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

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

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

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