attr2
method2();
}
}
Class 3
public class Wolf extends Mammal {
#Inject #NonNull private String attr1;
#Inject #NonNull private String attr2;
#Inject #NonNull private String attr3;
#Override
void method2() {
// some logic
}
}
Out there in the main program I have code that calls wolf.method1(). The problem here is that only wolf has all the attributes needed (due to Guice injections), whereas all the fields in Animal are undefined. I suspect I can probably do it in Vanilla Java, but things are going to get super messy (I have 6 attributes in Animal class and 5 more in Mammal). Is there a way to mix-and-match lombok's annotations (#NoArgsConstructor, #AllArgsConstructor, etc) to make this work?
Thanks.
Your abstract classes cannot be directly instantiated even if you add public constructors, because they are declared abstract. If you prefer, you can make the constructors protected to indicate they are only available to subclasses.
right that abstract classes can't really be instantiated, but why didn't the Java compiler catch this and stop complaining about not having a constructor?
Any class you write without an explicit constructor has an implicit no-args constructor. Any implicit no-args constructor implicitly calls its superclass's no-args constructor, even if that superclass is abstract. So if some class up the chain doesn't have a no-args constructor (because you explicitly gave it another constructor), then you code won't compile.
In the code you gave in your question, there are no explicit constructors, so every class does have an implicit no-args constructor. In your actual code, presumably you have written a constructor somewhere, which is why the implicit no-args constructor has not been added.
I propose:
use only interfaces for the entire hierarchy.
decouple behaviors and encapsulate them in separate classes.
use composition instead of inheritance, i.e. private SomeBehavior someBehavior; for every specific animal that needs it.
It will make your design better and solve the problem as well.
i.e.
public interface Mammals {
}
public interface Animal extends Mammals {
}
public interface Dog extends Animal {
}
and
public class TakeADump {
public void dump() {
}
}
public class TakeAPee {
public void pee() {
}
}
and then
public class Sheperd implements Dog {
private TakeADump dumpService;
private TakeApee peeService;
}
And now your dog can s... and p... :)
Also add
public class F... {
public void f...(<Animal> animal) {
// ... check it's an instance of the same or compatible animal or throw UnsupportedOperationException() if it's incompatible
}
}
:D
Of course it will make sense to create an abstract Animal.
i.e.
public class AbstractAnimal {
private TakeADump dumpService;
private TakeApee peeService;
private F... f...Service;
}
then
public abstract class AbstractDog extends AbstractAnimal implements Dog {
}
and
public class Sheperd extends AbstractDog {
public void lookAfterSheep() {
Sheep sheep = SheepLocator.findNearest();
// pee on a sheep
peeService.pee(sheep);
// dump on a sheep
dumpService.dump(sheep);
// f... a sheep
f...Service.mount(sheep);
}
}
So your mistake is using too much abstract when you can use interfaces.
When you are implementing concept of inheritance with some argument constructor it is good practice to have one default (no argument) constructor defined there. Because while creating object of child class compiler internally will call constructor of parent class.
e.g
class ABC {
}
class XYZ implements ABC{
}
public class Test{
XYZ obj= new XYZ() // this will internally call default constructor of XYZ and
//in that first statement will super()--> this will call default constructor of class ABC
}
if anyhow you have implemented argument constructor in parent class then compiler will not implement default constructor implicitly we need to define it explicit to make call to super constructor from child constructor. Or do a explicit call to argument constructor from child constructor.
I'm trying to loop through multiple objects of different types that are all extending the same parent class. For example:
ArrayList<Enchant> enchants = new ArrayList<>();
Inside of the enchants array would be objects of classes that extend an Enchant class.
For example:
public class JumpBoost extends Enchant{
public static Item generateItem(){
return something;
}
}
Each object in the enchants ArrayList would have a method called generateItem().
I would like to loop through it using:
for(Enchant enchant : enchants){
enchant.generateItem()
}
I've already tried doing this, but with no luck due to the Enchant class not having the generateItem() method.
I essentially just need a way to group and loop through multiple objects of different types.
The idiomatic approach is to define an abstract generateItem() on Enchant. If you can't modify Enchant for some reason, you could create an abstract subclass of Enchant, and make all your subclasses inherit from that.
You could also try making an interface with generateItem() that all the subclasses implement and store the Interface type in the ArrayList.
If you still can't do any of those for whatever reason, you should rethink your design. But as a last resort, you can use reflection to dynamically access the generateItem of each individual subclass. Just call .getClass() and then lookup the method and invoke it.
You can use polymorphism to solve that problem.
First define the base class/abstract class/interface with the method generateItem(), derive specified classes and override/implement generateItem() method.
Here is implementation with interface:
interface Base{
void generateItem();
}
class Derived1 implements Base{
#Override
public void generateItem() {
System.out.println("generateItem() from Derived1");
}
}
class Derived2 implements Base{
#Override
public void generateItem() {
System.out.println("generateItem() from Derived2");
}
}
class Main {
public static void main(String[] args) {
List<Base> list = new ArrayList<>();
list.add(new Derived1());
list.add(new Derived2());
list.forEach(Base::generateItem);
}
}
The output is:
generateItem() from Derived1
generateItem() from Derived2
For more, you can read about polymorphism here.
Give Enchant an abstract method that subclasses need to implement. Making it abstract means Enchant doesn't need a definition of the method, but subclasses do.
public class Enchant {
public abstract Item generateItem();
}
The method should not be a static method as you have it in your code. It should be an instance method, since you're calling it on instances of Enchant.
Make enchant as interface or abstract class and extend it in the derived classes
Just extending the question..
Same method in abstract class and interface
Suppose a class implements an interface and extends an abstract class and both have the same method (name+signature), but different return types. Now when i override the method it compiles only when i make the return type same as that of the interface declaration.
Also, what would happen if the method is declared as private or final in the abstract class or the interface?
**On a side note. Mr. Einstein stuck to this question for an abominable amount of time during an interview. Is there a popular scenario where we do this or he was just showing off?
If the method in abstract class is abstract too, you will have to provide its implementation in the first concrete class it extends. Additionally, you will have to provide implementation of interface. If both the methods differ only in return type, the concrete class will have overloaded methods which differ only in return type. And we can't have overloaded methods which differ only in return type, hence the error.
interface io {
public void show();
}
abstract class Demo {
abstract int show();
}
class Test extends Demo implements io {
void show () { //Overloaded method based on return type, Error
}
int show() { //Error
return 1;
}
public static void main (String args[]) {
}
}
No, same method names and parameters, but different return types is not possible in Java. The underlying Java type system is not able* to determine differences between calls to the methods at runtime.
(*I am sure someone will prove me wrong, but most likely the solution is considered bad style anyways.)
Regarding private/final: Since you have to implement those methods, neither the interface method nor the abstract method can be final. Interface methods are public by default. The abstract method can't be private, since it must be visible in the implementing class, otherwise you can never fulfill the method implementation, because your implementing class can't "see" the method.
With Interfaces the methods are abstract and public by default ,
so they cant have any other access specifier and they cant be final
With abstract class , abstract methods can have any access specifier other than private and because they are abstract they cant be final
While overriding , the method signature has to be same ; and covariant(subclass of the declared return type) return types are allowed
A class cannot implement two interfaces that have methods with same name but different return type. It will give compile time error.
Methods inside interface are by default public abstract they don't have any other specifier.
interface A
{
public void a();
}
interface B
{
public int a();
}
class C implements A,B
{
public void a() // error
{
//implementation
}
public int a() // error
{
//implementation
}
public static void main(String args[])
{
}
}
I have a class component
abstract class Component{
private componentType m_type;
public Component(componentType type)
{
m_type = type;
}
}
and 2 subclasses
class AmplifierComponent extends Component{
public AmplifierComponent()
{
super(componentType.Amp);
System.out.print(this.m_type);
}
}
class AttenuatorComponent extends Component{
public AttenuatorComponent()
{
super(componentType.Att);
System.out.print(this.m_type);
}
}
my problems are:
1.i can't instantiate any kind of component because m_type isn't visible(what that means?)
2.i need to make an array of all the components the user has insert to the chain. i can't manage to create an array of Component class.
can someone help me with the design?
or with some workarounds?
Thanks in advance
I don't understand why you need the type member. This looks redundant. You can instead simply do:
abstract class Component{
}
class AttenuatorComponent extends Component{
public AttenuatorComponent() {
// calls the default super constructor
}
}
and rely on polymorphism for your classes to behave appropriately. Having a type member to identify a hierarchy type is unnecessary when you've declared the corresponding classes. If you do have a member variable that is required to be visible in subclasses but not by clients, then you can make it protected rather than private.
Component could be an interface if there's no functionality/data associated with it.
Your array declaration would look like
Component[] components = new Component[20];
components[0] = new AttenuatorComponent();
Polymorphism would mean that you can iterate through this array of components, calling appropriate methods declared on (but not necessarily implemented by) the Component, and the suitable subclass methods would be called.
Set m_type to protected to be able to see it from child classes.
abstract class Component {
private componentType m_type;
public Component(componentType type)
{
m_type = type;
}
public componentType getType()
{
return this.m_type;
}
}
class AmplifierComponent extends Component{
public AmplifierComponent()
{
super(componentType.Amp);
System.out.print(super.getType());
}
}
class AttenuatorComponent extends Component{
public AttenuatorComponent()
{
super(componentType.Att);
System.out.print(super.getType());
}
}
That way you can read m_type, but cannot change it.
You could also make the getType() command protected, so it is only reachable through the classes which inherit it.
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