Abstract Classes

Vocab Bar
Abstract Class A class that represents an abstract concept, and is therefore only partially implemented.
Abstract Method A method that has no implementation code, just a header (essentially a placeholder).

Abstract classes are classes that are not fully implemented because some of their methods represent abstract concepts. For example, if I had a Geometry class that calculated the area and perimeter of different shapes, the methods would be specific to each different shape (calculating the area of a circle is different than calculating the area of a square). Therefore, the Geometry class would be an abstract class that had subclasses representing the different shapes (circles, triangles, squares, etc). Below is an example:

    public abstract class Geometry {

        private String shape;

        public Geometry(String shape) {

            this.shape = shape;


        // not all methods in abstract classes must be abstract

        public String getShapeName() {

            return shape;


        // abstract methods have no implementation code

        public abstract double perimeter();

        public abstract double area();


    /* classes which extend abstract classes must implement all of their abstract methods */

    public class Square extends Geometry {

        public double sideLength;

        public Square(double sideLength, String name) {


            this.sideLength = sideLength;



        public double perimeter() {

            return 4 * sideLength;


        public double area() {

            return Math.pow(side, 2);



Abstract classes have a key few rules:

  1. If a class has any abstract methods, it must be declared an abstract class (by using the abstract keyword).

  2. Any subclass of an abstract class must implement all of the superclass’s abstract methods.

  3. Instances of abstract classes cannot be created (because they are incomplete).

  4. Constructors are optional in abstract classes.

In order to avoid rule #3, create an object with the type of the abstract class and assign it to an instance of the subclass, as shown below:

    Geometry squareProperties = new Square(5, "square");

Overall, abstract classes are very useful when creating classes/methods that cannot be fully defined without more specific information.

Lesson Quiz

1. Which of the following is not a feature of abstract classes?

a. An abstract class can have both abstract and non-abstract methods.
b. A class must be an abstract class if it has any abstract methods.
c. An abstract subclass can choose which abstract methods to implement.
d. An abstract class or method must have the abstract keyword in its header.

2. If Car is an abstract class, what must be wrong with the below code?

    Car myCar; // variable declaration, line 1

    myCar = new Car("Toyota"); // variable assignment, line 2
a. It should not have any parameters in its instantiation.
b. myCar cannot be an object of type Car (line 1 is wrong).
c. myCar cannot equal an instance of the abstract class Car (line 2 is wrong).
d. The syntax of the code is incorrect.

Written by Chris Elliott

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