non abstract method in abstract class code example

Example 1: is it necessary for abstract class to have abstract method

No, abstract class can have zero abstract methods.

Example 2: 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();
   }
}

Example 3: can you declare an abstract method in a non abstract class

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

Example 4: Can we add a non-abstract method into abstract class?

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