Abstract Class Summary

Abstract Class Concept

• Class that cannot be instantiated
• May contain abstract and concrete methods
• Declared using abstract keyword
• Example:

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

Abstract Methods

• Methods without implementation
• Must be implemented by concrete subclasses
• Declared using abstract keyword
• Cannot be private or final
• Example:

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

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;
    }
  }
  

Abstract Class Rules

• Cannot create instance of abstract class
• Class must be abstract if it has abstract methods
• Abstract class can have concrete methods
• Abstract class can have fields
• Can have constructors (called by subclasses)

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();  // each animal sounds different
  
    void sleep() {             // common to all animals
      System.out.println("Zzz");
    }
  }
  

Best Practices

• Use abstract classes to define base behavior
• Keep abstract classes focused and cohesive
• Document expected behavior of abstract methods
• Consider interfaces for pure abstractions
• Use meaningful names for abstract classes
• Provide default implementations when possible