Inheriting from Abstraction

abstract Classes

We noted in the introduction to this section that inheritance is a way to share behaviors among classes. You’ll sometimes find yourself creating a base class as a way to share behaviors among related classes. However, in such situations, it is not always desirable for instances of the base class to be created.

For example, suppose we began coding two classes, HouseCat and Tiger. Upon writing the code, we realized that there was some common data and behaviors. For example, they both make a noise, come from the same biological family, and get hungry. In order to reduce code repetition, we combined those in Cat (as above).

public class Cat {
   // Cat class definition

public class HouseCat extends Cat {
   // HouseCat class definition

public class Tiger extends Cat {
   // Tiger class definition

In reality, though, we might not want objects of type Cat to be created since such a cat couldn’t actually exist (a real cat would have a specific genus and species, for example). We could prevent objects of type Cat from being created, while still enabling sharing of behavior among its subclasses, by making Cat an abstract class.

Change the signature on Cat:

public abstract class Cat {
   // Cat class definition

Now, in Main, if you try creating a new Cat object:

Cat salem = new Cat(8);

IntelliJ has your back with a handy error message that an abstract class cannot be instantiated.

In order to use the behavior of an abstract class, we must extend it.

abstract Methods

We have another tool that we may use here, which is an abstract method. An abstract method is a method in an abstract class that does not have a body. In other words, it does not have any associated code, only a signature. It must also be marked abstract.

In our abstract Cat class, it would make sense to make an abstract noise method since all types of cats make noise. By creating this abstract method, we force any class that extends Cat to provide its own implementation of that behavior.

public abstract String noise();

Now, classes such as HouseCat and Tiger, which both extend Cat, must provide their own version of noise(), with the exact same method signature.


Check Your Understanding


A class derived from an abstract class must implement all of the abstract methods it inherits.

  1. True

  2. False


When might a programmer want to make a class abstract?

  1. When a class has no real data or behavior

  2. When expressionism just won’t cut it

  3. When that class needs to be instantiated in more than one package

  4. When shared behavior is desired among a group of non-abstract classes