JAVA

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