Understand the idea

An abstract class can combine shared behavior with abstract operations that subclasses implement. You cannot directly instantiate the abstract class. The contract allows callers to work with the general concept instead of every implementation detail.

Explore the details

Abstract Classes and Methods

Data abstraction is the process of hiding certain details and showing only essential information to the user.
Abstraction can be achieved with either abstract classes or interfaces (which you will learn more about in the next chapter).

The abstract keyword is a non-access modifier, used for classes and methods:

• Abstract class: is a restricted class that cannot be used to create objects (to access it, it must be inherited from another class).
• Abstract method: can only be used in an abstract class, and it does not have a body. The body is provided by the subclass (inherited from).

An abstract class can have both abstract and regular methods:

From the example above, it is not possible to create an object of the Animal class:

To access the abstract class, it must be inherited from another class. Let's convert the Animal class we used in the Polymorphism chapter to an abstract class:

abstract class Shape {
    String color;
    abstract double area();  // no body — subclasses must provide it

    void describe() {
        System.out.println(color + " shape, area = " + area());
    }
}

class Circle extends Shape {
    double r;
    Circle(String c, double r) { this.color = c; this.r = r; }
    double area() { return 3.14159 * r * r; }
}

class Rectangle extends Shape {
    double w, h;
    Rectangle(String c, double w, double h) { this.color = c; this.w = w; this.h = h; }
    double area() { return w * h; }
}

new Circle("red", 5).describe();      // red shape, area = 78.53975
new Rectangle("blue", 4, 6).describe(); // blue shape, area = 24.0

A complete runnable example

This complete Main.java program focuses on the lesson idea. Reference fragments above may demonstrate individual statements; this example includes the imports, entry point, and supporting types it needs.

public class Main {
    public static void main(String[] args) throws Exception {
        Shape shape = new Square(4);
        System.out.println(shape.area());
    }
}
abstract class Shape { abstract int area(); }
class Square extends Shape {
    private final int side;
    Square(int side) { this.side = side; }
    @Override int area() { return side * side; }
}

16

Change it. Run it. Understand it.

Load this lesson example into the editor. Change one value or statement, predict the result, then press RUN to execute Java.

Make it your own

Your learning checkpoint

• Explain the example without reading the output first.
• Make the change requested in the practice task.
• Describe why the result changed—or why compilation failed.

Ready to move on? Follow the next lesson in course order, or return to the curriculum to review a prerequisite.