JAVA

What are abstract methods in java

An abstract method is the method which does’nt have any body. 
Abstract method is declared with
keyword abstract and semicolon in place of method body.

  public abstract void <method name>();
Ex : public abstract void getDetails();
It is the responsibility of subclass to provide implementation to 
abstract method defined in abstract class

is it necessary for abstract class to have abstract method

No, abstract class can have zero abstract methods.

can you declare an abstract method in a non abstract class

No. A normal class(non-abstract class) cannot have abstract methods.

is it necessary for abstract class to have abstract method

Abstract classes CAN have non-abstract methods. 

can abstract class have non abstract methods in java

abstract class AbstractDemo { // Abstract class
   private int i = 0;
   public void display() { // non-abstract method
      System.out.print("Welcome to Tutorials Point");
   }
}
public class InheritedClassDemo extends AbstractDemo {
   public static void main(String args[]) {
      AbstractDemo demo = new InheritedClassDemo();
      demo.display();
   }
}

Java Abstract Class and Method

abstract class Language {

  // method of abstract class
  public void display() {
    System.out.println("This is Java Programming");
  }
}

class Main extends Language {
  public static void main(String[] args) {
    
    // create an object of Main
    Main obj = new Main();

    // access method of abstract class
    // using object of Main class
    obj.display();
  }
}

Can we add a non-abstract method into abstract class?

Yes, we can. An abstract class can have both abstract and non-abstract methods