Abstract Class Summary

Abstract Class Concept

• Model an IS-A relationship (Circle IS-A Shape)
• Declared using abstract keyword
• Example:

  abstract class Shape {
    abstract double getArea();  // abstract method
  
    boolean isSymmetric() {     // concrete method
      return true;
    }
  }
  

Notes:

• Abstract class cannot be instantiated
• Class must be abstract if it has abstract methods
• Can contain abstract methods, concrete methods and fields
• Can have no abstract methods
• Can have constructors for subclasses to call

Abstract Methods

• Methods without implementation

• Declared using abstract keyword

• Example:

  abstract class Shape {
    abstract double getArea();  // subclasses must implement
  }
  

Notes:

• All abstract methods must be implemented by concrete subclasses
• Abstract methods cannot be private or final
• Subclasses must override all inherited abstract methods from that abstract class

Concrete Classes

• Classes that implement all abstract methods
• Can be instantiated
• Must override all inherited abstract methods
• Example:

  class Circle extends Shape {
    private double radius;
  
    @Override
    double getArea() {  // implements abstract method
      return Math.PI * radius * radius;
    }
  }
  

When to Use Abstract Classes

• Define common characteristics of related classes
• Share code among several related classes
• Want to declare non-public members
• Need to provide default behavior
• Example:

  abstract class Animal {
    private String name;
  
    public Animal(String name) {
      this.name = name;
    }
  
    abstract void makeSound();
  
    void sleep() {
      System.out.println("Zzz");
    }
  }
  

Best Practices

• Use abstract classes to define base behavior
• Keep abstract classes focused and cohesive (i.e., all methods and fields should serve a clear purpose)
• Document expected behavior of abstract methods
• Consider interfaces for pure abstractions
• Use meaningful names for abstract classes
• Provide default implementations when possible