Inheritance Summary

Inheritance Concept

• Mechanism to extend existing code
• Models "IS-A" relationship between classes
• Uses extends keyword in Java
• Example:

  class ColoredCircle extends Circle {
    private Color color;
  }
  

Key Characteristics

• Subclass inherits:
  • Public fields from parent
  • Public methods from parent
• Private members remain inaccessible
• super keyword accesses parent members
• Example:

  class ColoredCircle extends Circle {
    public ColoredCircle(Point center, double radius, Color color) {
      super(center, radius);  // call parent constructor
      this.color = color;
    }
  }
  

When to Use Inheritance

• Use when subclass IS-A superclass
• Subclass should be substitutable for superclass
• Example of proper use:

  // Square IS-A Shape
  class Square extends Shape { }
  

• Example of improper use:

  // Circle is not a Point
  class Circle extends Point { }
  

Composition vs Inheritance

• Composition: HAS-A relationship

  class Car {
    private Engine engine;  // Car HAS-A Engine
  }
  

• Inheritance: IS-A relationship

  class Car extends Vehicle {  // Car IS-A Vehicle
  }
  

Run-Time Type

• Compile-time type: Declared type of variable
• Run-time type: Actual type of object
• Example:

  Shape s = new Circle(p, 1.0);
  // Compile-time type: Shape
  // Run-time type: Circle
  

Best Practices

• Use inheritance sparingly
• Prefer composition over inheritance
• Ensure IS-A relationship exists
• Document inheritance relationships
• Keep inheritance hierarchies shallow
• Consider making classes final if not meant for inheritance