Summary - Abstract class
- Java course
- Intermediate
- Lesson 58/116
- 5 min
Combine abstract methods and sealed to write a geometry hierarchy.
Summary: Geometry hierarchy
In this summary you'll design a geometry hierarchy. The Shape abstract class defines the area abstract method; Circle and Square implement this contract. You close the hierarchy with sealed.
First you'll define abstract class Shape { abstract double area(); }. Then Circle and Square subclasses override the area method. Finally you close the hierarchy with sealed class Shape permits Circle, Square. This structure combines abstract methods and sealed in a real scenario. Each subclass writes its own area calculation; but all follow the Shape contract.
Practice Define the Shape abstract class and Circle
Define an abstract class Shape; add the abstract double area(); signature. Write class Circle extends Shape; add a double r field, a Circle(double r) constructor (assign the field) and a public double area() method whose body returns Math.PI * r * r;. In main create Circle c = new Circle(2); and run System.out.println(c.area());.
Starter code
// Write the abstract geometry class
// Add the Circle subclass and compute its area
// Print the result in mainSample solution
abstract class Shape {
abstract double area();
}
class Circle extends Shape {
double r;
Circle(double r) {
this.r = r;
}
public double area() {
return Math.PI * r * r;
}
}
class Main {
public static void main(String[] args) {
Circle c = new Circle(2);
System.out.println(c.area());
}
}Expected output
12.566370614359172
Practice Close the hierarchy with sealed Shape
In this file you'll make Shape sealed and add the Circle and Square subclasses. Define sealed abstract class Shape permits Circle, Square; add abstract double area(); inside. Write final class Circle extends Shape; add double r, Circle(double r) and public double area() (body return Math.PI * r * r;). Write final class Square extends Shape; add double s, Square(double s) and public double area() (body return s * s;). In main print new Circle(2).area() and new Square(3).area() on separate lines.
Starter code
abstract class Shape {
abstract double area();
}
class Circle extends Shape {
double r;
Circle(double r) {
this.r = r;
}
public double area() {
return Math.PI * r * r;
}
}
class Main {
public static void main(String[] args) {
Circle c = new Circle(2);
System.out.println(c.area());
}
}
// Close the hierarchy with sealed
// Add the Square subclass
// Print circle and square areas in mainSample solution
sealed abstract class Shape permits Circle, Square {
abstract double area();
}
final class Circle extends Shape {
double r;
Circle(double r) {
this.r = r;
}
public double area() {
return Math.PI * r * r;
}
}
final class Square extends Shape {
double s;
Square(double s) {
this.s = s;
}
public double area() {
return s * s;
}
}
class Main {
public static void main(String[] args) {
System.out.println(new Circle(2).area());
System.out.println(new Square(3).area());
}
}Expected output
12.566370614359172 9.0
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How should Shape be defined in the geometry hierarchy?
- interface
- record
- final class
- abstract class Correct answer
Explanation: Shape is an abstract class; subclasses inherit it.
2 Match the geometry hierarchy parts
| Term | Meaning |
|---|---|
Shape |
Abstract base |
Circle |
Circle subclass |
area |
Abstract method |
sealed |
Closing keyword |
3 Complete the abstract area signature
abstract class Shape {
____ double area();
}abstract class Shape {
abstract double area();
}Explanation: The abstract keyword declares a bodyless signature.