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Abstract Classes and Methods:

Data abstraction is the process of hiding certain details and showing only essential information to the user.
Abstraction can be achieved with either abstract classes or interfaces .

The abstract keyword is a non-access modifier, used for classes and methods:

• Abstract class: is a restricted class that cannot be used to create objects (to access it, it must be inherited from another class).
• Abstract method: can only be used in an abstract class, and it does not have a body. The body is provided by the subclass (inherited from).

Points to Remember:

• An abstract class must be declared with an abstract keyword.
• It can have abstract and non-abstract methods.[TBD]
• It cannot be instantiated.
• It can have constructors and static methods also.
• It can have final methods which will force the subclass not to change the body of the method.

Abstract Class:

A class which contains the abstract keyword in its declaration is known as abstract class.

• Abstract classes may or may not contain abstract methods, i.e., methods without body ( public void get(); )
• But, if a class has at least one abstract method, then the class must be declared abstract.
• If a class is declared abstract, it cannot be instantiated.
• To use an abstract class, you have to inherit it from another class, provide implementations to the abstract methods in it.
• If you inherit an abstract class, you have to provide implementations to all the abstract methods in it.
• An abstract class having both abstract methods and non-abstract methods.
• Declaring a class as abstract with no abstract methods means that we don’t allow it to be instantiated on its own.[TBD]
• An abstract class can extend only one class or one abstract class at a time.[TBD]

Java Abstract Method:

A method that doesn’t have its body is known as an abstract method. We use the same abstract keyword to create abstract methods.

abstract void display();

Advantage:

Abstract class having constructor, data member and methods:

An abstract class can have a data member, abstract method, method body (non-abstract method), constructor, and even main() method.

//Example of an abstract class that has abstract and non-abstract methods  
 abstract class Bike{  
   Bike(){System.out.println("bike is created");}  
   abstract void run();  
   void changeGear(){System.out.println("gear changed");}  
 }  
//Creating a Child class which inherits Abstract class  
 class Honda extends Bike{  
 void run(){System.out.println("running safely..");}  
 }  
//Creating a Test class which calls abstract and non-abstract methods  
 class TestAbstraction2{  
 public static void main(String args[]){  
  Bike obj = new Honda();         [TBD] 
  obj.run();  
  obj.changeGear();  
 }  
}  

Reference:

https://www.javatpoint.com/abstract-class-in-java#:~:text=in%20Java%20first.-,Abstraction%20in%20Java,text%20and%20send%20the%20message.

https://www.tutorialspoint.com/can-we-define-an-abstract-class-with-no-abstract-methods-in-java#:~:text=Declaring%20a%20class%20as%20abstract,object%20of%20the%20class%20directly.

https://www.tutorialspoint.com/java/java_abstraction.htm

https://www.w3schools.com/java/java_abstract.asp

https://www.programiz.com/java-programming/abstract-classes-methods