The definition of a class should not be instantiated since it only represents the abstract concept. For example, the instance of pet can not be seen. What you can see are the instances of cat, dog, pig or horse. The instance of pet can not be seen and does not make sense.
Abstract Classes
As with object-oriented programming, declaring abstract concepts might be desired, but there is no possibility that you can create an instance of same. For example, the abstract concept of numbers is represented by Number class available with the java.lang package. The Number class serves as a model for numbers used in programs but is impossible to create the generic object.
The Number class is parent class of number of other number classes like Integer and double which implements specific types of numbers. The abstracted class represented by the number class, which implements abstract concepts. Abstract classes can be subclassed but it cannot have instance of it.