What is an Abstract Class? !!!

What is an Abstract Class? How to use abstract classes in Java?
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Java provides you with a special type of class, called an abstract class, which can help you, organize your classes based on common methods. An abstract class lets you put the common method names in one abstract class without having to write the actual implementation code. Abstract keyword is used to define abstract class. Following are the key points of abstract class :

• Classes, which contain abstract methods, are called abstract classes.
• A program cannot instantiated an abstract class as an object.
• Abstract classes may contain a mixture of non-abstract and abstract methods. Non-abstract methods implement the method statements.
• Any subclass, which extends the class containing abstract methods, must provide the implementation for all the abstract methods; otherwise the subclass itself becomes an abstract class.

Example

java code

public class AbstractExample {

    public static void main(String[] args) {
        
         // Book myBook= new Book(); Error Book is abstract
        Book myBook = new TechBook();// Ok
        myBook.getInfo();
        myBook.readBook();
        myBook.writeBook(null);
    }
}

abstract class Book {

    public abstract String readBook();

    public abstract void writeBook(String text);

    public String getInfo() {
        return "This is a abstractBook";
    }
}

class TechBook extends Book
{

    @Override
    public String readBook() {
        System.out.println("readBook");
        return null;
    }

    @Override
    public void writeBook(String text) {
         System.out.println("writeBook");
    }
    
}

can a non-abstract class have abstract methods?

nope it can't , To have a abstract methods you must define your class as abstract .

updated.

Abstract class is a class that can not be instantiated, it exists extensively for inheritance and it must be inherited.

Unlike interfaces, abstract classes can contain fields that are not static and final, and they can contain implemented methods. Such abstract classes are similar to interfaces, except that they provide a partial implementation, leaving it to subclasses to complete the implementation. If an abstract class contains only abstract method declarations, it should be declared as an interface instead.

An abstract class may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be sub-classed. Syntax :
public abstract class GraphicObject
{
// declare fields
// declare non-abstract methods
abstract void draw();
}

No, We can't create abstract class without abstract method. abstract class must have at-least one abstract method.