Using abstract

• The keyword abstract is used when creating a class hierarchy.

• Often when designing the hierarchy, you will find that methods in the parent class don't actually do anything.

• However, if you do not write at least the signature of the method in the parent, then your hierarchy cannot take advantage of polymorphism.

• Making the method abstract takes care of this.

• Syntactically, by making a method abstract, you must make the class abstract.

  
  public abstract class Abc
  {
      public abstract void method();
  }
  

• The idea is that the class is used to organize the hierarchy, but is not meant to be instantiated. In fact, it cannot be.

• If a child class does not override all of the abstract methods, then it too must be abstract.

• At some point, a child class will override them all, then an object can be instantiated.

• The instantiated class is called the concrete object.