Abstract Classes and Methods

Written by PChan on 2017-04-25

Overview

Abstract Method
A method without a body (merely the return type and the signature).
Abstract Class
A class that represents something abstract, like an idea.

Some of the methods inside an abstract class may be undefined because there is no valid definition for them. These methods must be defined in the subclasses unless the subclass is abstract as well. This enforces a degree of uniformity between your classes.

Defining Abstract Classes and Methods

An abstract method is declared by adding the abstract keyword after the visibility and before the return type:

// General Syntax
public abstract returnType methodName();

// Example
public abstract double area();

Generally, an abstract method would not have an body as you would need to override the definition anyway. But, adding a body would not cause an error.

If you have at least one abstract method in a class, you must make the class abstract like so:

// General Syntax
public abstract class AbstractClass{

}

// Example
public abstract class Quadralateral{

}

Warning

An abstract method must be inside an abstract class. However, an abstract class does NOT have to contain an abstract method.

You would extend an abstract class the way you would extend any other classes:

public class SubClass extends AbstractClass{

}

Key Takeaways

  • Unless the subclass of an abstract class is declared abstract, it must implement all methods in the abstract class
  • An abstract class CAN NOT be instantiated directly; it may be used as the type of a variable
  • The keyword abstract is used to declare abstract methods and abstract classes
  • Abstract methods typically do not have any valid definitions and each subclass would have its own implementation
  • Abstract classes may include constructors, but they CAN NOT be used to create an instance of the class
  • Abstract methods must be inside abstract classes, but abstract classes need not contain abstract method