Understanding Class Hierarchies
Class hierarchies in Java enable structured relationships between classes through inheritance. This concept is foundational to object-oriented programming and enhances code reusability and organization, which simplifies managing complex systems.
In a class hierarchy, a superclass can pass down properties and methods to its subclasses. This allows subclasses to inherit functionality while also being able to define or modify behaviors that are specific to themselves. This leads to a more manageable code structure and helps in maintaining code consistency.
Components of Class Hierarchies
- Superclass: The parent class from which properties and methods are inherited.
- Subclass: A child class that inherits from the superclass and can override or extend its functionality.
- Polymorphism: The ability to treat objects of different classes through a common interface, allowing for method overriding.
Example Hierarchy
public class Shape {
protected String name;
public Shape(String name) {
this.name = name;
}
public double calc_area() {
return 0.0;
}
public void print_shape() {
System.out.println("Shape: " + name);
}
}
Derived Class: Rectangle
Now, let’s create a Rectangle class that inherits from Shape.
public class Rectangle extends Shape {
private int length;
private int width;
public Rectangle(String name, int length, int width) {
super(name);
this.length = length;
this.width = width;
}
}
class Circle extends Shape {
@Override
public double calc_area() {
return length * width;
}
}
Derived Class: Triangle
Next, let’s create a Triangle class that also inherits from Shape.
public class Triangle extends Shape {
private int side1;
private int side2;
private int side3;
public Triangle(String name, int s1, int s2, int s3) {
super(name);
this.side1 = s1;
this.side2 = s2;
this.side3 = s3;
}
}
class Square extends Shape {
@Override
public String draw() {
return "Drawing a square";
}
}
Testing Our Classes
Let’s create instances of Rectangle and Triangle and call their methods.
}
public class Main {
public static void main(String[] args) {
Shape myCircle = new Circle();
Shape mySquare = new Square();
System.out.println(myCircle.draw()); // Outputs: Drawing a circle
System.out.println(mySquare.draw()); // Outputs: Drawing a square
}
}
Shape: Rectangle
Area of rectangle: 20.0
Shape: Triangle
Area of triangle: 6.0
In the example above, Shape acts as the superclass, while Circle and Square are subclasses that override the draw method to provide specific behavior. This demonstrates polymorphism, enabling us to reference subclasses using a superclass type and invoke the overridden methods dynamically at runtime. This is particularly powerful as it allows for flexibility in the code, facilitating easier changes and enhancements.
Benefits of Using Class Hierarchies
- Code Reusability: Common behaviors are defined in the superclass, significantly reducing code duplication and making it easier to maintain and update.
- Organized Structure: A clear hierarchical structure in your code aids in understanding the relationships between classes, making the codebase more intuitive and manageable.
- Polymorphism: Allows for dynamic method resolution, enabling methods to be invoked on objects of subclasses through references of the superclass type.
Practical Exercise: Popcorn Hack 1
Let’s implement the Triangle subclass to deepen your understanding. Below is a half-completed method for the Triangle class. Your task is to complete the draw method:
```java class Shape {
public String draw() {
return "Drawing a shape";
} }
class Triangle extends Shape {
@Override
public String draw() {
// TODO: Implement this method
}
}
public class Main {
public static void main(String[] args) {
Shape myTriangle = new Triangle();
System.out.println(myTriangle.draw()); // Should output: "Drawing a triangle."
}
}
``` Make sure your implementation returns a unique string for the `Triangle` class. This exercise will help reinforce how subclasses can extend functionality.
class Shape {
public String draw() {
return "Drawing a shape";
}
}
class Triangle extends Shape {
@Override
public String draw() {
return "Drawing a triangle.";
}
}
public class Main {
public static void main(String[] args) {
Shape myTriangle = new Triangle();
System.out.println(myTriangle.draw()); // Should output: "Drawing a triangle."
}
}
// Execute the main method
new Main().main(null);
Drawing a triangle.
Expanding Your Skills: Adding a Rectangle Class
Next, let’s implement the Rectangle subclass. Below is the basic setup for it. Your task is to implement the draw method for the Rectangle class:
```java class Rectangle extends Shape {
@Override
public String draw() {
return "Drawing a rectangle.";
} }
public class Main { public static void main(String[] args) { Shape myRectangle = new Rectangle(); System.out.println(myRectangle.draw()); // Should output: “Drawing a rectangle.” } }
public static void main(String[] args) {
Shape myRectangle = new Rectangle();
System.out.println(myRectangle.draw()); // Should output: "Drawing a rectangle."
}
``` Complete the `draw` method in `Rectangle`, ensuring it returns a unique string. This will reinforce how multiple subclasses can have distinct implementations of the same method, enhancing your understanding of class hierarchies.
// Define the Shape class
abstract class Shape {
public abstract String draw();
}
// Define the Rectangle subclass
class Rectangle extends Shape {
@Override
public String draw() {
return "Drawing a rectangle.";
}
}
// Main execution
Shape myRectangle = new Rectangle();
System.out.println(myRectangle.draw()); // Should output: "Drawing a rectangle."
Drawing a rectangle.
Advanced Challenge: Area Calculation
Now, let’s enhance our Shape class to include an area calculation feature. Modify the Shape class to include an area method, and implement it in your subclasses. Below is a structure to help you get started:
```java class Shape {
public String draw() {
return "Drawing a shape";
}
public double area() {
return 0; // Default implementation
} }
class Circle extends Shape {
@Override
public double area() {
// TODO: Implement area calculation
}
}
class Square extends Shape {
@Override
public double area() {
// TODO: Implement area calculation
}
}
// Implement for Triangle and Rectangle as well
``` Ensure each subclass calculates and returns its area correctly. This will allow you to practice method overriding further and understand how different shapes can extend base functionalities.
import java.lang.Math;
abstract class Shape {
public String draw() {
return "Drawing a shape";
}
public abstract double area(); // Abstract method for area
}
class Circle extends Shape {
private double radius;
public Circle(double radius) {
this.radius = radius;
}
@Override
public double area() {
return Math.PI * radius * radius; // Area = πr²
}
}
class Square extends Shape {
private double side;
public Square(double side) {
this.side = side;
}
@Override
public double area() {
return side * side; // Area = side²
}
}
class Rectangle extends Shape {
private double length;
private double width;
public Rectangle(double length, double width) {
this.length = length;
this.width = width;
}
@Override
public double area() {
return length * width; // Area = length * width
}
}
class Triangle extends Shape {
private double base;
private double height;
public Triangle(double base, double height) {
this.base = base;
this.height = height;
}
@Override
public double area() {
return 0.5 * base * height; // Area = 0.5 * base * height
}
}
// Main execution block
Shape[] shapes = {
new Circle(5),
new Square(4),
new Rectangle(3, 6),
new Triangle(4, 5)
};
for (Shape shape : shapes) {
System.out.println(shape.getClass().getSimpleName() + " Area: " + shape.area());
}
Circle Area: 78.53981633974483
Square Area: 16.0
Rectangle Area: 18.0
Triangle Area: 10.0
Homework Hack
For your homework, create your own class hierarchy for shapes. You should have a base class called Shape with subclasses Triangle, Rectangle, and Hexagon. Each subclass should implement a method called draw(), returning a unique string for each shape type.
- `Triangle`: "Drawing a triangle."
- `Rectangle`: "Drawing a rectangle."
- `Hexagon`: "Drawing a hexagon."
Make sure to demonstrate polymorphism by creating an array of Shape types and iterating through it to call the draw() method. This will reinforce your understanding of class hierarchies and method overriding.
// Base class
abstract class Shape {
public abstract String draw();
}
// Subclass for Triangle
class Triangle extends Shape {
@Override
public String draw() {
return "Drawing a triangle.";
}
}
// Subclass for Rectangle
class Rectangle extends Shape {
@Override
public String draw() {
return "Drawing a rectangle.";
}
}
// Subclass for Hexagon
class Hexagon extends Shape {
@Override
public String draw() {
return "Drawing a hexagon.";
}
}
Shape[] shapes = {new Triangle(), new Rectangle(), new Hexagon()};
for (Shape shape : shapes) {
System.out.println(shape.draw());
}
Drawing a triangle.
Drawing a rectangle.
Drawing a hexagon.